“Some day we will build a thinking machine. It will be a truly intelligent machine. One that can see and hear and speak. A machine that will be proud of us.”

Yet 5 years later it filed for bankruptcy protection under Chapter 11. The site inc.com has a fascinating article titled "The Rise and Fall of Thinking Machines". The article covers the growth of the industry and how a cosy relationship with Thinking Machines and DARPA over-heated the market, to the point of collapse. It explains how and why commerce moved away from AI and towards more practical number-crunching super computers.

The 5th Generation Computer project was a USD 400 million project in Japan to build a next generation computer. Valves (or Tubes) was the first generation, transistors the second, integrated circuits the third and finally microprocessors was the fourth. The fifth was intended to be a machine capable of effective Artificial Intelligence. This project spurred an "arms" race with the UK and USA, that caused much of the AI bubble. The 5GP would provide massive multi-cpu parallel processing hardware along with powerful knowledge representation and reasoning software via Prolog; a type of expert system. By 1992 the project was considered a failure and cancelled. It was the largest and most visible commercial venture for Prolog, and many of the failures are pinned on the problems of trying to run a logic based programming language concurrently on multi CPU hardware with effective results. Some believe that the failure of the 5GP project tainted Prolog and resigned it academia, see "Whatever Happened to Prolog" by John C. Dvorak.

However while research funding dried up and the term AI became less used, many green shoots where planted and continued more quietly under discipline specific names: cognitive systems, machine learning, intelligent systems, knowledge representation and reasoning. Offshoots of these then made their way into commercial systems, such as expert systems in the Business Rules Management System (BRMS) market.

Imperative, system based languages, languages such as C, C++, Java and C#/.Net have dominated the last 20 years, enabled by the practicality of the languages and ability to run with good performance on commodity hardware. However many believe there is renaissance underway in the field of AI, spurred by advances in hardware capabilities and AI research. In 2005 Heather Havenstein authored "Spring comes to AI winter" which outlines a case for this resurgence, which she refers to as a spring. Norvig and Russel dedicate several pages to what factors allowed the industry to over come it's problems and the research that came about as a result:

“Recent years have seen a revolution in both the content and the methodology of work in artificial intelligence. It is now more common to build on existing theories than to propose brand-new ones, to base claims on rigorous theorems or hard experimental evidence rather than on intuition, and to show relevance to real-world applications rather than toy examples.” (Artificial Intelligence: A Modern Approach.)

Computer vision, neural networks, machine learning and knowledge representation and reasoning (KRR) have made great strides towards becoming practical in commercial environments. For example, vision-based systems can now fully map out and navigate their environments with strong recognition skills. As a result we now have self-driving cars about to enter the commercial market. Ontological research, based around description logic, has provided very rich semantics to represent our world. Algorithms such as the tableaux algorithm have made it possible to effectively use those rich semantics in large complex ontologies. Early KRR systems, like Prolog in 5GP, were dogged by the limited semantic capabilities and memory restrictions on the size of those ontologies.

1.1.2. Knowledge Representation and Reasoning

In A Little History talks about AI as a broader subject and touches on Knowledge Representation and Reasoning (KRR) and also Expert Systems, I'll come back to Expert Systems later.

KRR is about how we represent our knowledge in symbolic form, i.e. how we describe something. Reasoning is about how we go about the act of thinking using this knowledge. System based object-oriented languages, like C++, Java and C#, have data definitions called classes for describing the composition and behaviour of modeled entities. In Java we call exemplars of these described things beans or instances. However those classification systems are limited to ensure computational efficiency. Over the years researchers have developed increasingly sophisticated ways to represent our world. Many of you may already have heard of OWL (Web Ontology Language). There is always a gap between what can be theoretically represented and what can be used computationally in practically timely manner, which is why OWL has different sub-languages from Lite to Full. It is not believed that any reasoning system can support OWL Full. Although each year algorithmic advances try to narrow that gap and improve the expressiveness available to reasoning engines.

There are also many approaches to how these systems go about thinking. You may have heard discussions comparing the merits of forward chaining, which is reactive and data driven, with backward chaining, which is passive and query driven. Many other types of reasoning techniques exist, each of which enlarges the scope of the problems we can tackle declaratively. To list just a few: imperfect reasoning (fuzzy logic, certainty factors), defeasible logic, belief systems, temporal reasoning and correlation. You don't need to understand all these terms to understand and use Drools. They are just there to give an idea of the range of scope of research topics, which is actually far more extensive, and continues to grow as researchers push new boundaries.

KRR is often referred to as the core of Artificial Intelligence. Even when using biological approaches like neural networks, which model the brain and are more about pattern recognition than thinking, they still build on KRR theory. My first endeavours with Drools were engineering oriented, as I had no formal training or understanding of KRR. Learning KRR has allowed me to get a much wider theoretical background. Allowing me to better understand both what I've done and where I'm going, as it underpins nearly all of the theoretical side to our Drools R&D. It really is a vast and fascinating subject that will pay dividends for those who take the time to learn. I know it did and still does for me. Bracham and Levesque have written a seminal piece of work, called "Knowledge Representation and Reasoning" that is a must read for anyone wanting to build strong foundations. I would also recommend the Russel and Norvig book "Artificial Intelligence, a modern approach" which also covers KRR.

1.1.3. Rule Engines and Production Rule Systems (PRS)

We've now covered a brief history of AI and learnt that the core of AI is formed around KRR. We've shown than KRR is vast and fascinating subject which forms the bulk of the theory driving Drools R&D.

The rule engine is the computer program that delivers KRR functionality to the developer. At a high level it has three components:

Ontology
Rules
Data

As previously mentioned the ontology is the representation model we use for our "things". It could use records or Java classes or full-blown OWL based ontologies. The rules perform the reasoning, i.e., they facilitate "thinking". The distinction between rules and ontologies blurs a little with OWL based ontologies, whose richness is rule based.

The term "rules engine" is quite ambiguous in that it can be any system that uses rules, in any form, that can be applied to data to produce outcomes. This includes simple systems like form validation and dynamic expression engines. The book "How to Build a Business Rules Engine" (2004) by Malcolm Chisholm exemplifies this ambiguity. The book is actually about how to build and alter a database schema to hold validation rules. The book then shows how to generate VB code from those validation rules to validate data entry. While perfectly valid, this is very different to what we are talking about.

Drools started life as a specific type of rule engine called a Production Rule System (PRS) and was based around the Rete algorithm (usually pronounced as two syllables, e.g., REH-te or RAY-tay). The Rete algorithm, developed by Charles Forgy in 1974, forms the brain of a Production Rule System and is able to scale to a large number of rules and facts. A Production Rule is a two-part structure: the engine matches facts and data against Production Rules - also called Productions or just Rules - to infer conclusions which result in actions.

when

    <conditions>

then

    <actions>;

The process of matching the new or existing facts against Production Rules is called pattern matching, which is performed by the inference engine. Actions execute in response to changes in data, like a database trigger; we say this is a data driven approach to reasoning. The actions themselves can change data, which in turn could match against other rules causing them to fire; this is referred to as forward chaining

Drools implements and extends the Rete algorithm;. The Drools Rete implementation is called ReteOO, signifying that Drools has an enhanced and optimized implementation of the Rete algorithm for object oriented systems. Our more recent work goes well beyond Rete. Other Rete based engines also have marketing terms for their proprietary enhancements to Rete, like RetePlus and Rete III. The most common enhancements are covered in "Production Matching for Large Learning Systems" (1995) by Robert B. Doorenbos' thesis, which presents Rete/UL. A PRS using another algorithm called "Leaps" used to be provided as well, but was retired as it became unmaintained. The good news is that our research is close to producing an algorithm that merges the benefits of Leaps with Rete.

The Rules are stored in the Production Memory and the facts that the Inference Engine matches against are kept in the Working Memory. Facts are asserted into the Working Memory where they may then be modified or retracted. A system with a large number of rules and facts may result in many rules being true for the same fact assertion; these rules are said to be in conflict. The Agenda manages the execution order of these conflicting rules using a Conflict Resolution strategy.

Figure 1.1. High-level View of a Production Rule System

1.1.4. Hybrid Reasoning Systems (HRS)

You may have read discussions comparing the merits of forward chaining (reactive and data driven) or backward chaining(passive query). Here is a quick explanation of these two main types of reasoning.

Forward chaining is "data-driven" and thus reactionary, with facts being asserted into working memory, which results in one or more rules being concurrently true and scheduled for execution by the Agenda. In short, we start with a fact, it propagates through the rules, and we end in a conclusion.

Figure 1.2. Forward Chaining

Backward chaining is "goal-driven", meaning that we start with a conclusion which the engine tries to satisfy. If it can't, then it searches for conclusions that it can satisfy. These are known as subgoals, that will help satisfy some unknown part of the current goal. It continues this process until either the initial conclusion is proven or there are no more subgoals. Prolog is an example of a Backward Chaining engine. Drools can also do backward chaining, which we refer to as derivation queries.

Figure 1.3. Backward Chaining

Historically you would have to make a choice between systems like OPS5 (forward) or Prolog (backward). Nowadays many modern systems provide both types of reasoning capabilities. There are also many other types of reasoning techniques, each of which enlarges the scope of the problems we can tackle declaratively. To list just a few: imperfect reasoning (fuzzy logic, certainty factors), defeasible logic, belief systems, temporal reasoning and correlation. Modern systems are merging these capabilities, and others not listed, to create hybrid reasoning systems (HRS).

While Drools started out as a PRS, 5.x introduced Prolog style backward chaining reasoning as well as some functional programming styles. For this reason we now prefer the term Hybrid Reasoning System when describing Drools.

Drools currently provides crisp reasoning, but imperfect reasoning is almost ready. Initially this will be imperfect reasoning with fuzzy logic; later we'll add support for other types of uncertainty. Work is also under way to bring OWL based ontological reasoning, which will integrate with our traits system. We also continue to improve our functional programming capabilities.

1.1.5. Expert Systems

You will often hear the terms expert systems used to refer to production rule systems or Prolog-like systems. While this is normally acceptable, it's technically incorrect as these are frameworks to build expert systems with, rather than expert systems themselves. It becomes an expert system once there is an ontological model to represent the domain and there are facilities for knowledge acquisition and explanation.

Mycin is the most famous expert system, built during the 70s. It is still heavily covered in academic literature, such as the recommended book "Expert Systems" by Peter Jackson.

Figure 1.4. Early History of Expert Systems

1.1.6. Recommended Reading

General AI, KRR and Expert System Books

For those wanting to get a strong theoretical background in KRR and expert systems, I'd strongly recommend the following books. "Artificial Intelligence: A Modern Approach" is a must have, for anyone's bookshelf.

Introduction to Expert Systems
- Peter Jackson

Expert Systems: Principles and Programming
- Joseph C. Giarratano, Gary D. Riley

Knowledge Representation and Reasoning
- Ronald J. Brachman, Hector J. Levesque

Artificial Intelligence : A Modern Approach.
- Stuart Russell and Peter Norvig

Figure 1.5. Recommended Reading

Papers

Here are some recommended papers that cover interesting areas in rule engine research:

Production Matching for Large Learning Systems: Rete/UL (1993)
- Robert B. Doorenbos
Advances In Rete Pattern Matching
- Marshall Schor, Timothy P. Daly, Ho Soo Lee, Beth R. Tibbitts (AAAI 1986)
Collection-Oriented Match
- Anurag Acharya and Milind Tambe (1993)
The Leaps Algorithm
- Don Batery (1990)
Gator: An Optimized Discrimination Network for Active Database Rule Condition Testing
- Eric Hanson , Mohammed S. Hasan (1993)

Drools Books

There are currently three Drools books, all from Packt Publishing.

JBoss Drools Business Rules
- Paul Brown
Drools JBoss Rules 5.0 Developers Guide
- Michali Bali
Drools Developer's Cookbook
- Lucas Amador

Figure 1.6. Recommended Reading

1.2. Why use a Rule Engine?

Some frequently asked questions:

When should you use a rule engine?
What advantage does a rule engine have over hand coded "if...then" approaches?
Why should you use a rule engine instead of a scripting framework, like BeanShell?

We will attempt to address these questions below.

	It seems to me that in the excitement of working with rules engines, that people forget that a rules engine is only one piece of a complex application or solution. Rules engines are not really intended to handle workflow or process executions nor are workflow engines or process management tools designed to do rules. Use the right tool for the job. Sure, a pair of pliers can be used as a hammering tool in a pinch, but that's not what it's designed for.
	--Dave Hamu

As rule engines are dynamic (dynamic in the sense that the rules can be stored and managed and updated as data), they are often looked at as a solution to the problem of deploying software. (Most IT departments seem to exist for the purpose of preventing software being rolled out.) If this is the reason you wish to use a rule engine, be aware that rule engines work best when you are able to write declarative rules. As an alternative, you can consider data-driven designs (lookup tables), or script processing engines where the scripts are managed in a database and are able to be updated on the fly.

1.2.4. Scripting or Process Engines

Hopefully the preceding sections have explained when you may want to use a rule engine.

Alternatives are script-based engines that provide the drive for "changes on the fly", and there are many such solutions.

Alternatively Process Engines (also capable of workflow) such as jBPM allow you to graphically (or programmatically) describe steps in a process. Those steps can also involve decision points which are in themselves a simple rule. Process engines and rules often can work nicely together, so they are not mutually exclusive.

One key point to note with rule engines is that some rule engines are really scripting engines. The downside of scripting engines is that you are tightly coupling your application to the scripts. If they are rules, you are effectively calling rules directly and this may cause more difficulty in future maintenance, as they tend to grow in complexity over time. The upside of scripting engines is that they can be easier to implement initially, producing results quickly, and are conceptually simpler for imperative programmers.

Many people have also implemented data-driven systems successfully in the past (where there are control tables that store meta-data that changes your applications behavior) - these can work well when the control can remain very limited. However, they can quickly grow out of control if extended too much (such that only the original creators can change the applications behavior) or they cause the application to stagnate as they are too inflexible.

1.2.5. Strong and Loose Coupling

No doubt you have heard terms like "tight coupling" and "loose coupling" in systems design. Generally people assert that "loose" or "weak" coupling is preferable in design terms, due to the added flexibility it affords. Similarly, you can have "strongly coupled" and "weakly coupled" rules. Strongly coupled in this sense means that one rule "firing" will clearly result in another rule firing, and so on; in other words, there is a clear (probably obvious) chain of logic. If your rules are all strongly coupled, the chances are that the will turn out to be inflexible, and, more significantly, that a rule engine is an overkill. A clear chain can be hard coded, or implemented using a Decision Tree. This is not to say that strong coupling is inherently bad, but it is a point to keep in mind when considering a rule engine and the way you capture the rules. "Loosely" coupled rules should result in a system that allows rules to be changed, removed and added without requiring changes to other, unrelated rules.

1.3. Rete Algorithm

The Rete algorithm was invented by Dr. Charles Forgy and documented in his PhD thesis in 1978-79. A simplified version of the paper was published in 1982 (http://citeseer.ist.psu.edu/context/505087/0). The latin word "rete" means "net" or "network". The Rete algorithm can be broken into 2 parts: rule compilation and runtime execution.

The compilation algorithm describes how the Rules in the Production Memory are processed to generate an efficient discrimination network. In non-technical terms, a discrimination network is used to filter data as it propagates through the network. The nodes at the top of the network would have many matches, and as we go down the network, there would be fewer matches. At the very bottom of the network are the terminal nodes. In Dr. Forgy's 1982 paper, he described 4 basic nodes: root, 1-input, 2-input and terminal.

Figure 1.7. Rete Nodes

The root node is where all objects enter the network. From there, it immediately goes to the ObjectTypeNode. The purpose of the ObjectTypeNode is to make sure the engine doesn't do more work than it needs to. For example, say we have 2 objects: Account and Order. If the rule engine tried to evaluate every single node against every object, it would waste a lot of cycles. To make things efficient, the engine should only pass the object to the nodes that match the object type. The easiest way to do this is to create an ObjectTypeNode and have all 1-input and 2-input nodes descend from it. This way, if an application asserts a new Account, it won't propagate to the nodes for the Order object. In Drools when an object is asserted it retrieves a list of valid ObjectTypesNodes via a lookup in a HashMap from the object's Class; if this list doesn't exist it scans all the ObjectTypeNodes finding valid matches which it caches in the list. This enables Drools to match against any Class type that matches with an instanceof check.

Figure 1.8. ObjectTypeNodes

ObjectTypeNodes can propagate to AlphaNodes, LeftInputAdapterNodes and BetaNodes. AlphaNodes are used to evaluate literal conditions. Although the 1982 paper only covers equality conditions, many RETE implementations support other operations. For example, Account.name == "Mr Trout" is a literal condition. When a rule has multiple literal conditions for a single object type, they are linked together. This means that if an application asserts an Account object, it must first satisfy the first literal condition before it can proceed to the next AlphaNode. In Dr. Forgy's paper, he refers to these as IntraElement conditions. The following diagram shows the AlphaNode combinations for Cheese( name == "cheddar", strength == "strong" ):

Figure 1.9. AlphaNodes

Drools extends Rete by optimizing the propagation from ObjectTypeNode to AlphaNode using hashing. Each time an AlphaNode is added to an ObjectTypeNode it adds the literal value as a key to the HashMap with the AlphaNode as the value. When a new instance enters the ObjectType node, rather than propagating to each AlphaNode, it can instead retrieve the correct AlphaNode from the HashMap,thereby avoiding unnecessary literal checks.

There are two two-input nodes, JoinNode and NotNode, and both are types of BetaNodes. BetaNodes are used to compare 2 objects, and their fields, to each other. The objects may be the same or different types. By convention we refer to the two inputs as left and right. The left input for a BetaNode is generally a list of objects; in Drools this is a Tuple. The right input is a single object. Two Nodes can be used to implement 'exists' checks. BetaNodes also have memory. The left input is called the Beta Memory and remembers all incoming tuples. The right input is called the Alpha Memory and remembers all incoming objects. Drools extends Rete by performing indexing on the BetaNodes. For instance, if we know that a BetaNode is performing a check on a String field, as each object enters we can do a hash lookup on that String value. This means when facts enter from the opposite side, instead of iterating over all the facts to find valid joins, we do a lookup returning potentially valid candidates. At any point a valid join is found the Tuple is joined with the Object; which is referred to as a partial match; and then propagated to the next node.

Figure 1.10. JoinNode

To enable the first Object, in the above case Cheese, to enter the network we use a LeftInputNodeAdapter - this takes an Object as an input and propagates a single Object Tuple.

Terminal nodes are used to indicate a single rule having matched all its conditions; at this point we say the rule has a full match. A rule with an 'or' conditional disjunctive connective results in subrule generation for each possible logically branch; thus one rule can have multiple terminal nodes.

Drools also performs node sharing. Many rules repeat the same patterns, and node sharing allows us to collapse those patterns so that they don't have to be re-evaluated for every single instance. The following two rules share the first pattern, but not the last:

rule
when
    Cheese( $cheddar : name == "cheddar" )
    $person : Person( favouriteCheese == $cheddar )
then
    System.out.println( $person.getName() + " likes cheddar" );
end

rule
when
    Cheese( $cheddar : name == "cheddar" )
    $person : Person( favouriteCheese != $cheddar )
then
    System.out.println( $person.getName() + " does not like cheddar" );
end

As you can see below, the compiled Rete network shows that the alpha node is shared, but the beta nodes are not. Each beta node has its own TerminalNode. Had the second pattern been the same it would have also been shared.

Figure 1.11. Node Sharing

Chapter 2. User Guide

2.1. The Basics

2.1.1. Stateless Knowledge Session

So where do we get started? There are so many use cases and so much functionality in a rule engine such as Drools that it becomes beguiling. Have no fear my intrepid adventurer, the complexity is layered and you can ease yourself into with simple use cases.

Stateless session, not utilising inference, forms the simplest use case. A stateless session can be called like a function passing it some data and then receiving some results back. Some common use cases for stateless sessions are, but not limited to:

Validation
- Is this person eligible for a mortgage?
Calculation
- Compute a mortgage premium.
Routing and Filtering
- Filter incoming messages, such as emails, into folders.
- Send incoming messages to a destination.

So let's start with a very simple example using a driving license application.

public class Applicant {

    private String name;

    private int age;

    private boolean valid;

    // getter and setter methods here

}

Now that we have our data model we can write our first rule. We assume that the application uses rules to reject invalid applications. As this is a simple validation use case we will add a single rule to disqualify any applicant younger than 18.

package com.company.license

rule "Is of valid age"
when
    $a : Applicant( age < 18 )
then
    $a.setValid( false );
end

To make the engine aware of data, so it can be processed against the rules, we have to insert the data, much like with a database. When the Applicant instance is inserted into the engine it is evaluated against the constraints of the rules, in this case just two constraints for one rule. We say two because the type Applicant is the first object type constraint, and age < 18 is the second field constraint. An object type constraint plus its zero or more field constraints is referred to as a pattern. When an inserted instance satisfies both the object type constraint and all the field constraints, it is said to be matched. The $a is a binding variable which permits us to reference the matched object in the consequence. There its properties can be updated. The dollar character ('$') is optional, but it helps to differentiate variable names from field names. The process of matching patterns against the inserted data is, not surprisingly, often referred to as pattern matching.

Let's assume that the rules are in the same folder as the classes, so we can use the classpath resource loader to build our first KnowledgeBase. A Knowledge Base is what we call our collection of compiled definitions, such as rules and processes, which are compiled using the KnowledgeBuilder. Both the KnowledgeBuilder and KnowledgeBase can be created from the factories KnowledgeBuilderFactory and KnowledgeBaseFactory.

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

kbuilder.add( ResourceFactory.newClassPathResource( "licenseApplication.drl", 

              getClass() ), ResourceType.DRL );

if ( kbuilder.hasErrors() ) {

    System.err.println( kbuilder.getErrors().toString() );

}

KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );

The above code snippet looks on the classpath for the licenseApplication.drl file, using the method newClassPathResource(). The resource type is DRL, short for "Drools Rule Language". Once the DRL file has been added we can check the Knowledge Builder object for any errors. If there are no errors, we can add the resulting packages to our Knowledge Base. Now we are ready to build our session and execute against some data:

StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

Applicant applicant = new Applicant( "Mr John Smith", 16 );

assertTrue( applicant.isValid() );

ksession.execute( applicant );

assertFalse( applicant.isValid() );

The preceding code executes the data against the rules. Since the applicant is under the age of 18, the application is marked as invalid.

So far we've only used a single instance, but what if we want to use more than one? We can execute against any object implementing Iterable, such as a collection. Let's add another class called Application, which has the date of the application, and we'll also move the boolean valid field to the Application class.

public class Applicant {

    private String name;

    private int age;

    // getter and setter methods here

}


public class Application {

    private Date dateApplied;

    private boolean valid;

    // getter and setter methods here

}

We can also add another rule to validate that the application was made within a period of time.

package com.company.license

rule "Is of valid age"
when
    Applicant( age < 18 )
    $a : Application()     
then
    $a.setValid( false );
end

rule "Application was made this year"
when
    $a : Application( dateApplied > "01-jan-2009" )     
then
    $a.setValid( false );
end

Unfortunately a Java elements does not implement the Iterable interface, so we have to use the JDK converter method Arrays.asList(...). The code shown below executes against an iterable list, where all collection elements are inserted before any matched rules are fired.

StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

Applicant applicant = new Applicant( "Mr John Smith", 16 );

Application application = new Application();

assertTrue( application.isValid() );

ksession.execute( Arrays.asList( new Object[] { application, applicant } ) );

assertFalse( application.isValid() );

The two execute methods execute(Object object) and execute(Iterable objects) are actually convenience methods for the interface BatchExecutor's method execute(Command command).

A CommandFactory is used to create commands, so that the following is equivalent to execute(Iterable it):

ksession.execute( CommandFactory.newInsertIterable( new Object[] { application, applicant } ) );

Batch Executor and Command Factory are particularly useful when working with multiple Commands and with output identifiers for obtaining results.

List<Command> cmds = new ArrayList<Command>();

cmds.add( CommandFactory.newInsert( new Person( "Mr John Smith" ), "mrSmith" );

cmds.add( CommandFactory.newInsert( new Person( "Mr John Doe" ), "mrDoe" );

BatchExecutionResults results = ksession.execute( CommandFactory.newBatchExecution( cmds ) );

assertEquals( new Person( "Mr John Smith" ), results.getValue( "mrSmith" ) );

CommandFactory supports many other Commands that can be used in the BatchExecutor like StartProcess, Query, and SetGlobal.

2.1.2. Stateful Knowledge Session

Stateful Sessions are longer lived and allow iterative changes over time. Some common use cases for Stateful Sessions are, but not limited to:

Monitoring
- Stock market monitoring and analysis for semi-automatic buying.
Diagnostics
- Fault finding, medical diagnostics
Logistics
- Parcel tracking and delivery provisioning
Compliance
- Validation of legality for market trades.

In contrast to a Stateless Session, the dispose() method must be called afterwards to ensure there are no memory leaks, as the Knowledge Base contains references to Stateful Knowledge Sessions when they are created. StatefulKnowledgeSession also supports the BatchExecutor interface, like StatelessKnowledgeSession, the only difference being that the FireAllRules command is not automatically called at the end for a Stateful Session.

We illustrate the monitoring use case with an example for raising a fire alarm. Using just four classes, we represent rooms in a house, each of which has one sprinkler. If a fire starts in a room, we represent that with a single Fire instance.

public class Room {

    private String name

    // getter and setter methods here

}

public class Sprinkler {

    private Room room;

    private boolean on;

    // getter and setter methods here

}

public class Fire {

    private Room room;

    // getter and setter methods here

}

public class Alarm {

}

In the previous section on Stateless Sessions the concepts of inserting and matching against data were introduced. That example assumed that only a single instance of each object type was ever inserted and thus only used literal constraints. However, a house has many rooms, so rules must express relationships between objects, such as a sprinkler being in a certain room. This is best done by using a binding variable as a constraint in a pattern. This "join" process results in what is called cross products, which are covered in the next section.

When a fire occurs an instance of the Fire class is created, for that room, and inserted into the session. The rule uses a binding on the room field of the Fire object to constrain matching to the sprinkler for that room, which is currently off. When this rule fires and the consequence is executed the sprinkler is turned on.

rule "When there is a fire turn on the sprinkler"
when
    Fire($room : room)
    $sprinkler : Sprinkler( room == $room, on == false )
then
    modify( $sprinkler ) { setOn( true ) };
    System.out.println( "Turn on the sprinkler for room " + $room.getName() );
end

Whereas the Stateless Session uses standard Java syntax to modify a field, in the above rule we use the modify statement, which acts as a sort of "with" statement. It may contain a series of comma separated Java expressions, i.e., calls to setters of the object selected by the modify statement's control expression. This modifies the data, and makes the engine aware of those changes so it can reason over them once more. This process is called inference, and it's essential for the working of a Stateful Session. Stateless Sessions typically do not use inference, so the engine does not need to be aware of changes to data. Inference can also be turned off explicitly by using the sequential mode.

So far we have rules that tell us when matching data exists, but what about when it does not exist? How do we determine that a fire has been extinguished, i.e., that there isn't a Fire object any more? Previously the constraints have been sentences according to Propositional Logic, where the engine is constraining against individual instances. Drools also has support for First Order Logic that allows you to look at sets of data. A pattern under the keyword not matches when something does not exist. The rule given below turns the sprinkler off as soon as the fire in that room has disappeared.

rule "When the fire is gone turn off the sprinkler"
when
    $room : Room( )
    $sprinkler : Sprinkler( room == $room, on == true )
    not Fire( room == $room )
then
    modify( $sprinkler ) { setOn( false ) };
    System.out.println( "Turn off the sprinkler for room " + $room.getName() );
end

While there is one sprinkler per room, there is just a single alarm for the building. An Alarm object is created when a fire occurs, but only one Alarm is needed for the entire building, no matter how many fires occur. Previously not was introduced to match the absence of a fact; now we use its complement exists which matches for one or more instances of some category.

rule "Raise the alarm when we have one or more fires"
when
    exists Fire()
then
    insert( new Alarm() );
    System.out.println( "Raise the alarm" );
end

Likewise, when there are no fires we want to remove the alarm, so the not keyword can be used again.

rule "Cancel the alarm when all the fires have gone"
when
    not Fire()
    $alarm : Alarm()
then
    retract( $alarm );
    System.out.println( "Cancel the alarm" );
end

Finally there is a general health status message that is printed when the application first starts and after the alarm is removed and all sprinklers have been turned off.

rule "Status output when things are ok"
when
    not Alarm()
    not Sprinkler( on == true ) 
then
    System.out.println( "Everything is ok" );
end

The above rules should be placed in a single DRL file and saved to some directory on the classpath and using the file name fireAlarm.drl, as in the Stateless Session example. We can then build a Knowledge Base, as before, just using the new name fireAlarm.drl. The difference is that this time we create a Stateful Session from the Knowledge Base, whereas before we created a Stateless Session.

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

kbuilder.add( ResourceFactory.newClassPathResource( "fireAlarm.drl", getClass() ),

              ResourceType.DRL );

if ( kbuilder.hasErrors() ) {

    System.err.println( kbuilder.getErrors().toString() );

}

kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );

StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();

With the session created it is now possible to iteratively work with it over time. Four Room objects are created and inserted, as well as one Sprinkler object for each room. At this point the engine has done all of its matching, but no rules have fired yet. Calling ksession.fireAllRules() allows the matched rules to fire, but without a fire that will just produce the health message.

String[] names = new String[]{"kitchen", "bedroom", "office", "livingroom"};

Map<String,Room> name2room = new HashMap<String,Room>();

for( String name: names ){

    Room room = new Room( name );

    name2room.put( name, room );

    ksession.insert( room );

    Sprinkler sprinkler = new Sprinkler( room );

    ksession.insert( sprinkler );

}


ksession.fireAllRules();

> Everything is ok

We now create two fires and insert them; this time a reference is kept for the returned FactHandle. A Fact Handle is an internal engine reference to the inserted instance and allows instances to be retracted or modified at a later point in time. With the fires now in the engine, once fireAllRules() is called, the alarm is raised and the respective sprinklers are turned on.

Fire kitchenFire = new Fire( name2room.get( "kitchen" ) );

Fire officeFire = new Fire( name2room.get( "office" ) );


FactHandle kitchenFireHandle = ksession.insert( kitchenFire );

FactHandle officeFireHandle = ksession.insert( officeFire );


ksession.fireAllRules();

> Raise the alarm
> Turn on the sprinkler for room kitchen
> Turn on the sprinkler for room office

After a while the fires will be put out and the Fire instances are retracted. This results in the sprinklers being turned off, the alarm being cancelled, and eventually the health message is printed again.

ksession.retract( kitchenFireHandle );

ksession.retract( officeFireHandle );


ksession.fireAllRules();

> Cancel the alarm
> Turn off the sprinkler for room office
> Turn off the sprinkler for room kitchen
> Everything is ok

Everyone still with me? That wasn't so hard and already I'm hoping you can start to see the value and power of a declarative rule system.

2.1.3. Methods versus Rules

People often confuse methods and rules, and new rule users regular ask, "How do I call a rule?" After the last section, you are now feeling like a rule expert and the answer to that is obvious, but let's summarize the differences nonetheless.

public void helloWorld(Person person) {

    if ( person.getName().equals( "Chuck" ) ) {

        System.out.println( "Hello Chuck" );

    }

}

Methods are called directly.
Specific instances are passed.
One call results in a single execution.

rule "Hello World"
    when
        Person( name == "Chuck" )
    then
        System.out.println( "Hello Chuck" );
        end

Rules execute by matching against any data as long it is inserted into the engine.
Rules can never be called directly.
Specific instances cannot be passed to a rule.
Depending on the matches, a rule may fire once or several times, or not at all.

2.1.4. Cross Products

Earlier the term "cross product" was mentioned, which is the result of a join. Imagine for a moment that the data from the fire alarm example were used in combination with the following rule where there ar no field constraints:

rule
when
    $room : Room()
    $sprinkler : Sprinkler()
then
    System.out.println( "room:" + $room.getName() +
                        " sprinkler:" + $sprinkler.getRoom().getName() );
end

In SQL terms this would be like doing select * from Room, Sprinkler and every row in the Room table would be joined with every row in the Sprinkler table resulting in the following output:

room:office sprinkler:office
room:office sprinkler:kitchen
room:office sprinkler:livingroom
room:office sprinkler:bedroom
room:kitchen sprinkler:office
room:kitchen sprinkler:kitchen
room:kitchen sprinkler:livingroom
room:kitchen sprinkler:bedroom
room:livingroom sprinkler:office
room:livingroom sprinkler:kitchen
room:livingroom sprinkler:livingroom
room:livingroom sprinkler:bedroom
room:bedroom sprinkler:office
room:bedroom sprinkler:kitchen
room:bedroom sprinkler:livingroom
room:bedroom sprinkler:bedroom

These cross products can obviously become huge, and they may very well contain spurious data. The size of cross products is often the source of performance problems for new rule authors. From this it can be seen that it's always desirable to constrain the cross products, which is done with the variable constraint.

rule
when
    $room : Room()
    $sprinkler : Sprinkler( room == $room )
then
    System.out.println( "room:" + $room.getName() +
                        " sprinkler:" + $sprinkler.getRoom().getName() );
end

This results in just four rows of data, with the correct Sprinkler for each Room. In SQL (actually HQL) the corresponding query would be select * from Room, Sprinkler where Room == Sprinkler.room.

room:office sprinkler:office
room:kitchen sprinkler:kitchen
room:livingroom sprinkler:livingroom
room:bedroom sprinkler:bedroom

2.2. Inference

2.2.1. Bus Pass Example

Inference has a bad name these days, as something not relevant to business use cases and just too complicated to be useful. It is true that contrived and complicated examples occur with inference, but that should not detract from the fact that simple and useful ones exist too. But more than this, correct use of inference can crate more agile and less error prone businesses with easier to maintain software.

So what is inference? Something is inferred when we gain knowledge of something from using previous knowledge. For example, given a Person fact with an age field and a rule that provides age policy control, we can infer whether a Person is an adult or a child and act on this.

rule "Infer Adult"
when
  $p : Person( age >= 18 )
then
  insert( new IsAdult( $p ) )
end

due to the preceding rule, every Person who is 18 or over will have an instance of IsAdult inserted for them. This fact is special in that it is known as a relation. We can use this inferred relation in any rule:

$p : Person()
IsAdult( person == $p )

So now we know what inference is, and have a basic example, how does this facilitate good rule design and maintenance?

Let's take a government department that are responsible for issuing ID cards when children become adults, henceforth referred to as ID department. They might have a decision table that includes logic like this, which says when an adult living in London is 18 or over, issue the card:

However the ID department does not set the policy on who an adult is. That's done at a central government level. If the central government were to change that age to 21, this would initiate a change management process. Someone would have to liaise with the ID department and make sure their systems are updated, in time for the law going live.

This change management process and communication between departments is not ideal for an agile environment, and change becomes costly and error prone. Also the card department is managing more information than it needs to be aware of with its "monolithic" approach to rules management which is "leaking" information better placed elsewhere. By this I mean that it doesn't care what explicit "age >= 18" information determines whether someone is an adult, only that they are an adult.

In contrast to this, let's pursue an approach where we split (de-couple) the authoring responsibilities, so that both the central government and the ID department maintain their own rules.

It's the central government's job to determine who is an adult. If they change the law they just update their central repository with the new rules, which others use:

The IsAdult fact, as discussed previously, is inferred from the policy rules. It encapsulates the seemingly arbitrary piece of logic "age >= 18" and provides semantic abstractions for its meaning. Now if anyone uses the above rules, they no longer need to be aware of explicit information that determines whether someone is an adult or not. They can just use the inferred fact:

While the example is very minimal and trivial it illustrates some important points. We started with a monolithic and leaky approach to our knowledge engineering. We created a single decision table that had all possible information in it and that leaks information from central government that the ID department did not care about and did not want to manage.

We first de-coupled the knowledge process so each department was responsible for only what it needed to know. We then encapsulated this leaky knowledge using an inferred fact IsAdult. The use of the term IsAdult also gave a semantic abstraction to the previously arbitrary logic "age >= 18".

So a general rule of thumb when doing your knowledge engineering is:

Bad
- Monolithic
- Leaky
Good

2.3. Truth Maintenance with Logical Objects

2.3.1. Overview

After regular inserts you have to retract facts explicitly. With logical assertions, the fact that was asserted will be automatically retracted when the conditions that asserted it in the first place are no longer true. Actually, it's even cleverer then that, because it will be retracted only if there isn't any single condition that supports the logical assertion.

Normal insertions are said to be stated, i.e., just like the intuitive meaning of "stating a fact" implies. Using a HashMap and a counter, we track how many times a particular equality is stated; this means we count how many different instances are equal.

When we logically insert an object during a RHS execution we are said to justify it, and it is considered to be justified by the firing rule. For each logical insertion there can only be one equal object, and each subsequent equal logical insertion increases the justification counter for this logical assertion. A justification is removed by the LHS of the creating rule becoming untrue, and the counter is decreased accordingly. As soon as we have no more justifications the logical object is automatically retracted.

If we try to logically insert an object when there is an equal stated object, this will fail and return null. If we state an object that has an existing equal object that is justified we override the Fact; how this override works depends on the configuration setting WM_BEHAVIOR_PRESERVE. When the property is set to discard we use the existing handle and replace the existing instance with the new Object, which is the default behavior; otherwise we override it to stated but we create an new FactHandle.

This can be confusing on a first read, so hopefully the flow charts below help. When it says that it returns a new FactHandle, this also indicates the Object was propagated through the network.

Figure 2.1. Stated Insertion

Figure 2.2. Logical Insertion

2.3.1.1. Bus Pass Example With Inference and TMS

The previous example was issuing ID cards to over 18s, in this example we now issue bus passes, either a child or adult pass.

rule "Issue Child Bus Pass" when
  $p : Person( age < 16 )
then
  insert(new ChildBusPass( $p ) );
end
 
rule "Issue Adult Bus Pass" when
  $p : Person( age >= 16 )
then
  insert(new AdultBusPass( $p ) );
end

As before the above example is considered monolithic, leaky and providing poor separation of concerns.

As before we can provide a more robust application with a separation of concerns using inference. Notice this time we don't just insert the inferred object, we use "insertLogical":

rule "Infer Child" when
    $p : Person( age < 16 )
then
    insertLogical( new IsChild( $p ) )
end
rule "Infer Adult" when
    $p : Person( age >= 16 )
then
    insertLogical( new IsAdult( $p ) )
end

A "insertLogical" is part of the Drools Truth Maintenance System (TMS). Here the fact is logically inserted, this fact is dependant on the truth of the "when" clause. It means that when the rule becomes false the fact is automatically retracted. This works particularly well as the two rules are mutually exclusive. So in the above rules if the person is under 16 it inserts an IsChild fact, once the person is 16 or over the IsChild fact is automatically retracted and the IsAdult fact inserted.

We can now bring back in the code to issue the passes, these two can also be logically inserted, as the TMS supports chaining of logical insertions for a cascading set of retracts.

rule "Issue Child Bus Pass" when
  $p : Person( )
       IsChild( person == $p )
then
    insertLogical(new ChildBusPass( $p ) );
end
 
rule "Issue Adult Bus Pass" when
  $p : Person( age >= 16 )
       IsAdult( person =$p )
then
    insertLogical(new AdultBusPass( $p ) );
end

Now when the person changes from being 15 to 16, not only is the IsChild fact automatically retracted, so is the person's ChildBusPass fact. For bonus points we can combine this with the 'not' conditional element to handle notifications, in this situation a request for the returning of the pass. So when the TMS automatically retracts the ChildBusPass object, this rule triggers and sends a request to the person:

rule "Return ChildBusPass Request "when
  $p : Person( )
       not( ChildBusPass( person == $p ) )
then
    requestChildBusPass( $p );
end

2.3.1.2. Lazy Truth Maintenance

You no longer need to enable or disable truth maintenance, via the kbase configuration. It is now handled automatically and turned on only when needed. This was done along with the code changes so that all entry points use the same code, previous to this the default entry point and named entry points used different code, to avoid TMS overhead for event processing.

2.3.1.3. Important note: Equality for Java objects

It is important to note that for Truth Maintenance (and logical assertions) to work at all, your Fact objects (which may be JavaBeans) must override equals and hashCode methods (from java.lang.Object) correctly. As the truth maintenance system needs to know when two different physical objects are equal in value, both equals and hashCode must be overridden correctly, as per the Java standard.

Two objects are equal if and only if their equals methods return true for each other and if their hashCode methods return the same values. See the Java API for more details (but do keep in mind you MUST override both equals and hashCode).

TMS behaviour is not affected by theruntime configuration of Identity vs Equality, TMS is always equality.

2.4. Decision Tables in Spreadsheets

Decision tables are a "precise yet compact" (ref. Wikipedia) way of representing conditional logic, and are well suited to business level rules.

Drools supports managing rules in a spreadsheet format. Supported formats are Excel (XLS), and CSV, which means that a variety of spreadsheet programs (such as Microsoft Excel, OpenOffice.org Calc amongst others) can be utilized. It is expected that web based decision table editors will be included in a near future release.

Decision tables are an old concept (in software terms) but have proven useful over the years. Very briefly speaking, in Drools decision tables are a way to generate rules driven from the data entered into a spreadsheet. All the usual features of a spreadsheet for data capture and manipulation can be taken advantage of.

2.4.1. When to Use Decision Tables

Consider decision tables as a course of action if rules exist that can be expressed as rule templates and data: each row of a decision table provides data that is combined with a template to generate a rule.

Many businesses already use spreadsheets for managing data, calculations, etc. If you are happy to continue this way, you can also manage your business rules this way. This also assumes you are happy to manage packages of rules in .xls or .csv files. Decision tables are not recommended for rules that do not follow a set of templates, or where there are a small number of rules (or if there is a dislike towards software like Excel or OpenOffice.org). They are ideal in the sense that there can be control over what parameters of rules can be edited, without exposing the rules directly.

Decision tables also provide a degree of insulation from the underlying object model.

2.4.2. Overview

Here are some examples of real world decision tables (slightly edited to protect the innocent).

In the above examples, the technical aspects of the decision table have been collapsed away (using a standard spreadsheet feature).

The rules start from row 17, with each row resulting in a rule. The conditions are in columns C, D, E, etc., the actions being off-screen. The values in the cells are quite simple, and their meaning is indicated by the headers in Row 16. Column B is just a description. It is customary to use color to make it obvious what the different areas of the table mean.

Note

Note that although the decision tables look like they process top down, this is not necessarily the case. Ideally, rules are authored without regard for the order of rows, simply because this makes maintenance easier, as rows will not need to be shifted around all the time.

As each row is a rule, the same principles apply. As the rule engine processes the facts, any rules that match may fire. (Some people are confused by this. It is possible to clear the agenda when a rule fires and simulate a very simple decision table where only the first match effects an action.) Also note that you can have multiple tables on one spreadsheet. This way, rules can be grouped where they share common templates, yet at the end of the day they are all combined into one rule package. Decision tables are essentially a tool to generate DRL rules automatically.

2.4.3. How Decision Tables Work

The key point to keep in mind is that in a decision table each row is a rule, and each column in that row is either a condition or action for that rule.

The spreadsheet looks for the RuleTable keyword to indicate the start of a rule table (both the starting row and column). Other keywords are also used to define other package level attributes (covered later). It is important to keep the keywords in one column. By convention the second column ("B") is used for this, but it can be any column (convention is to leave a margin on the left for notes). In the following diagram, C is actually the column where it starts. Everything to the left of this is ignored.

If we expand the hidden sections, it starts to make more sense how it works; note the keywords in column C.

Now the hidden magic which makes it work can be seen. The RuleSet keyword indicates the name to be used in the rule package that will encompass all the rules. This name is optional, using a default, but it must have the RuleSet keyword in the cell immediately to the right.

The other keywords visible in Column C are Import and Sequential which will be covered later. The RuleTable keyword is important as it indicates that a chunk of rules will follow, based on some rule templates. After the RuleTable keyword there is a name, used to prefix the names of the generated rules. The sheet name and row numbers are appended to guarantee unique rule names.

Warning

The RuleTable name combined with the sheet name must be unique across all spreadsheet files in the same KnowledgeBase. If that's not the case, some rules might have the same name and only 1 of them will be applied. To show such ignored rules, raise the severity of such rule name conflicts.

The column of RuleTable indicates the column in which the rules start; columns to the left are ignored.

Note

In general the keywords make up name-value pairs.

Referring to row 14 (the row immediately after RuleTable), the keywords CONDITION and ACTION indicate that the data in the columns below are for either the LHS or the RHS parts of a rule. There are other attributes on the rule which can also be optionally set this way.

Row 15 contains declarations of ObjectTypes. The content in this row is optional, but if this option is not in use, the row must be left blank; however this option is usually found to be quite useful. When using this row, the values in the cells below (row 16) become constraints on that object type. In the above case, it generates Person(age=="42") and Cheese(type=="stilton"), where 42 and "stilton" come from row 18. In the above example, the "==" is implicit; if just a field name is given the translator assumes that it is to generate an exact match.

Note

An ObjectType declaration can span columns (via merged cells), meaning that all columns below the merged range are to be combined into one set of constraints within a single pattern matching a single fact at a time, as opposed to non-merged cells containing the same ObjectType, but resulting in different patterns, potentially matching different or identical facts.

Row 16 contains the rule templates themselves. They can use the "$param" placeholder to indicate where data from the cells below should be interpolated. (For multiple insertions, use "$1", "$2", etc., indicating parameters from a comma-separated list in a cell below.) Row 17 is ignored; it may contain textual descriptions of the column's purpose.

Rows 18 and 19 show data, which will be combined (interpolated) with the templates in row 15, to generate rules. If a cell contains no data, then its template is ignored. (This would mean that some condition or action does not apply for that rule row.) Rule rows are read until there is a blank row. Multiple RuleTables can exist in a sheet. Row 20 contains another keyword, and a value. The row positions of keywords like this do not matter (most people put them at the top) but their column should be the same one where the RuleTable or RuleSet keywords should appear. In our case column C has been chosen to be significant, but any other column could be used instead.

In the above example, rules would be rendered like the following (as it uses the "ObjectType" row):

//row 18
rule "Cheese_fans_18"
when
    Person(age=="42")
    Cheese(type=="stilton")
then
    list.add("Old man stilton");
end

Note

The constraints age=="42" and type=="stilton" are interpreted as single constraints, to be added to the respective ObjectType in the cell above. If the cells above were spanned, then there could be multiple constraints on one "column".

Warning

Very large decision tables may have very large memory requirements.

2.4.4. Spreadsheet Syntax

2.4.4.1. Spreadsheet Structure

There are two types of rectangular areas defining data that is used for generating a DRL file. One, marked by a cell labelled RuleSet, defines all DRL items except rules. The other one may occur repeatedly and is to the right and below a cell whose contents begin with RuleTable. These areas represent the actual decision tables, each area resulting in a set of rules of similar structure.

A Rule Set area may contain cell pairs, one below the RuleSet cell and containing a keyword designating the kind of value contained in the other one that follows in the same row.

The columns of a Rule Table area define patterns and constraints for the left hand sides of the rules derived from it, actions for the consequences of the rules, and the values of individual rule attributes. Thus, a Rule Table area should contain one or more columns, both for conditions and actions, and an arbitrary selection of columns for rule attributes, at most one column for each of these. The first four rows following the row with the cell marked with RuleTable are earmarked as header area, mostly used for the definition of code to construct the rules. It is any additional row below these four header rows that spawns another rule, with its data providing for variations in the code defined in the Rule Table header.

All keywords are case insensitive.

Only the first worksheet is examined for decision tables.

2.4.4.2. Rule Set Entries

Entries in a Rule Set area may define DRL constructs (except rules), and specify rule attributes. While entries for constructs may be used repeatedly, each rule attribute may be given at most once, and it applies to all rules unless it is overruled by the same attribute being defined within the Rule Table area.

Entries must be given in a vertically stacked sequence of cell pairs. The first one contains a keyword and the one to its right the value, as shown in the table below. This sequence of cell pairs may be interrupted by blank rows or even a Rule Table, as long as the column marked by RuleSet is upheld as the one containing the keyword.

Table 2.1. Entries in the Rule Set area

Keyword	Value	Usage
RuleSet	The package name for the generated DRL file. Optional, the default is `rule_table`.	Must be First entry.
Sequential	"true" or "false". If "true", then salience is used to ensure that rules fire from the top down.	Optional, at most once. If omitted, no firing order is imposed.
EscapeQuotes	"true" or "false". If "true", then quotation marks are escaped so that they appear literally in the DRL.	Optional, at most once. If omitted, quotation marks are escaped.
Import	A comma-separated list of Java classes to import.	Optional, may be used repeatedly.
Variables	Declarations of DRL globals, i.e., a type followed by a variable name. Multiple global definitions must be separated with a comma.	Optional, may be used repeatedly.
Functions	One or more function definitions, according to DRL syntax.	Optional, may be used repeatedly.
Queries	One or more query definitions, according to DRL syntax.	Optional, may be used repeatedly.
Declare	One or more declarative types, according to DRL syntax.	Optional, may be used repeatedly.

Warning

In some locales, MS Office, LibreOffice and OpenOffice will encode a double quoth " differently, which will cause a compilation error. The difference is often hard to see. For example: “A” will fail, but "A" will work.

For defining rule attributes that apply to all rules in the generated DRL file you can use any of the entries in the following table. Notice, however, that the proper keyword must be used. Also, each of these attributes may be used only once.

Table 2.2. Rule attribute entries in the Rule Set area

Keyword	Initial	Value
PRIORITY	P	An integer defining the "salience" value for the rule. Overridden by the "Sequential" flag.
DURATION	D	A long integer value defining the "duration" value for the rule.
TIMER	T	A timer definition. See "Timers and Calendars".
CALENDARS	E	A calendars definition. See "Timers and Calendars".
NO-LOOP	U	A Boolean value. "true" inhibits looping of rules due to changes made by its consequence.
LOCK-ON-ACTIVE	L	A Boolean value. "true" inhibits additional activations of all rules with this flag set within the same ruleflow or agenda group.
AUTO-FOCUS	F	A Boolean value. "true" for a rule within an agenda group causes activations of the rule to automatically give the focus to the group.
ACTIVATION-GROUP	X	A string identifying an activation (or XOR) group. Only one rule within an activation group will fire, i.e., the first one to fire cancels any existing activations of other rules within the same group.
AGENDA-GROUP	G	A string identifying an agenda group, which has to be activated by giving it the "focus", which is one way of controlling the flow between groups of rules.
RULEFLOW-GROUP	R	A string identifying a rule-flow group.

2.4.4.3. Rule Tables

All Rule Tables begin with a cell containing "RuleTable", optionally followed by a string within the same cell. The string is used as the initial part of the name for all rules derived from this Rule Table, with the row number appended for distinction. (This automatic naming can be overridden by using a NAME column.) All other cells defining rules of this Rule Table are below and to the right of this cell.

The next row defines the column type, with each column resulting in a part of the condition or the consequence, or providing some rule attribute, the rule name or a comment. The table below shows which column headers are available; additional columns may be used according to the table showing rule attribute entries given in the preceding section. Note that each attribute column may be used at most once. For a column header, either use the keyword or any other word beginning with the letter given in the "Initial" column of these tables.

Table 2.3. Column Headers in the Rule Table

Keyword	Initial	Value	Usage
NAME	N	Provides the name for the rule generated from that row. The default is constructed from the text following the RuleTable tag and the row number.	At most one column
DESCRIPTION	I	A text, resulting in a comment within the generated rule.	At most one column
CONDITION	C	Code snippet and interpolated values for constructing a constraint within a pattern in a condition.	At least one per rule table
ACTION	A	Code snippet and interpolated values for constructing an action for the consequence of the rule.	At least one per rule table
METADATA	@	Code snippet and interpolated values for constructing a metadata entry for the rule.	Optional, any number of columns

Given a column headed CONDITION, the cells in successive lines result in a conditional element.

Text in the first cell below CONDITION develops into a pattern for the rule condition, with the snippet in the next line becoming a constraint. If the cell is merged with one or more neighbours, a single pattern with multiple constraints is formed: all constraints are combined into a parenthesized list and appended to the text in this cell. The cell may be left blank, which means that the code snippet in the next row must result in a valid conditional element on its own.
To include a pattern without constraints, you can write the pattern in front of the text for another pattern.
The pattern may be written with or without an empty pair of parentheses. A "from" clause may be appended to the pattern.
If the pattern ends with "eval", code snippets are supposed to produce boolean expressions for inclusion into a pair of parentheses after "eval".
Text in the second cell below CONDITION is processed in two steps.
1. The code snippet in this cell is modified by interpolating values from cells farther down in the column. If you want to create a constraint consisting of a comparison using "==" with the value from the cells below, the field selector alone is sufficient. Any other comparison operator must be specified as the last item within the snippet, and the value from the cells below is appended. For all other constraint forms, you must mark the position for including the contents of a cell with the symbol $param. Multiple insertions are possible by using the symbols $1, $2, etc., and a comma-separated list of values in the cells below.
  A text according to the pattern forall(delimiter){snippet} is expanded by repeating the snippet once for each of the values of the comma-separated list of values in each of the cells below, inserting the value in place of the symbol $ and by joining these expansions by the given delimiter. Note that the forall construct may be surrounded by other text.
2. If the cell in the preceding row is not empty, the completed code snippet is added to the conditional element from that cell. A pair of parentheses is provided automatically, as well as a separating comma if multiple constraints are added to a pattern in a merged cell.
  If the cell above is empty, the interpolated result is used as is.
Text in the third cell below CONDITION is for documentation only. It should be used to indicate the column's purpose to a human reader.
From the fourth row on, non-blank entries provide data for interpolation as described above. A blank cell results in the omission of the conditional element or constraint for this rule.

Given a column headed ACTION, the cells in successive lines result in an action statement.

Text in the first cell below ACTION is optional. If present, it is interpreted as an object reference.
Text in the second cell below ACTION is processed in two steps.
1. The code snippet in this cell is modified by interpolating values from cells farther down in the column. For a singular insertion, mark the position for including the contents of a cell with the symbol $param. Multiple insertions are possible by using the symbols $1, $2, etc., and a comma-separated list of values in the cells below.
  A method call without interpolation can be achieved by a text without any marker symbols. In this case, use any non-blank entry in a row below to include the statement.
  The forall construct is available here, too.
2. If the first cell is not empty, its text, followed by a period, the text in the second cell and a terminating semicolon are stringed together, resulting in a method call which is added as an action statement for the consequence.
  If the cell above is empty, the interpolated result is used as is.
Text in the third cell below ACTION is for documentation only. It should be used to indicate the column's purpose to a human reader.
From the fourth row on, non-blank entries provide data for interpolation as described above. A blank cell results in the omission of the action statement for this rule.

Note

Using $1 instead of $param works in most cases, but it will fail if the replacement text contains a comma: then, only the part preceding the first comma is inserted. Use this "abbreviation" judiciously.

Given a column headed METADATA, the cells in successive lines result in a metadata annotation for the generated rules.

Text in the first cell below METADATA is ignored.
Text in the second cell below METADATA is subject to interpolation, as described above, using values from the cells in the rule rows. The metadata marker character @ is prefixed automatically, and thus it should not be included in the text for this cell.
Text in the third cell below METADATA is for documentation only. It should be used to indicate the column's purpose to a human reader.
From the fourth row on, non-blank entries provide data for interpolation as described above. A blank cell results in the omission of the metadata annotation for this rule.

2.4.4.4. Examples

The various interpolations are illustrated in the following example.

Example 2.1. Interpolating cell data

If the template is Foo(bar == $param) and the cell is 42, then the result is Foo(bar == 42).

If the template is Foo(bar < $1, baz == $2) and the cell contains 42,43, the result will be Foo(bar < 42, baz ==43).

The template forall(&&){bar != $} with a cell containing 42,43 results in bar != 42 && bar != 43.

The next example demonstrates the joint effect of a cell defining the pattern type and the code snippet below it.

This spreadsheet section shows how the Person type declaration spans 2 columns, and thus both constraints will appear as Person(age == ..., type == ...). Since only the field names are present in the snippet, they imply an equality test.

In the following example the marker symbol $param is used.

The result of this column is the pattern Person(age == "42")). You may have noticed that the marker and the operator "==" are redundant.

The next example illustrates that a trailing insertion marker can be omitted.

Here, appending the value from the cell is implied, resulting in Person(age < "42")).

You can provide the definition of a binding variable, as in the example below. .

Here, the result is c: Cheese(type == "stilton"). Note that the quotes are provided automatically. Actually, anything can be placed in the object type row. Apart from the definition of a binding variable, it could also be an additional pattern that is to be inserted literally.

A simple construction of an action statement with the insertion of a single value is shown below.

The cell below the ACTION header is left blank. Using this style, anything can be placed in the consequence, not just a single method call. (The same technique is applicable within a CONDITION column as well.)

Below is a comprehensive example, showing the use of various column headers. It is not an error to have no value below a column header (as in the NO-LOOP column): here, the attribute will not be applied in any of the rules.

And, finally, here is an example of Import, Variables and Functions.

Multiple package names within the same cell must be separated by a comma. Also, the pairs of type and variable names must be comma-separated. Functions, however, must be written as they appear in a DRL file. This should appear in the same column as the "RuleSet" keyword; it could be above, between or below all the rule rows.

Note

It may be more convenient to use Import, Variables, Functions and Queries repeatedly rather than packing several definitions into a single cell.

2.4.5. Creating and integrating Spreadsheet based Decision Tables

The API to use spreadsheet based decision tables is in the drools-decisiontables module. There is really only one class to look at: SpreadsheetCompiler. This class will take spreadsheets in various formats, and generate rules in DRL (which you can then use in the normal way). The SpreadsheetCompiler can just be used to generate partial rule files if it is wished, and assemble it into a complete rule package after the fact (this allows the separation of technical and non-technical aspects of the rules if needed).

To get started, a sample spreadsheet can be used as a base. Alternatively, if the plug-in is being used (Rule Workbench IDE), the wizard can generate a spreadsheet from a template (to edit it an xls compatible spreadsheet editor will need to be used).

2.4.6. Managing Business Rules in Decision Tables

2.4.6.1. Workflow and Collaboration

Spreadsheets are well established business tools (in use for over 25 years). Decision tables lend themselves to close collaboration between IT and domain experts, while making the business rules clear to business analysts, it is an ideal separation of concerns.

Typically, the whole process of authoring rules (coming up with a new decision table) would be something like:

Business analyst takes a template decision table (from a repository, or from IT)
Decision table business language descriptions are entered in the table(s)
Decision table rules (rows) are entered (roughly)
Decision table is handed to a technical resource, who maps the business language (descriptions) to scripts (this may involve software development of course, if it is a new application or data model)
Technical person hands back and reviews the modifications with the business analyst.
The business analyst can continue editing the rule rows as needed (moving columns around is also fine etc).
In parallel, the technical person can develop test cases for the rules (liaising with business analysts) as these test cases can be used to verify rules and rule changes once the system is running.

2.4.6.2. Using spreadsheet features

Features of applications like Excel can be used to provide assistance in entering data into spreadsheets, such as validating fields. Lists that are stored in other worksheets can be used to provide valid lists of values for cells, like in the following diagram.

Some applications provide a limited ability to keep a history of changes, but it is recommended to use an alternative means of revision control. When changes are being made to rules over time, older versions are archived (many open source solutions exist for this, such as Subversion or Git).

2.4.7. Rule Templates

With Rule Templates the data is separated from the rule and there are no restrictions on which part of the rule is data-driven. So whilst you can do everything you could do in decision tables you can also do the following:

store your data in a database (or any other format)
conditionally generate rules based on the values in the data
use data for any part of your rules (e.g. condition operator, class name, property name)
run different templates over the same data

As an example, a more classic decision table is shown, but without any hidden rows for the rule meta data (so the spreadsheet only contains the raw data to generate the rules).

See the ExampleCheese.xls in the examples download for the above spreadsheet.

If this was a regular decision table there would be hidden rows before row 1 and between rows 1 and 2 containing rule metadata. With rule templates the data is completely separate from the rules. This has two handy consequences - you can apply multiple rule templates to the same data and your data is not tied to your rules at all. So what does the template look like?

1  template header
2  age
3  type
4  log
5
6  package org.drools.examples.templates;
7
8  global java.util.List list;
9
10 template "cheesefans"
11
12 rule "Cheese fans_@{row.rowNumber}"
13 when
14    Person(age == @{age})
15    Cheese(type == "@{type}")
16 then
17    list.add("@{log}");
18 end
19
20 end template

Annotations to the preceding program listing:

Line 1: All rule templates start with template header.
Lines 2-4: Following the header is the list of columns in the order they appear in the data. In this case we are calling the first column age, the second type and the third log.
Line 5: An empty line signifies the end of the column definitions.
Lines 6-9: Standard rule header text. This is standard rule DRL and will appear at the top of the generated DRL. Put the package statement and any imports and global and function definitions into this section.
Line 10: The keyword template signals the start of a rule template. There can be more than one template in a template file, but each template should have a unique name.
Lines 11-18: The rule template - see below for details.
Line 20: The keywords end template signify the end of the template.

The rule templates rely on MVEL to do substitution using the syntax @{token_name}. There is currently one built-in expression, @{row.rowNumber} which gives a unique number for each row of data and enables you to generate unique rule names. For each row of data a rule will be generated with the values in the data substituted for the tokens in the template. With the example data above the following rule file would be generated:

package org.drools.examples.templates;

global java.util.List list;

rule "Cheese fans_1"
when
  Person(age == 42)
  Cheese(type == "stilton")
then
  list.add("Old man stilton");
end

rule "Cheese fans_2"
when
  Person(age == 21)
  Cheese(type == "cheddar")
then
  list.add("Young man cheddar");
end

The code to run this is simple:



DecisionTableConfiguration dtableconfiguration =

    KnowledgeBuilderFactory.newDecisionTableConfiguration();

dtableconfiguration.setInputType( DecisionTableInputType.XLS );


KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();


kbuilder.add( ResourceFactory.newClassPathResource( getSpreadsheetName(),

                                                    getClass() ),

              ResourceType.DTABLE,

              dtableconfiguration );

2.5. Templates

If you discover that you have a group of rules following the same arrangement of patterns, constraints and actions on the RHS, differing only in constants or names for objects or fields, you might think of employing Drools's rule template feature for generating the actual rules. You would write a rule template file, containing the textual skeleton of your rule and use the Drools template compiler in combination with a collection of objects providing the actual values for the "flesh" of the rules for their instantiation.

The mechanism is very similar to what a macro processor does. The major advantage proffered by template expansion is that it's nicely integrated in the overall handling of Knowledge Resources.

Caution

This is an experimental feature. In particular, the API is subject to change.

2.5.1. The Rule Template File

A rule template file begins with a header defining the placeholders, or formal template parameters for the strings that are to be inserted during instantiation. After the first line, which invariably contains template header, you should write a number of lines, each of which contains a single parameter name.

Example 2.2. Rule template file: template header

template header
parameter-name-1
...
parameter-name-n
...

The template header is followed by the text that is to be replicated and interpolated with the actual parameters. It may begin with a package statement, followed by some additional lines. These may be sectioned into one or more templates, each of them between a pair of matching template and end template statements. The template takes an argument, which puts a name to the template. The name can be a simple unquoted name or an arbitrary string enclosed in double quotes. The template text between these lines may contain one or more rules, constituting the "raw material" for the expansion.

Example 2.3. Rule template file: templates

template header
parameter-name-1
...
parameter-name-n
package ...     // optional
header text    // optional
template template-name
...
// template text
...
end template
...

The resulting text will begin with the package line and the header text following it, if present. Then, each template text will be expanded individually, yielding one set of rules for each of the actual parameter sets. Therefore, the structure of the template sections affect the order of the generated rules, since the generator iterates over the sections and then over the set of actual parameters.

Any interpolation takes place between a pair of template and end template statements, when this template is expanded. The template text is scanned for occurrences of parameter expansions written according to:

@{parameter-name}

The name between '@{' and '}' should be one of the parameter names defined in the template header. The substitution is effected anywhere, even within string literals.

An important parameter is available without having to be included in the data source providing the actual values. The parameter substitution @{row.rowNumber} expands to the integers 0, 1, 2, etc., providing a unique distinction for the instantiation derived from a parameter set. You would use this as part of each rule name, because, without this precaution, there would be duplicate rule names. (You are, of course, free to use your own identification included as an extra parameter.)

2.5.2. Expanding a Template

To expand a template, you must prepare a data source. This can be a spreadsheet, as explained in the previous section. Here, we'll concentrate on expansion driven by Java objects. There are two straightforward ways of supplying values for a fixed set of names: Java objects, in the JavaBeans style, and Maps. Both of them can be arranged in a Collection, whose elements will be processed during the expansion, resulting in an instantiation for each element.

2.5.2.1. Instantiation from Java Objects

You may use a Java object that provides getter methods corresponding to all of the parameter names of your template file. If, for instance, you have defined a header

template header
type
limit
word

the following Java class could be used:



public class ParamSet {

    //...

    public ParamSet( String t, int l, boolean w ) {

        //...

    }

    public String  getType(){...}

    public int     getLimit(){...}

    public boolean isWord(){...}

}

Although interpolation is pure text manipulation, the actual values supplied may be of any type, just as long as this type provides a reasonable toString() method. (For simple types, the eponymous static method of the related class from java.lang is used.)

Assuming that we have created a Collection<ParamSet> for a template file template.drl, we can now proceed to request its expansion.



Collection<ParamSet> paramSets = new ArrayList<ParamSet>();

// populate paramSets

paramSets.add( new ParamSet( "Foo", 42, true ) );

paramSets.add( new ParamSet( "Bar", 13, false ) );

ObjectDataCompiler converter = new ObjectDataCompiler();

InputStream templateStream =

    this.getClass().getResourceAsStream( "template.drl" );

String drl = converter.compile( objs, templateStream );

The resulting string contains the expanded rules text. You could write it to a file and proceed as usual, but it's also possible to feed this to a KnowledgeBuilder and continue with the resulting Knowledge Packages.



KnowledgeBase kieBaseModel = KnowledgeBaseFactory.newKnowledgeBase();

KnowledgeBuilder kBuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

Reader rdr = new StringReader( drl );

kBuilder.add( ResourceFactory.newReaderResource( rdr ), ResourceType.DRL );

if( kBuilder.hasErrors() ){

    // ...

    throw new IllegalStateException( "DRL errors" );

}

kieBaseModel.addKnowledgePackages( kBuilder.getKnowledgePackages() );

2.5.2.2. Instantiation from Maps

A Map that provides the values for substituting template parameters should have a (string) key set matching all of the parameter names. Again, values could be from any class, as long as they provide a good toString() method. The expansion would use the same approach, just differing in the way the map collection is composed.



Collection<Map<String,Object>> paramMaps = new ArrayList<Map<String,Object>>();

// populate paramMaps

ObjectDataCompiler converter = new ObjectDataCompiler();

InputStream templateStream =

    this.getClass().getResourceAsStream( "template.drl" );

String drl = converter.compile( objs, templateStream );

2.5.3. Example

The following example illustrates template expansion. It is based on simple objects of class Item containing a couple of integer fields and an enum field of type ItemCode.



public class Item {

    // ...

    public Item( String n, int p, int w, ItemCode c ){...}


    public String   getName() {...}

    public int      getWeight() {...}

    public int      getPrice() {...}

    public ItemCode getCode() {...}

}


public enum ItemCode {

    LOCK,

    STOCK,

    BARREL;

}

The rule template contains a single rule. Notice that the field name for the range test is a parameter, which enables us to instantiate the template for different fields.

template header
field
lower
upper
codes

package range;
template "inRange"
rule "is in range @{row.rowNumber}"
when
    Item( $name : name, $v : @{field} >= @{lower} && <= @{upper}, $code : code @{codes} )
then
    System.out.println( "Item " + $name + " @{field} in range: " + $v + " code: " + $code );
end
end template

The next code snippet is from the application, where several parameter sets have to be set up. First, there is class ParamSet, for storing a set of actual parameters.



public class ParamSet {

    //...

    private EnumSet<ItemCode> codeSet;


    public ParamSet( String f, int l, int u, EnumSet<ItemCode> cs ){...}


    public String getField() { return field; }

    public int getLower() { return lower; }

    public int getUpper() { return upper; }


    public String getCodes(){

        StringBuilder sb = new StringBuilder();

        String conn = "";

        for( ItemCode ic: codeSet ){

             sb.append( conn ).append( " == ItemCode." ).append( ic );

             conn = " ||";

        }

        return sb.toString();

    }

}

Note that the method getCodes() does returns the EnumSet<ItemCode> field value as a String value representing a multiple restriction, i.e., a test for one out of a list of values.

The task of expanding a template, passing the resulting DRL text to a Knowledge Builder and adding the resulting Knowledge Packages to a Knowledge Base is generic. The utility class Expander takes care of this, using a Knowledge Base, the InputStream with the rule template and the collection of parameter sets.



public class Expander {


    public void expand( KnowledgeBase kieBaseModel, InputStream is, Collection<?> act )

        throws Exception {

        ObjectDataCompiler converter = new ObjectDataCompiler();

        String drl = converter.compile( act, is );

        

        KnowledgeBuilder kBuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

        Reader rdr = new StringReader( drl );

        kBuilder.add( ResourceFactory.newReaderResource( rdr ), ResourceType.DRL );

        if( kBuilder.hasErrors() ){

            for( KnowledgeBuilderError err: kBuilder.getErrors() ){

                System.err.println( err.toString() );

            }

            throw new IllegalStateException( "DRL errors" );

        }

        kieBaseModel.addKnowledgePackages( kBuilder.getKnowledgePackages() );

    }

}

We are now all set to prepare the Knowledge Base with some generated rules. First, we define several parameter sets, constructed as ParamSet objects, and add them to a List, which is passed to the expand method shown above. Then we launch a stateful session, insert a few Item, and watch what happens.



Collection<ParamSet> cfl = new ArrayList<ParamSet>();

cfl.add( new ParamSet( "weight",  10,  99, EnumSet.of( ItemCode.LOCK, ItemCode.STOCK ) ) );

cfl.add( new ParamSet( "price",   10,  50, EnumSet.of( ItemCode.BARREL ) ) );


KnowledgeBase kieBaseModel = KnowledgeBaseFactory.newKnowledgeBase();

Expander ex = new Expander();

InputStream dis = new FileInputStream( new File( "rangeTemp.drl" ) );

ex.expand( kieBaseModel, dis, cfl );

        

StatefulKnowledgeSession session = kieBaseModel.newStatefulKnowledgeSession();

session.insert( new Item( "A", 130,  42, ItemCode.LOCK ) );

session.insert( new Item( "B",  44, 100, ItemCode.STOCK ) );

session.insert( new Item( "C", 123, 180, ItemCode.BARREL ) );

session.insert( new Item( "D",  85,   9, ItemCode.LOCK ) );

        

session.fireAllRules();

Notice that the two resulting rules deal with different fields, one with an item's weight, the other one with its price. - Below is the output.

Item E price in range: 25 code: BARREL
Item A weight in range: 42 code: LOCK

2.6. More on building and deploying

2.6.1. Knowledge Base by Configuration Using Changesets

So far, the programmatic API has been used to build a Knowledge Base. Quite often it's more desirable to do this via configuration. To facilitate this, Drools supports the "Changeset" feature. The file changeset.xml contains a list of resources, and it may also point recursively to another changeset XML file. Currently there is no XML schema for the changeset XML, but we hope to add one soon. A few examples will be shown to give you the gist of things. A resource approach is employed that uses a prefix to indicate the protocol. All the protocols provided by java.net.URL, such as "file" and "http", are supported, as well as an additional "classpath". Currently the type attribute must always be specified for a resource, as it is not inferred from the file name extension. Here is a simple example that points to a http location for some rules.


<change-set xmlns='http://drools.org/drools-5.0/change-set'

             xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

             xs:schemaLocation='http://drools.org/drools-5.0/change-set http://anonsvn.jboss.org/repos/labs/labs/jbossrules/trunk/drools-api/src/main/resources/change-set-1.0.0.xsd' >

   <add>

       <resource source='http:org/domain/myrules.drl' type='DRL' />

   </add>

 </change-set>

To use the above XML, the code is almost identical as before, except we change the resource type to CHANGE_SET.

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

kbuilder.add( ResourceFactory.newClasspathResource( "myChangeSet.xml", getClass() ),

              ResourceType.CHANGE_SET );

if ( kbuilder.hasErrors() ) {

    System.err.println( builder.getErrors().toString() );

}

Changesets can include any number of resources, and they even support additional configuration information, which currently is only needed for decision tables. The example below is expanded to load the rules from a http URL location, and an Excel decision table from the classpath.


<change-set xmlns='http://drools.org/drools-5.0/change-set'

             xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

             xs:schemaLocation='http://drools.org/drools-5.0/change-set.xsd http://anonsvn.jboss.org/repos/labs/labs/jbossrules/trunk/drools-api/src/main/resources/change-set-1.0.0.xsd' >

   <add>

       <resource source='http:org/domain/myrules.drl' type='DRL' />

       <resource source='classpath:data/IntegrationExampleTest.xls' type="DTABLE">

           <decisiontable-conf input-type="XLS" worksheet-name="Tables_2" />

       </resource>

   </add>

 </change-set>

It is also possible to specify a directory, to add the contents of that directory. It is expected that all the files are of the specified type, since type is not yet inferred from the file name extensions.


<change-set xmlns='http://drools.org/drools-5.0/change-set'

             xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

             xs:schemaLocation='http://drools.org/drools-5.0/change-set.xsd http://anonsvn.jboss.org/repos/labs/labs/jbossrules/trunk/drools-api/src/main/resources/change-set-1.0.0.xsd' >

   <add>

       <resource source='file://myfolder/' type='DRL' />

   </add>

 </change-set>

2.6.2. Knowledge Agent

The Knowledge Agent provides automatic loading, caching and re-loading of resources and is configured from a properties files. The Knowledge Agent can update or rebuild this Knowledge Base as the resources it uses are changed. The strategy for this is determined by the configuration given to the factory, but it is typically pull-based using regular polling. We hope to add push-based updates and rebuilds in future versions.

KnowledgeAgent kagent = KnowledgeAgentFactory.newKnowledgeAgent( "MyAgent" );

kagent.applyChangeSet( ResourceFactory.newUrlResource( url ) );

KnowledgeBase kbase = kagent.getKnowledgeBase();

A KnowledgeAgent object will continuously scan all the added resources, using a default polling interval of 60 seconds and, when some last modification date is updated, it will applied the changes into the cached Knowledge Base using the new resources. Note that the previous KnowledgeBase reference will still exist and you'll have to call getKnowledgeBase() to access the newly built KnowledgeBase. If a directory is specified as part of the change set, the entire contents of that directory will be scanned for changes. The way modifications are applied depends on drools.agent.newInstance property present in the KnowledgeAgentConfiguration object passed to the agent.

For polling to occur, the polling and notifier services must be started:

ResourceFactory.getResourceChangeNotifierService().start();

ResourceFactory.getResourceChangeScannerService().start();

2.6.2.1. Knowledge Agent and Custom ClassLoaders

Because Knowledge Agent could scan and process remote resources, it could ends up failing when compiling or executing rules, queries, functions, etc. that use classes outside the agent's classloader. If this is your case, you could take 2 approach: use a custom classloader for agent's kbuilder or force the agent to use the same classloader that its kbase has.

2.6.2.1.1. Custom ClassLoaders from KnowledgeBuilder

Knowledge Agent uses KnowledgeBuilder internally in order to compile managed resources. If you need to pass custom configuration to these compilers you could pass a KnowledgeBuilderConfiguration object to KnowledgeAgentFactory.newKnowledgeAgent(). This object will be used in every builder the agent creates. Using a KnowledgeBuilderConfiguration you can specify a custom classloader.

Important

The usage of a KnowledgeBuilder to specify the CL the agent must use is discouraged. See the next section for the best solution to this problem.

Warning

If the agent is handling binary resources (packages), then using a KnowledgeBuilder to specify the CL that must be used will not work. In this case the only solution is to pass a kbase that uses the custom CL. See next section for more details.

2.6.2.1.2. Reuse KnowledgeBase ClassLoader

Most of the times, the classloader you want to use in the compilation process of remote resources is the same needed in the agent's kbase, so the rules could be executed. If you want to use this approach, you will need to setup the desired ClassLoader to the agent kbase and use the "drools.agent.useKBaseClassLoaderForCompiling" property of KnowledgeAgentConfiguration object.

This approach lets you modify agent's kbuilder classloader in runtime by modifying the classloader the agent's kbase uses. This will serve also when not using incremental change set processing (see the section below). When the kbase is recreated its configuration is reused, so the classloader is maintained.

KnowledgeBaseConfiguration kbaseConfig =

    KnowledgeBaseFactory.newKnowledgeBaseConfiguration(null, customClassLoader);

KnowledgeBase kbase =

    KnowledgeBaseFactory.newKnowledgeBase(kbaseConfig); //kbase with custom classloader

KnowledgeAgentConfiguration aconf =

    KnowledgeAgentFactory.newKnowledgeAgentConfiguration();

aconf.setProperty("drools.agent.newInstance", "false"); //incremental change set processing enabled

aconf.setProperty("drools.agent.useKBaseClassLoaderForCompiling", "true");

KnowledgeAgent kagent = KnowledgeAgentFactory.newKnowledgeAgent(

                "test agent", kbase, aconf);

2.6.2.2. Incremental Change Set Processing

Knowledge Agent can process change sets in two different ways: recreating the knowledge base every time a new change set is processed or applying the change set in the cached knowledge base without destroying it. This behavior is controlled by the "newInstance" property of the KnowledgeAgentConfiguration object passed to the Agent's constructor.

When "newInstance" is set to true (the default value), the agent will destroy the cached Knowledge Base it contains and populate a new one containing the change set modifications. When "newInstance" is set to "false" change sets are applied directly to the cached Knowledge Base. The rule that were not modified in the change sets' resources are not replaced in the Knowledge Base, the modified or deleted rules are modified or deleted from the cached Knowledge Base. Functions, Queries and Definition Types are always replaced in the cached Knowledge Base whether they are modified or not.

The following code snippet creates a new Knowledge Agent with its "newInstance" property set to false

KnowledgeAgentConfiguration aconf = KnowledgeAgentFactory.newKnowledgeAgentConfiguration();

aconf.setProperty("drools.agent.newInstance", "false");

KnowledgeAgent kagent = KnowledgeAgentFactory.newKnowledgeAgent("test agent", null, aconf);

2.6.2.3. Remote HTTP resource caching

A note on remote HTTP Url Resources: if your knowledge agent is "pulling" resources from a http(s) URL, then you might rightly be concerned if that resource (remote web server) suddenly disappears. To survive a restart when a resource is no longer available remotely (eg the remote server is being restarted) then you can set a System Property: drools.resource.urlcache to a directory that has write permissions for the application: the Knowledge Agent will cache copies of the remote resources in that local directory.

For example, using the java command line: -Ddrools.resource.urlcache=/users/someone/KnowledgeCache - will keep local copies of the resources (rules, packages etc) in that directory, for the agent to use should it be restarted (when a remote resource becomes available, and is updated, it will automatically update the local cache copy).

2.7. Logging

One way to illuminate the black box that is a rule engine, is to play with the logging level.

Everything is logged to SLF4J, which is a simple logging facade that can delegate any log to Logback, Apache Commons Logging, Log4j or java.util.logging. Add a dependency to the logging adaptor for your logging framework of choice. If you're not using any logging framework yet, you can use Logback by adding this Maven dependency:


    <dependency>

      <groupId>ch.qos.logback</groupId>

      <artifactId>logback-classic</artifactId>

      <version>1.x</version>

    </dependency>

Note

If you're developing for an ultra light environment, use slf4j-nop or slf4j-simple instead.

Configure the logging level on the package org.drools. For example:

In Logback, configure it in your logback.xml file:


<configuration>



    <logger name="org.drools" level="debug"/>



    ...



<configuration>

In Log4J, configure it in your log4j.xml file:


<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">



    <category name="org.drools">

      <priority value="debug" />

    </category>



    ...



</log4j:configuration>

Chapter 3. API Reference

3.1. Building

Figure 3.1. org.drools.core.builder

3.1.1. Building Using Code

The KnowledgeBuilder is responsible for taking source files, such as a DRL file or an Excel file, and turning them into a Knowledge Package of rule and process definitions which a Knowledge Base can consume. An object of the class ResourceType indicates the type of resource the builder is being asked to process.

The ResourceFactory provides capabilities to load resources from a number of sources, such as a java.io.Reader, the classpath, a URL, a java.io.File, or a byte elements. Binary files, such as decision tables (Excel's' .xls files), should not be passed in with Reader, which is only suitable for text based resources.

Figure 3.2. KnowledgeBuilder

The KnowledgeBuilder is created using the KnowledgeBuilderFactory.

Figure 3.3. KnowledgeBuilderFactory

A KnowledgeBuilder can be created using the default configuration.

Example 3.1. Creating a new KnowledgeBuilder

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

A configuration can be created using the KnowledgeBuilderFactory. This allows the behavior of the Knowledge Builder to be modified. The most common usage is to provide a custom class loader so that the KnowledgeBuilder object can resolve classes that are not in the default classpath. The first parameter is for properties and is optional, i.e., it may be left null, in which case the default options will be used. The options parameter can be used for things like changing the dialect or registering new accumulator functions.

Example 3.2. Creating a new KnowledgeBuilder with a custom ClassLoader

KnowledgeBuilderConfiguration kbuilderConf = KnowledgeBuilderFactory.newKnowledgeBuilderConfiguration(null, classLoader );

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder(kbuilderConf);

Resources of any type can be added iteratively. Below, a DRL file is added. The Knowledge Builder can handle multiple namespaces, so you can combine resources regardless of their namespace.

Example 3.3. Adding DRL Resources

kbuilder.add( ResourceFactory.newFileResource( "/project/myrules.drl" ),

              ResourceType.DRL);

It is a best practice to check the compilation results after each resource addition. The KnowledgeBuilder can report compilation results of 3 different severities: ERROR, WARNING and INFO.

An ERROR indicates that the compilation of the resource failed. You should not add more resources or retrieve the Knowledge Packages if there are errors. getKnowledgePackages() returns an empty list if there are errors.

WARNING and INFO results can be ignored, but are available for inspection nonetheless.

To check and retrieve the build results for a list of severities, the KnowledgeBuilder API offers a couple of methods:

Example 3.4. KnowledgeBuilder result inspection methods

    /**
     * Return the knowledge builder results for the listed severities.
     * @param severities
     * @return
     */
    KnowledgeBuilderResults getResults(ResultSeverity... severities);
    
    /**
     * Checks if the builder generated any results of the listed severities
     * @param severities
     * @return
     */
    boolean hasResults(ResultSeverity... severities ;

The KnowledgeBuilder API also has two helper methods to inspect for errors only: hasErrors() and getErrors():

Example 3.5. Validating

if( kbuilder.hasErrors() ) {

    System.out.println( kbuilder.getErrors() );

    return;

}

When all the resources have been added and there are no errors the collection of Knowledge Packages can be retrieved. It is a java.util.Collection because there is one KnowledgePackage per package namespace. These Knowledge Packages are serializable and often used as a unit of deployment.

Example 3.6. Getting the KnowledgePackages

Collection<KnowledgePackage> kpkgs = kbuilder.getKnowledgePackages();

The final example puts it all together.

Example 3.7. Putting it all together

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

if( kbuilder.hasErrors() ) {

    System.out.println( kbuilder.getErrors() );

    return;

}


KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

kbuilder.add( ResourceFactory.newFileResource( "/project/myrules1.drl" ),

              ResourceType.DRL);

kbuilder.add( ResourceFactory.newFileResource( "/project/myrules2.drl" ),

              ResourceType.DRL);


if( kbuilder.hasErrors() ) {

    System.out.println( kbuilder.getErrors() );

    return;

}


Collection<KnowledgePackage> kpkgs = kbuilder.getKnowledgePackages();

The KnowledgeBuilder also has a batch mode, with a fluent interface, that allows to build multiple DRLs at once as in the following example:

Example 3.8. Using KnowledgeBuilder in batch mode

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

kbuilder.batch()

        .add( ResourceFactory.newFileResource( "/project/myrules1.drl" ), ResourceType.DRL )

        .add( ResourceFactory.newFileResource( "/project/myrules2.drl" ), ResourceType.DRL )

        .add( ResourceFactory.newFileResource( "/project/mytypes1.drl" ), ResourceType.DRL )

        .build();

In this way it is no longer necessary to build the DRL files in the right order (e.g., first the DRLs containing the type declarations and then the ones with the rules using them) and it will also be possible to have circular references among them.

Moreover, the KnowledgeBuilder (regardless if you are using the batch mode or not) also allows to discard what has been added with the last DRL(s) build. This can be useful to recover from having added an erroneous DRL to the KnowledgeBuilder, as shown below.

Example 3.9. Discard the build of the last added DRL

kbuilder.add( ResourceFactory.newFileResource( "/project/wrong.drl" ), ResourceType.DRL );

if ( kbuilder.hasErrors() ) {

    kbuilder.undo();

}

3.1.2. Building Using Configuration and the ChangeSet XML

Instead of adding the resources to create definitions programmatically it is also possible to do it by configuration, via the ChangeSet XML. The simple XML file supports three elements: add, remove, and modify, each of which has a sequence of <resource> subelements defining a configuration entity. The following XML schema is not normative and intended for illustration only.

Example 3.10. XML Schema for ChangeSet XML (not normative)


<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"

           xmlns="http://drools.org/drools-5.0/change-set"

           targetNamespace="http://drools.org/drools-5.0/change-set">



  <xs:element name="change-set" type="ChangeSet"/>



  <xs:complexType name="ChangeSet">

    <xs:choice maxOccurs="unbounded">

      <xs:element name="add"    type="Operation"/>

      <xs:element name="remove" type="Operation"/>

      <xs:element name="modify" type="Operation"/>

    </xs:choice>

  </xs:complexType>



  <xs:complexType name="Operation">

    <xs:sequence>

      <xs:element name="resource" type="Resource"

                  maxOccurs="unbounded"/>

    </xs:sequence>

  </xs:complexType>



  <xs:complexType name="Resource">

    <xs:sequence>

      




      <xs:element name="decisiontable-conf" type="DecTabConf"

                  minOccurs="0"/>

    </xs:sequence>

    




    <xs:attribute name="source" type="xs:string"/>

    <xs:attribute name="type"   type="ResourceType"/>

  </xs:complexType>



  <xs:complexType name="DecTabConf">

    <xs:attribute name="input-type"     type="DecTabInpType"/>

    <xs:attribute name="worksheet-name" type="xs:string"

                  use="optional"/>

  </xs:complexType>



  




  <xs:simpleType name="ResourceType">

    <xs:restriction base="xs:string">

      <xs:enumeration value="DRL"/>

      <xs:enumeration value="XDRL"/>

      <xs:enumeration value="DSL"/>

      <xs:enumeration value="DSLR"/>

      <xs:enumeration value="DRF"/>

      <xs:enumeration value="DTABLE"/>

      <xs:enumeration value="PKG"/>

      <xs:enumeration value="BRL"/>

      <xs:enumeration value="CHANGE_SET"/>

    </xs:restriction>

  </xs:simpleType>



  




  <xs:simpleType name="DecTabInpType">

    <xs:restriction base="xs:string">

      <xs:enumeration value="XLS"/>

      <xs:enumeration value="CSV"/>

    </xs:restriction>

  </xs:simpleType>



</xs:schema>

Currently only the add element is supported, but the others will be implemented to support iterative changes. The following example loads a single DRL file.

Example 3.11. Simple ChangeSet XML


<change-set xmlns='http://drools.org/drools-5.0/change-set'

             xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

             xs:schemaLocation='http://drools.org/drools-5.0/change-set.xsd http://anonsvn.jboss.org/repos/labs/labs/jbossrules/trunk/drools-api/src/main/resources/change-set-1.0.0.xsd' >

   <add>

      <resource source='file:/project/myrules.drl' type='DRL' />

   </add>

</change-set>

Notice the file: prefix, which signifies the protocol for the resource. The Change Set supports all the protocols provided by java.net.URL, such as "file" and "http", as well as an additional "classpath". Currently the type attribute must always be specified for a resource, as it is not inferred from the file name extension. Using the ClassPath resource loader in Java allows you to specify the Class Loader to be used to locate the resource but this is not possible from XML. Instead, the Class Loader will default to the one used by the Knowledge Builder unless the ChangeSet XML is itself loaded by the ClassPath resource, in which case it will use the Class Loader specified for that resource.

Currently you still need to use the API to load that ChangeSet, but we will add support for containers such as Spring in the future, so that the process of creating a Knowledge Base can be done completely by XML configuration. Loading resources using an XML file couldn't be simpler, as it's just another resource type.

Example 3.12. Loading the ChangeSet XML

kbuilder.add( ResourceFactory.newUrlResource( url ), ResourceType.CHANGE_SET );

Change Sets can include any number of resources, and they even support additional configuration information, which currently is only needed for decision tables. Below, the example is expanded to load rules via HTTP from a URL location, and an Excel decision table from the classpath.

Example 3.13. ChangeSet XML with resource configuration


<change-set xmlns='http://drools.org/drools-5.0/change-set'

             xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

             xs:schemaLocation='http://drools.org/drools-5.0/change-set.xsd http://anonsvn.jboss.org/repos/labs/labs/jbossrules/trunk/drools-api/src/main/resources/change-set-1.0.0.xsd' >

  <add>

       <resource source='http:org/domain/myrules.drl' type='DRL' />

       <resource source='classpath:data/IntegrationExampleTest.xls' type="DTABLE">

           <decisiontable-conf input-type="XLS" worksheet-name="Tables_2" />

       </resource>

   </add>

 </change-set>

The Change Set is especially useful when working with a Knowledge Agent, as it allows for change notification and automatic rebuilding of the Knowledge Base, which is covered in more detail in the section on the Knowledge Agent, under Deploying.

Directories can also be specified, to add all resources in that folder. Currently it is expected that all resources in that folder are of the same type. If you use the Knowledge Agent it will provide a continous scanning for added, modified or removed resources and rebuild the cached Knowledge Base. The KnowledgeAgent provides more information on this.

Example 3.14. ChangeSet XML which adds a directory's contents


<change-set xmlns='http://drools.org/drools-5.0/change-set'

            xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

            xs:schemaLocation='http://drools.org/drools-5.0/change-set.xsd http://anonsvn.jboss.org/repos/labs/labs/jbossrules/trunk/drools-api/src/main/resources/change-set-1.0.0.xsd' >

   <add>

      <resource source='file:/projects/myproject/myrules' type='DRL' />

   </add>

</change-set>

3.1.3. Changing the Default Build Result Severity

In some cases, it is possible to change the default severity of a type of build result. For instance, when a new rule with the same name of an existing rule is added to a package, the default behavior is to replace the old rule by the new rule and report it as an INFO. This is probably ideal for most use cases, but in some deployments the user might want to prevent the rule update and report it as an error.

Changing the default severity for a result type is configured like any other option in Drools and can be done by API calls, system properties or configuration files. As of this version, Drools supports configurable result severity for rule updates and function updates. To configure it using system properties or configuration files, the user has to use the following properties:

Example 3.15. Setting the severity using properties

// sets the severity of rule updates
drools.kbuilder.severity.duplicateRule = <INFO|WARNING|ERROR>
// sets the severity of function updates
drools.kbuilder.severity.duplicateFunction = <INFO|WARNING|ERROR>

To configure it through the API:

Example 3.16. Setting the severity using the API

KnowledgeBuilderConfiguration kbconf = ...

// sets the severity of rule updates to error
kbconf.setOption( KBuilderSeverityOption.get( "drools.kbuilder.severity.duplicateRule", ResultSeverity.ERROR ) ); 
// sets the severity of function updates to error
kbconf.setOption( KBuilderSeverityOption.get( "drools.kbuilder.severity.duplicateFunction", ResultSeverity.ERROR ) );

3.2. Deploying

3.2.1. KnowledgePackage and Knowledge Definitions

A Knowledge Package is a collection of Knowledge Definitions, such as rules and processes. It is created by the Knowledge Builder, as described in the chapter "Building". Knowledge Packages are self-contained and serializable, and they currently form the basic deployment unit.

Figure 3.4. KnowledgePackage

Knowledge Packages are added to the Knowledge Base. However, a Knowledge Package instance cannot be reused once it's added to the Knowledge Base. If you need to add it to another Knowledge Base, try serializing it first and using the "cloned" result. We hope to fix this limitation in future versions of Drools.

3.2.2. KnowledgeBase

The Knowledge Base is a repository of all the application's knowledge definitions. It may contain rules, processes, functions, and type models. The Knowledge Base itself does not contain instance data, known as facts; instead, sessions are created from the Knowledge Base into which data can be inserted and where process instances may be started. Creating the Knowledge Base can be heavy, whereas session creation is very light, so it is recommended that Knowledge Bases be cached where possible to allow for repeated session creation.

Figure 3.5. KnowledgeBase

A KnowledgeBase object is also serializable, and some people may prefer to build and then store a KnowledgeBase, treating it also as a unit of deployment, instead of the Knowledge Packages.

The KnowledgeBase is created using the KnowledgeBaseFactory.

Figure 3.6. KnowledgeBaseFactory

A KnowledgeBase can be created using the default configuration.

Example 3.17. Creating a new KnowledgeBase

KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

If a custom class loader was used with the KnowledgeBuilder to resolve types not in the default class loader, then that must also be set on the KnowledgeBase. The technique for this is the same as with the KnowledgeBuilder.

Example 3.18. Creating a new KnowledgeBase with a custom ClassLoader

KnowledgeBaseConfiguration kbaseConf =

    KnowledgeBaseFactory.createKnowledgeBaseConfiguration( null, cl );

KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase( kbaseConf );

3.2.3. In-Process Building and Deployment

This is the simplest form of deployment. It compiles the knowledge definitions and adds them to the Knowledge Base in the same JVM. This approach requires drools-core.jar and drools-compiler.jar to be on the classpath.

Example 3.19. Add KnowledgePackages to a KnowledgeBase

Collection<KnowledgePackage> kpkgs = kbuilder.getKnowledgePackages();


KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

kbase.addKnowledgePackages( kpkgs );

Note that the addKnowledgePackages(kpkgs) method can be called iteratively to add additional knowledge.

3.2.4. Building and Deployment in Separate Processes

Both the KnowledgeBase and the KnowledgePackage are units of deployment and serializable. This means you can have one machine do any necessary building, requiring drools-compiler.jar, and have another machine deploy and execute everything, needing only drools-core.jar.

Although serialization is standard Java, we present an example of how one machine might write out the deployment unit and how another machine might read in and use that deployment unit.

Example 3.20. Writing the KnowledgePackage to an OutputStream

ObjectOutputStream out = new ObjectOutputStream( new FileOutputStream( fileName ) );

out.writeObject( kpkgs );

out.close();

Example 3.21. Reading the KnowledgePackage from an InputStream

ObjectInputStream in = new ObjectInputStream( new FileInputStream( fileName ) );

// The input stream might contain an individual

// package or a collection.

@SuppressWarnings( "unchecked" )

Collection<KnowledgePackage> kpkgs =

    ()in.readObject( Collection<KnowledgePackage> );

in.close();


KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

kbase.addKnowledgePackages( kpkgs );

The KnowledgeBase is also serializable and some people may prefer to build and then store the KnowledgeBase itself, instead of the Knowledge Packages.

Drools Guvnor, our server side management system, uses this deployment approach. After Guvnor has compiled and published serialized Knowledge Packages on a URL, Drools can use the URL resource type to load them.

3.2.5. StatefulknowledgeSessions and KnowledgeBase Modifications

Stateful Knowledge Sessions will be discussed in more detail in section "Running". The KnowledgeBase creates and returns StatefulKnowledgeSession objects, and it may optionally keep references to those. When KnowledgeBase modifications occur those modifications are applied against the data in the sessions. This reference is a weak reference and it is also optional, which is controlled by a boolean flag.

3.2.6. KnowledgeAgent

The KnowlegeAgent provides automatic loading, caching and re-loading of resources and is configured from a properties files. The Knowledge Agent can update or rebuild this Knowlege Base as the resources it uses are changed. The strategy for this is determined by the configuration given to the factory, but it is typically pull-based using regular polling. We hope to add push-based updates and rebuilds in future versions. The Knowledge Agent will continuously scan all the added resources, using a default polling interval of 60 seconds. If their date of the last modification is updated it will rebuild the cached Knowledge Base using the new resources.

Figure 3.7. KnowledgeAgent

The KnowlegeBuilder is created using a KnowledgeBuilderFactory object. The agent must specify a name, which is used in the log files to associate a log entry with the corresponding agent.

Example 3.22. Creating the KnowledgeAgent

KnowledgeAgent kagent = KnowledgeAgentFactory.newKnowledgeAgent( "MyAgent" );

Figure 3.8. KnowledgeAgentFactory

The following example constructs an agent that will build a new KnowledgeBase from the specified ChangeSet. (See section "Building" for more details on the ChangeSet format.) Note that the method can be called iteratively to add new resources over time. The Knowledge Agent polls the resources added from the ChangeSet every 60 seconds, the default interval, to see if they are updated. Whenever changes are found it will construct a new Knowledge Base or apply the modifications to the existing Knowledge Base according to its configuration. If the change set specifies a resource that is a directory its contents will be scanned for changes, too.

Example 3.23. Writing the KnowledgePackage to an OutputStream

KnowledgeAgent kagent = KnowledgeAgentFactory.newKnowledgeAgent( "MyAgent" );

kagent.applyChangeSet( ResourceFactory.newUrlResource( url ) );

KnowledgeBase kbase = kagent.getKnowledgeBase();

Resource scanning is not on by default, it's a service and must be started, and the same is true for notification. Both can be done via the ResourceFactory.

Example 3.24. Starting the Scanning and Notification Services

ResourceFactory.getResourceChangeNotifierService().start();

ResourceFactory.getResourceChangeScannerService().start();

The default resource scanning period may be changed via the ResourceChangeScannerService. A suitably updated ResourceChangeScannerConfiguration object is passed to the service's configure() method, which allows for the service to be reconfigured on demand.

Example 3.25. Changing the Scanning Intervals

ResourceChangeScannerConfiguration sconf =

    ResourceFactory.getResourceChangeScannerService().newResourceChangeScannerConfiguration();

// Set the disk scanning interval to 30s, default is 60s.

sconf.setProperty( "drools.resource.scanner.interval", "30" ); 

ResourceFactory.getResourceChangeScannerService().configure( sconf );

Knowledge Agents can take an empty Knowledge Base or a populated one. If a populated Knowledge Base is provided, the Knowledge Agent will run an iterator from Knowledge Base and subscribe to the resources that it finds. While it is possible for the Knowledge Builder to build all resources found in a directory, that information is lost by the Knowledge Builder so that those directories will not be continuously scanned. Only directories specified as part of the applyChangeSet(Resource) method are monitored.

One of the advantages of providing KnowledgeBase as the starting point is that you can provide it with a KnowledgeBaseConfiguration. When resource changes are detected and a new KnowledgeBase object is instantiated, it will use the KnowledgeBaseConfiguration of the previous KnowledgeBase object.

Example 3.26. Using an existing KnowledgeBase

KnowledgeBaseConfiguration kbaseConf =

    KnowledgeBaseFactory.createKnowledgeBaseConfiguration( null, cl );

KnowledgeBase kbase KnowledgeBaseFactory.newKnowledgeBase( kbaseConf );

// Populate kbase with resources here.


KnowledgeAgent kagent =

    KnowledgeAgentFactory.newKnowledgeAgent( "MyAgent", kbase );

KnowledgeBase kbase = kagent.getKnowledgeBase();

In the above example getKnowledgeBase() will return the same provided kbase instance until resource changes are detected and a new Knowledge Base is built. When the new Knowledge Base is built, it will be done with the KnowledgeBaseConfiguration that was provided to the previous KnowledgeBase.

As mentioned previously, a ChangeSet XML can specify a directory and all of its contents will be added. If this ChangeSet XML is used with the applyChangeSet() method it will also add any directories to the scanning process. When the directory scan detects an additional file, it will be added to the Knowledge Base; any removed file is removed from the Knowledge Base, and modified files will be removed from the Knowledge Base.

Example 3.27. ChangeSet XML which adds a directories contents


<change-set xmlns='http://drools.org/drools-5.0/change-set'

            xmlns:xs='http://www.w3.org/2001/XMLSchema-instance'

            xs:schemaLocation='http://drools.org/drools-5.0/change-set.xsd' >

   <add>

      <resource source='file:/projects/myproject/myrules' type='PKG' />

   </add>

</change-set>

Note that for the resource type PKG the drools-compiler dependency is not needed as the Knowledge Agent is able to handle those with just drools-core.

The KnowledgeAgentConfiguration can be used to modify a Knowledge Agent's default behavior. You could use this to load the resources from a directory, while inhibiting the continuous scan for changes of that directory.

Example 3.28. Change the Scanning Behavior

KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();


KnowledgeAgentConfiguration kaconf =

    KnowledgeAgentFactory.newKnowledgeAgentConfiguation();

// Do not scan directories, just files.

kaconf.setProperty( "drools.agent.scanDirectories", "false" );

KnowledgeAgent kagent =

    KnowledgeAgentFactory.newKnowledgeAgent( "test agent", kaconf );

Previously we mentioned Drools Guvnor and how it can build and publish serialized Knowledge Packages on a URL, and that the ChangeSet XML can handle URLs and Packages. Taken together, this forms an importanty deployment scenario for the Knowledge Agent.

3.3. Running

3.3.1. KnowledgeBase

The KnowledgeBase is a repository of all the application's knowledge definitions. It will contain rules, processes, functions, and type models. The Knowledge Base itself does not contain data; instead, sessions are created from the KnowledgeBase into which data can be inserted and from which process instances may be started. Creating the KnowledgeBase can be heavy, whereas session creation is very light, so it is recommended that KnowledgeBases be cached where possible to allow for repeated session creation.

Example 3.29. Creating a new KnowledgeBase

KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

3.3.2. StatefulKnowledgeSession

The StatefulKnowledgeSession stores and executes on the runtime data. It is created from the KnowledgeBase.

Figure 3.9. StatefulKnowledgeSession

Example 3.30. Create a StatefulKnowledgeSession from a KnowledgeBase

StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();

3.3.3. KnowledgeRuntime

3.3.3.1. WorkingMemoryEntryPoint

The WorkingMemoryEntryPoint provides the methods around inserting, updating and retrieving facts. The term "entry point" is related to the fact that we have multiple partitions in a Working Memory and you can choose which one you are inserting into, although this use case is aimed at event processing and covered in more detail in the Fusion manual. Most rule based applications will work with the default entry point alone.

The KnowledgeRuntime interface provides the main interaction with the engine. It is available in rule consequences and process actions. In this manual the focus is on the methods and interfaces related to rules, and the methods pertaining to processes will be ignored for now. But you'll notice that the KnowledgeRuntime inherits methods from both the WorkingMemory and the ProcessRuntime, thereby providing a unified API to work with processes and rules. When working with rules, three interfaces form the KnowledgeRuntime: WorkingMemoryEntryPoint, WorkingMemory and the KnowledgeRuntime itself.

Figure 3.10. WorkingMemoryEntryPoint

3.3.3.1.1. Insertion

Insertion is the act of telling the WorkingMemory about a fact, which you do by ksession.insert(yourObject), for example. When you insert a fact, it is examined for matches against the rules. This means all of the work for deciding about firing or not firing a rule is done during insertion; no rule, however, is executed until you call fireAllRules(), which you call after you have finished inserting your facts. It is a common misunderstanding for people to think the condition evaluation happens when you call fireAllRules(). Expert systems typically use the term assert or assertion to refer to facts made available to the system. However, due to "assert" being a keyword in most languages, we have decided to use the insert keyword; however, expect to hear the two terms used interchangeably.

When an Object is inserted it returns a FactHandle. This FactHandle is the token used to represent your inserted object within the WorkingMemory. It is also used for interactions with the WorkingMemory when you wish to retract or modify an object.

Cheese stilton = new Cheese("stilton");

FactHandle stiltonHandle = ksession.insert( stilton );

As mentioned in the Knowledge Base section, a Working Memory may operate in two assertion modes, i.e., equality or identity, with identity being the default.

Identity means that the Working Memory uses an IdentityHashMap to store all asserted objects. New instance assertions always result in the return of new FactHandle, but if an instance is asserted again then it returns the original fact handle, i.e., it ignores repeated insertions for the same object.

Equality means that the Working Memory uses a HashMap to store all asserted objects. An object instance assertion will only return a new FactHandle if the inserted object is not equal (according to its equal method) to an already existing fact.

3.3.3.1.2. Retraction

Retraction is the removal of a fact from Working Memory, which means that it will no longer track and match that fact, and any rules that are activated and dependent on that fact will be cancelled. Note that it is possible to have rules that depend on the nonexistence of a fact, in which case retracting a fact may cause a rule to activate. (See the not and exists keywords.) Retraction may be done using the FactHandle that was returned by the insert call. On the right hand side of a rule the retract statement is used, which works with a simple object reference.

Cheese stilton = new Cheese("stilton");

FactHandle stiltonHandle = ksession.insert( stilton );

....

ksession.retract( stiltonHandle );

3.3.3.1.3. Update

The Rule Engine must be notified of modified facts, so that they can be reprocessed. You must use the update() method to notify the WorkingMemory of changed objects for those objects that are not able to notify the WorkingMemory themselves. Notice that update() always takes the modified object as a second parameter, which allows you to specify new instances for immutable objects. On the right hand side of a rule the modify statement is recommended, as it makes the changes and notifies the engine in a single statement. Alternatively, after changing a fact object's field values through calls of setter methods you must invoke update immediately, event before changing another fact, or you will cause problems with the indexing within the rule engine. The modify statement avoids this problem.

Cheese stilton = new Cheese("stilton");

FactHandle stiltonHandle = workingMemory.insert( stilton );

...

stilton.setPrice( 100 );

workingMemory.update( stiltonHandle, stilton );

3.3.3.2. WorkingMemory

The WorkingMemory provides access to the Agenda, permits query executions, and lets you access named Entry Points.

Figure 3.11. WorkingMemory

3.3.3.2.1. Query

Queries are used to retrieve fact sets based on patterns, as they are used in rules. Patterns may make use of optional parameters. Queries can be defined in the Knowledge Base, from where they are called up to return the matching results. While iterating over the result collection, any identifier bound in the query can be used to access the corresponding fact or fact field by calling the get method with the binding variable's name as its argument. If the binding refers to a fact object, its FactHandle can be retrieved by calling getFactHandle, again with the variable's name as the parameter.

Figure 3.12. QueryResults

Figure 3.13. QueryResultsRow

Example 3.31. Simple Query Example

QueryResults results =

    ksession.getQueryResults( "my query", new Object[] { "string" } );

for ( QueryResultsRow row : results ) {

    System.out.println( row.get( "varName" ) );

}

3.3.3.2.2. Live Queries

Invoking queries and processing the results by iterating over the returned set is not a good way to monitor changes over time.

To alleviate this, Drools provides Live Queries, which have a listener attached instead of returning an iterable result set. These live queries stay open by creating a view and publishing change events for the contents of this view. To activate, you start your query with parameters and listen to changes in the resulting view. The dispose method terminates the query and discontinues this reactive scenario.

Example 3.32. Implementing ViewChangedEventListener

final List updated = new ArrayList();

final List removed = new ArrayList();

final List added = new ArrayList();

 

ViewChangedEventListener listener = new ViewChangedEventListener() {           

 public void rowUpdated(Row row) {

  updated.add( row.get( "$price" ) );

 }

  

 public void rowRemoved(Row row) {

  removed.add( row.get( "$price" ) );

 }

  

 public void rowAdded(Row row) {

  added.add( row.get( "$price" ) );

 }

};       

 

// Open the LiveQuery

LiveQuery query = ksession.openLiveQuery( "cheeses",

                                          new Object[] { "cheddar", "stilton" },

                                          listener );

...

...

query.dispose() // calling dispose to terminate the live query

A Drools blog article contains an example of Glazed Lists integration for live queries:

http://blog.athico.com/2010/07/glazed-lists-examples-for-drools-live.html

3.3.3.3. KnowledgeRuntime

The KnowledgeRuntime provides further methods that are applicable to both rules and processes, such as setting globals and registering channels. ("Exit point" is an obsolete synonym for "channel".)

Figure 3.14. KnowledgeRuntime

3.3.3.3.1. Globals

Globals are named objects that are made visible to the rule engine, but in a way that is fundamentally different from the one for facts: changes in the object backing a global do not trigger reevaluation of rules. Still, globals are useful for providing static information, as an object offering services that are used in the RHS of a rule, or as a means to return objects from the rule engine. When you use a global on the LHS of a rule, make sure it is immutable, or, at least, don't expect changes to have any effect on the behavior of your rules.

A global must be declared in a rules file, and then it needs to be backed up with a Java object.

global java.util.List list

With the Knowledge Base now aware of the global identifier and its type, it is now possible to call ksession.setGlobal() with the global's name and an object, for any session, to associate the object with the global. Failure to declare the global type and identifier in DRL code will result in an exception being thrown from this call.

List list = new ArrayList();

ksession.setGlobal("list", list);

Make sure to set any global before it is used in the evaluation of a rule. Failure to do so results in a NullPointerException.

3.3.3.4. StatefulRuleSession

The StatefulRuleSession is inherited by the StatefulKnowledgeSession and provides the rule related methods that are relevant from outside of the engine.

Figure 3.15. StatefulRuleSession

3.3.3.4.1. Agenda Filters

Figure 3.16. AgendaFilters

AgendaFilter objects are optional implementations of the filter interface which are used to allow or deny the firing of an activation. What you filter on is entirely up to the implementation. Drools 4.0 used to supply some out of the box filters, which have not be exposed in drools 5.0 knowledge-api, but they are simple to implement and the Drools 4.0 code base can be referred to.

To use a filter specify it while calling fireAllRules(). The following example permits only rules ending in the string "Test". All others will be filtered out.

ksession.fireAllRules( new RuleNameEndsWithAgendaFilter( "Test" ) );

3.3.4. Agenda

The Agenda is a Rete feature. During actions on the WorkingMemory, rules may become fully matched and eligible for execution; a single Working Memory Action can result in multiple eligible rules. When a rule is fully matched an Activation is created, referencing the rule and the matched facts, and placed onto the Agenda. The Agenda controls the execution order of these Activations using a Conflict Resolution strategy.

The engine cycles repeatedly through two phases:

Working Memory Actions. This is where most of the work takes place, either in the Consequence (the RHS itself) or the main Java application process. Once the Consequence has finished or the main Java application process calls fireAllRules() the engine switches to the Agenda Evaluation phase.
Agenda Evaluation. This attempts to select a rule to fire. If no rule is found it exits, otherwise it fires the found rule, switching the phase back to Working Memory Actions.

Figure 3.17. Two Phase Execution

The process repeats until the agenda is clear, in which case control returns to the calling application. When Working Memory Actions are taking place, no rules are being fired.

Figure 3.18. Agenda

3.3.4.1. Conflict Resolution

Conflict resolution is required when there are multiple rules on the agenda. (The basics to this are covered in chapter "Quick Start".) As firing a rule may have side effects on the working memory, the rule engine needs to know in what order the rules should fire (for instance, firing ruleA may cause ruleB to be removed from the agenda).

The default conflict resolution strategies employed by Drools are: Salience and LIFO (last in, first out).

The most visible one is salience (or priority), in which case a user can specify that a certain rule has a higher priority (by giving it a higher number) than other rules. In that case, the rule with higher salience will be preferred. LIFO priorities are based on the assigned Working Memory Action counter value, with all rules created during the same action receiving the same value. The execution order of a set of firings with the same priority value is arbitrary.

As a general rule, it is a good idea not to count on rules firing in any particular order, and to author the rules without worrying about a "flow". However when a flow is needed a number of possibilities exist, including but not limited to: agenda groups, rule flow groups, activation groups, control/semaphore facts. These are discussed in later sections.

Drools 4.0 supported custom conflict resolution strategies; while this capability still exists in Drools it has not yet been exposed to the end user via knowledge-api in Drools 5.0.

3.3.4.2. AgendaGroup

Figure 3.19. AgendaGroup

Agenda groups are a way to partition rules (activations, actually) on the agenda. At any one time, only one group has "focus" which means that activations for rules in that group only will take effect. You can also have rules with "auto focus" which means that the focus is taken for its agenda group when that rule's conditions are true.

Agenda groups are known as "modules" in CLIPS terminology. While it best to design rules that do not need control flow, this is not always possible. Agenda groups provide a handy way to create a "flow" between grouped rules. You can switch the group which has focus either from within the rule engine, or via the API. If your rules have a clear need for multiple "phases" or "sequences" of processing, consider using agenda-groups for this purpose.

Each time setFocus() is called it pushes that Agenda Group onto a stack. When the focus group is empty it is popped from the stack and the focus group that is now on top evaluates. An Agenda Group can appear in multiple locations on the stack. The default Agenda Group is "MAIN", with all rules which do not specify an Agenda Group being in this group. It is also always the first group on the stack, given focus initially, by default.

ksession.getAgenda().getAgendaGroup( "Group A" ).setFocus();

3.3.4.3. ActivationGroup

Figure 3.20. ActivationGroup

An activation group is a set of rules bound together by the same "activation-group" rule attribute. In this group only one rule can fire, and after that rule has fired all the other rules are cancelled from the agenda. The clear() method can be called at any time, which cancels all of the activations before one has had a chance to fire.

ksession.getAgenda().getActivationGroup( "Group B" ).clear();

3.3.4.4. RuleFlowGroup

Figure 3.21. RuleFlowGroup

A rule flow group is a group of rules associated by the "ruleflow-group" rule attribute. These rules can only fire when the group is activate. The group itself can only become active when the elaboration of the ruleflow diagram reaches the node representing the group. Here too, the clear() method can be called at any time to cancels all activations still remaining on the Agenda.

ksession.getAgenda().getRuleFlowGroup( "Group C" ).clear();

3.3.5. Event Model

The event package provides means to be notified of rule engine events, including rules firing, objects being asserted, etc. This allows you, for instance, to separate logging and auditing activities from the main part of your application (and the rules).

The KnowlegeRuntimeEventManager interface is implemented by the KnowledgeRuntime which provides two interfaces, WorkingMemoryEventManager and ProcessEventManager. We will only cover the WorkingMemoryEventManager here.

Figure 3.22. KnowledgeRuntimeEventManager

The WorkingMemoryEventManager allows for listeners to be added and removed, so that events for the working memory and the agenda can be listened to.

Figure 3.23. WorkingMemoryEventManager

The following code snippet shows how a simple agenda listener is declared and attached to a session. It will print activations after they have fired.

Example 3.33. Adding an AgendaEventListener

ksession.addEventListener( new DefaultAgendaEventListener() {

   public void afterMatchFired(AfterActivationFiredEvent event) {

       super.afterMatchFired( event );

       System.out.println( event );

   }

});

Drools also provides DebugWorkingMemoryEventListener and DebugAgendaEventListener which implement each method with a debug print statement. To print all Working Memory events, you add a listener like this:

Example 3.34. Creating a new KnowledgeBuilder

ksession.addEventListener( new DebugWorkingMemoryEventListener() );

All emitted events implement the KnowlegeRuntimeEvent interface which can be used to retrieve the actual KnowlegeRuntime the event originated from.

Figure 3.24. KnowlegeRuntimeEvent

The events currently supported are:

ActivationCreatedEvent
ActivationCancelledEvent
BeforeActivationFiredEvent
AfterActivationFiredEvent
AgendaGroupPushedEvent
AgendaGroupPoppedEvent
ObjectInsertEvent
ObjectRetractedEvent
ObjectUpdatedEvent
ProcessCompletedEvent
ProcessNodeLeftEvent
ProcessNodeTriggeredEvent
ProcessStartEvent

3.3.6. KnowledgeRuntimeLogger

The KnowledgeRuntimeLogger uses the comprehensive event system in Drools to create an audit log that can be used to log the execution of an application for later inspection, using tools such as the Eclipse audit viewer.

Figure 3.25. KnowledgeRuntimeLoggerFactory

Example 3.35. FileLogger

KnowledgeRuntimeLogger logger =

  KnowledgeRuntimeLoggerFactory.newFileLogger(ksession, "logdir/mylogfile");

...

logger.close();

3.3.7. StatelessKnowledgeSession

The StatelessKnowledgeSession wraps the StatefulKnowledgeSession, instead of extending it. Its main focus is on decision service type scenarios. It avoids the need to call dispose(). Stateless sessions do not support iterative insertions and the method call fireAllRules() from Java code; the act of calling execute() is a single-shot method that will internally instantiate a StatefulKnowledgeSession, add all the user data and execute user commands, call fireAllRules(), and then call dispose(). While the main way to work with this class is via the BatchExecution (a subinterface of Command) as supported by the CommandExecutor interface, two convenience methods are provided for when simple object insertion is all that's required. The CommandExecutor and BatchExecution are talked about in detail in their own section.

Figure 3.26. StatelessKnowledgeSession

Our simple example shows a stateless session executing a given collection of Java objects using the convenience API. It will iterate the collection, inserting each element in turn.

Example 3.36. Simple StatelessKnowledgeSession execution with a Collection

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

kbuilder.add( ResourceFactory.newFileSystemResource( fileName ), ResourceType.DRL );

if (kbuilder.hasErrors() ) {

    System.out.println( kbuilder.getErrors() );

} else {

    KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

    kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );

    StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

    ksession.execute( collection );

}

If this was done as a single Command it would be as follows:

Example 3.37. Simple StatelessKnowledgeSession execution with InsertElements Command

ksession.execute( CommandFactory.newInsertElements( collection ) );

If you wanted to insert the collection itself, and the collection's individual elements, then CommandFactory.newInsert(collection) would do the job.

Methods of the CommandFactory create the supported commands, all of which can be marshalled using XStream and the BatchExecutionHelper. BatchExecutionHelper provides details on the XML format as well as how to use Drools Pipeline to automate the marshalling of BatchExecution and ExecutionResults.

StatelessKnowledgeSession supports globals, scoped in a number of ways. I'll cover the non-command way first, as commands are scoped to a specific execution call. Globals can be resolved in three ways.

The Stateless Knowledge Session method getGlobals() returns a Globals instance which provides access to the session's globals. These are shared for all execution calls. Exercise caution regarding mutable globals because execution calls can be executing simultaneously in different threads.

Example 3.38. Session scoped global

StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

// Set a global hbnSession, that can be used for DB interactions in the rules.

ksession.setGlobal( "hbnSession", hibernateSession );

// Execute while being able to resolve the "hbnSession" identifier.  

ksession.execute( collection );

Using a delegate is another way of global resolution. Assigning a value to a global (with setGlobal(String, Object)) results in the value being stored in an internal collection mapping identifiers to values. Identifiers in this internal collection will have priority over any supplied delegate. Only if an identifier cannot be found in this internal collection, the delegate global (if any) will be used.
The third way of resolving globals is to have execution scoped globals. Here, a Command to set a global is passed to the CommandExecutor.

The CommandExecutor interface also offers the ability to export data via "out" parameters. Inserted facts, globals and query results can all be returned.

Example 3.39. Out identifiers

// Set up a list of commands

List cmds = new ArrayList();

cmds.add( CommandFactory.newSetGlobal( "list1", new ArrayList(), true ) );

cmds.add( CommandFactory.newInsert( new Person( "jon", 102 ), "person" ) );

cmds.add( CommandFactory.newQuery( "Get People" "getPeople" );


// Execute the list

ExecutionResults results =

  ksession.execute( CommandFactory.newBatchExecution( cmds ) );


// Retrieve the ArrayList

results.getValue( "list1" );

// Retrieve the inserted Person fact

results.getValue( "person" );

// Retrieve the query as a QueryResults instance.

results.getValue( "Get People" );

3.3.7.1. Sequential Mode

With Rete you have a stateful session where objects can be asserted and modified over time, and where rules can also be added and removed. Now what happens if we assume a stateless session, where after the initial data set no more data can be asserted or modified and rules cannot be added or removed? Certainly it won't be necessary to re-evaluate rules, and the engine will be able to operate in a simplified way.

Order the Rules by salience and position in the ruleset (by setting a sequence attribute on the rule terminal node).
Create an elements, one element for each possible rule activation; element position indicates firing order.
Turn off all node memories, except the right-input Object memory.
Disconnect the Left Input Adapter Node propagation, and let the Object plus the Node be referenced in a Command object, which is added to a list on the Working Memory for later execution.
Assert all objects, and, when all assertions are finished and thus right-input node memories are populated, check the Command list and execute each in turn.
All resulting Activations should be placed in the elements, based upon the determined sequence number of the Rule. Record the first and last populated elements, to reduce the iteration range.
Iterate the elements of Activations, executing populated element in turn.
If we have a maximum number of allowed rule executions, we can exit our network evaluations early to fire all the rules in the elements.

The LeftInputAdapterNode no longer creates a Tuple, adding the Object, and then propagate the Tuple – instead a Command object is created and added to a list in the Working Memory. This Command object holds a reference to the LeftInputAdapterNode and the propagated object. This stops any left-input propagations at insertion time, so that we know that a right-input propagation will never need to attempt a join with the left-inputs (removing the need for left-input memory). All nodes have their memory turned off, including the left-input Tuple memory but excluding the right-input object memory, which means that the only node remembering an insertion propagation is the right-input object memory. Once all the assertions are finished and all right-input memories populated, we can then iterate the list of LeftInputAdatperNode Command objects calling each in turn. They will propagate down the network attempting to join with the right-input objects, but they won't be remembered in the left input as we know there will be no further object assertions and thus propagations into the right-input memory.

There is no longer an Agenda, with a priority queue to schedule the Tuples; instead, there is simply an elements for the number of rules. The sequence number of the RuleTerminalNode indicates the element within the elements where to place the Activation. Once all Command objects have finished we can iterate our elements, checking each element in turn, and firing the Activations if they exist. To improve performance, we remember the first and the last populated cell in the elements. The network is constructed, with each RuleTerminalNode being given a sequence number based on a salience number and its order of being added to the network.

Typically the right-input node memories are Hash Maps, for fast object retraction; here, as we know there will be no object retractions, we can use a list when the values of the object are not indexed. For larger numbers of objects indexed Hash Maps provide a performance increase; if we know an object type has only a few instances, indexing is probably not advantageous, and a list can be used.

Sequential mode can only be used with a Stateless Session and is off by default. To turn it on, either call RuleBaseConfiguration.setSequential(true), or set the rulebase configuration property drools.sequential to true. Sequential mode can fall back to a dynamic agenda by calling setSequentialAgenda with SequentialAgenda.DYNAMIC. You may also set the "drools.sequential.agenda" property to "sequential" or "dynamic".

3.3.8. Commands and the CommandExecutor

Drools has the concept of stateful or stateless sessions. We've already covered stateful sessions, which use the standard working memory that can be worked with iteratively over time. Stateless is a one-off execution of a working memory with a provided data set. It may return some results, with the session being disposed at the end, prohibiting further iterative interactions. You can think of stateless as treating a rule engine like a function call with optional return results.

In Drools 4 we supported these two paradigms but the way the user interacted with them was different. StatelessSession used an execute(...) method which would insert a collection of objects as facts. StatefulSession didn't have this method, and insert used the more traditional insert(...) method. The other issue was that the StatelessSession did not return any results, so that users themselves had to map globals to get results, and it wasn't possible to do anything besides inserting objects; users could not start processes or execute queries.

Drools 5.0 addresses all of these issues and more. The foundation for this is the CommandExecutor interface, which both the stateful and stateless interfaces extend, creating consistency and ExecutionResults:

Figure 3.27. CommandExecutor

Figure 3.28. ExecutionResults

The CommandFactory allows for commands to be executed on those sessions, the only difference being that the Stateless Knowledge Session executes fireAllRules() at the end before disposing the session. The currently supported commands are:

FireAllRules
GetGlobal
SetGlobal
InsertObject
InsertElements
Query
StartProcess
BatchExecution

InsertObject will insert a single object, with an optional "out" identifier. InsertElements will iterate an Iterable, inserting each of the elements. What this means is that a Stateless Knowledge Session is no longer limited to just inserting objects, it can now start processes or execute queries, and do this in any order.

Example 3.40. Insert Command

StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

ExecutionResults bresults =

  ksession.execute( CommandFactory.newInsert( new Cheese( "stilton" ), "stilton_id" ) );

Stilton stilton = bresults.getValue( "stilton_id" );

The execute method always returns an ExecutionResults instance, which allows access to any command results if they specify an out identifier such as the "stilton_id" above.

Example 3.41. InsertElements Command

StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();

Command cmd = CommandFactory.newInsertElements( Arrays.asList( Object[] { 

                  new Cheese( "stilton" ),

                  new Cheese( "brie" ),

                  new Cheese( "cheddar" ),

              });

ExecutionResults bresults = ksession.execute( cmd );

The execute method only allows for a single command. That's where BatchExecution comes in, which represents a composite command, created from a list of commands. Now, execute will iterate over the list and execute each command in turn. This means you can insert some objects, start a process, call fireAllRules and execute a query, all in a single execute(...) call, which is quite powerful.

As mentioned previosly, the Stateless Knowledge Session will execute fireAllRules() automatically at the end. However the keen-eyed reader probably has already noticed the FireAllRules command and wondered how that works with a StatelessKnowledgeSession. The FireAllRules command is allowed, and using it will disable the automatic execution at the end; think of using it as a sort of manual override function.

Commands support out identifiers. Any command that has an out identifier set on it will add its results to the returned ExecutionResults instance. Let's look at a simple example to see how this works.

Example 3.42. BatchExecution Command

StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();


List cmds = new ArrayList();        

cmds.add( CommandFactory.newInsertObject( new Cheese( "stilton", 1), "stilton") );

cmds.add( CommandFactory.newStartProcess( "process cheeses" ) );

cmds.add( CommandFactory.newQuery( "cheeses" ) );

ExecutionResults bresults = ksession.execute( CommandFactory.newBatchExecution( cmds ) );

Cheese stilton = ( Cheese ) bresults.getValue( "stilton" );

QueryResults qresults = ( QueryResults ) bresults.getValue( "cheeses" );

In the above example multiple commands are executed, two of which populate the ExecutionResults. The query command defaults to use the same identifier as the query name, but it can also be mapped to a different identifier.

A custom XStream marshaller can be used with the Drools Pipeline to achieve XML scripting, which is perfect for services. Here are two simple XML samples, one for the BatchExecution and one for the ExecutionResults.

Example 3.43. Simple BatchExecution XML


<batch-execution>

   <insert out-identifier='outStilton'>

      <org.drools.compiler.Cheese>

         <type>stilton</type>

         <price>25</price>

         <oldPrice>0</oldPrice>

      </org.drools.compiler.Cheese>

   </insert>

</batch-execution>

Example 3.44. Simple ExecutionResults XML


<execution-results>

   <result identifier='outStilton'>

      <org.drools.compiler.Cheese>

         <type>stilton</type>

         <oldPrice>25</oldPrice>        

         <price>30</price>

      </org.drools.compiler.Cheese>

   </result>

</execution-results>

Spring and Camel, covered in the integrations book, facilitate declarative services.

Example 3.45. BatchExecution Marshalled to XML


<batch-execution>

  <insert out-identifier="stilton">

    <org.drools.compiler.Cheese>

      <type>stilton</type>

      <price>1</price>

      <oldPrice>0</oldPrice>

    </org.drools.compiler.Cheese>

  </insert>

  <query out-identifier='cheeses2' name='cheesesWithParams'>

    <string>stilton</string>

    <string>cheddar</string>

  </query>

</batch-execution>

The CommandExecutor returns an ExecutionResults, and this is handled by the pipeline code snippet as well. A similar output for the <batch-execution> XML sample above would be:

Example 3.46. ExecutionResults Marshalled to XML


<execution-results>

  <result identifier="stilton">

    <org.drools.compiler.Cheese>

      <type>stilton</type>

      <price>2</price>

    </org.drools.compiler.Cheese>

  </result>        

  <result identifier='cheeses2'>

    <query-results>

      <identifiers>

        <identifier>cheese</identifier>

      </identifiers>

      <row>

        <org.drools.compiler.Cheese>

          <type>cheddar</type>

          <price>2</price>

          <oldPrice>0</oldPrice>

        </org.drools.compiler.Cheese>

      </row>

      <row>

        <org.drools.compiler.Cheese>

          <type>cheddar</type>

          <price>1</price>

          <oldPrice>0</oldPrice>

        </org.drools.compiler.Cheese>

      </row>

    </query-results>

  </result>

</execution-results>

The BatchExecutionHelper provides a configured XStream instance to support the marshalling of Batch Executions, where the resulting XML can be used as a message format, as shown above. Configured converters only exist for the commands supported via the Command Factory. The user may add other converters for their user objects. This is very useful for scripting stateless or stateful knowledge sessions, especially when services are involved.

There is currently no XML schema to support schema validation. The basic format is outlined here, and the drools-pipeline module has an illustrative unit test in the XStreamBatchExecutionTest unit test. The root element is <batch-execution> and it can contain zero or more commands elements.

Example 3.47. Root XML element


<batch-execution>

...

</batch-execution>

This contains a list of elements that represent commands, the supported commands is limited to those Commands provided by the Command Factory. The most basic of these is the <insert> element, which inserts objects. The contents of the insert element is the user object, as dictated by XStream.

Example 3.48. Insert


<batch-execution>

   <insert>

      ...<!-- any user object -->

   </insert>

</batch-execution>

The insert element features an "out-identifier" attribute, demanding that the inserted object will also be returned as part of the result payload.

Example 3.49. Insert with Out Identifier Command


<batch-execution>

   <insert out-identifier='userVar'>

      ...

   </insert>

</batch-execution>

It's also possible to insert a collection of objects using the <insert-elements> element. This command does not support an out-identifier. The org.domain.UserClass is just an illustrative user object that XStream would serialize.

Example 3.50. Insert Elements command


<batch-execution>

   <insert-elements>

      <org.domain.UserClass>

         ...

      </org.domain.UserClass>

      <org.domain.UserClass>

         ...

      </org.domain.UserClass>

      <org.domain.UserClass>

         ...

      </org.domain.UserClass>

   </insert-elements>

</batch-execution>

Next, there is the <set-global> element, which sets a global for the session.

Example 3.51. Insert Elements command


<batch-execution>

   <set-global identifier='userVar'>

      <org.domain.UserClass>

         ...

      </org.domain.UserClass>

   </set-global>

</batch-execution>

<set-global> also supports two other optional attributes, out and out-identifier. A true value for the boolean out will add the global to the <batch-execution-results> payload, using the name from the identifier attribute. out-identifier works like out but additionally allows you to override the identifier used in the <batch-execution-results> payload.

Example 3.52. Set Global Command


<batch-execution>

   <set-global identifier='userVar1' out='true'>

      <org.domain.UserClass>

         ...

      </org.domain.UserClass>

   </set-global>

   <set-global identifier='userVar2' out-identifier='alternativeUserVar2'>

      <org.domain.UserClass>

         ...

      </org.domain.UserClass>

   </set-global>

</batch-execution>

There is also a <get-global> element, without contents, with just an out-identifier attribute. (There is no need for an out attribute because retrieving the value is the sole purpose of a <get-global> element.

Example 3.53. Get Global Command


<batch-execution>

   <get-global identifier='userVar1' />

   <get-global identifier='userVar2' out-identifier='alternativeUserVar2'/>

</batch-execution>

While the out attribute is useful in returning specific instances as a result payload, we often wish to run actual queries. Both parameter and parameterless queries are supported. The name attribute is the name of the query to be called, and the out-identifier is the identifier to be used for the query results in the <execution-results> payload.

Example 3.54. Query Command


<batch-execution>

   <query out-identifier='cheeses' name='cheeses'/>

   <query out-identifier='cheeses2' name='cheesesWithParams'>

      <string>stilton</string>

      <string>cheddar</string>

   </query>

</batch-execution>

The <start-process> command accepts optional parameters. Other process related methods will be added later, like interacting with work items.

Example 3.55. Start Process Command


<batch-execution>

   <startProcess processId='org.drools.actions'>

      <parameter identifier='person'>

         <org.drools.TestVariable>

            <name>John Doe</name>

         </org.drools.TestVariable>

      </parameter>

   </startProcess>

</batch-execution

Example 3.56. Signal Event Command


<signal-event process-instance-id='1' event-type='MyEvent'>

   <string>MyValue</string>

</signal-event>

Example 3.57. Complete Work Item Command


<complete-work-item id='" + workItem.getId() + "' >

   <result identifier='Result'>

      <string>SomeOtherString</string>

   </result>

</complete-work-item>

Example 3.58. Abort Work Item Command


<abort-work-item id='21' />

Support for more commands will be added over time.

3.3.9. Marshalling

The MarshallerFactory is used to marshal and unmarshal Stateful Knowledge Sessions.

Figure 3.29. MarshallerFactory

At the simplest the MarshallerFactory can be used as follows:

Example 3.59. Simple Marshaller Example

// ksession is the StatefulKnowledgeSession

// kbase is the KnowledgeBase

ByteArrayOutputStream baos = new ByteArrayOutputStream();

Marshaller marshaller = MarshallerFactory.newMarshaller( kbase );

marshaller.marshall( baos, ksession );

baos.close();

However, with marshalling you need more flexibility when dealing with referenced user data. To achieve this we have the ObjectMarshallingStrategy interface. Two implementations are provided, but users can implement their own. The two supplied strategies are IdentityMarshallingStrategy and SerializeMarshallingStrategy. SerializeMarshallingStrategy is the default, as used in the example above, and it just calls the Serializable or Externalizable methods on a user instance. IdentityMarshallingStrategy instead creates an integer id for each user object and stores them in a Map, while the id is written to the stream. When unmarshalling it accesses the IdentityMarshallingStrategy map to retrieve the instance. This means that if you use the IdentityMarshallingStrategy, it is stateful for the life of the Marshaller instance and will create ids and keep references to all objects that it attempts to marshal. Below is he code to use an Identity Marshalling Strategy.

Example 3.60. IdentityMarshallingStrategy

ByteArrayOutputStream baos = new ByteArrayOutputStream();

ObjectMarshallingStrategy oms = MarshallerFactory.newIdentityMarshallingStrategy()

Marshaller marshaller =

  MarshallerFactory.newMarshaller( kbase, new ObjectMarshallingStrategy[]{ oms } );

marshaller.marshall( baos, ksession );

baos.close();

For added flexability we can't assume that a single strategy is suitable. Therefore we have added the ObjectMarshallingStrategyAcceptor interface that each Object Marshalling Strategy contains. The Marshaller has a chain of strategies, and when it attempts to read or write a user object it iterates the strategies asking if they accept responsability for marshalling the user object. One of the provided implementations is ClassFilterAcceptor. This allows strings and wild cards to be used to match class names. The default is "*.*", so in the above example the Identity Marshalling Strategy is used which has a default "*.*" acceptor.

Assuming that we want to serialize all classes except for one given package, where we will use identity lookup, we could do the following:

Example 3.61. IdentityMarshallingStrategy with Acceptor

ByteArrayOutputStream baos = new ByteArrayOutputStream();

ObjectMarshallingStrategyAcceptor identityAcceptor =

  MarshallerFactory.newClassFilterAcceptor( new String[] { "org.domain.pkg1.*" } );

ObjectMarshallingStrategy identityStrategy =

  MarshallerFactory.newIdentityMarshallingStrategy( identityAcceptor );

ObjectMarshallingStrategy sms = MarshallerFactory.newSerializeMarshallingStrategy();

Marshaller marshaller =

  MarshallerFactory.newMarshaller( kbase,

                                   new ObjectMarshallingStrategy[]{ identityStrategy, sms } );

marshaller.marshall( baos, ksession );

baos.close();

Note that the acceptance checking order is in the natural order of the supplied elements.

Also note that if you are using scheduled activations (i.e. some of your rules use timers or calendars) they are marshallable only if, before you use it, you configure your StatefulKnowledgeSession to use a trackable timer job factory manager as it follows:

Example 3.62. Configuring a trackable timer job factory manager

KnowledgeSessionConfiguration ksconf = KnowledgeBaseFactory.newKnowledgeSessionConfiguration();

ksconf.setOption(TimerJobFactoryOption.get("trackable"));

StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession(ksconf, null);

3.3.10. Persistence and Transactions

Longterm out of the box persistence with Java Persistence API (JPA) is possible with Drools. You will need to have some implementation of the Java Transaction API (JTA) installed. For development purposes we recommend the Bitronix Transaction Manager, as it's simple to set up and works embedded, but for production use JBoss Transactions is recommended.

Example 3.63. Simple example using transactions

Environment env = KnowledgeBaseFactory.newEnvironment();

env.set( EnvironmentName.ENTITY_MANAGER_FACTORY,

         Persistence.createEntityManagerFactory( "emf-name" ) );

env.set( EnvironmentName.TRANSACTION_MANAGER,

         TransactionManagerServices.getTransactionManager() );

          

// KnowledgeSessionConfiguration may be null, and a default will be used

StatefulKnowledgeSession ksession =

  JPAKnowledgeService.newStatefulKnowledgeSession( kbase, null, env );

int sessionId = ksession.getId();

 

UserTransaction ut =

  (UserTransaction) new InitialContext().lookup( "java:comp/UserTransaction" );

ut.begin();

ksession.insert( data1 );

ksession.insert( data2 );

ksession.startProcess( "process1" );

ut.commit();

To use a JPA, the Environment must be set with both the EntityManagerFactory and the TransactionManager. If rollback occurs the ksession state is also rolled back, so you can continue to use it after a rollback. To load a previously persisted Stateful Knowledge Session you'll need the id, as shown below:

Example 3.64. Loading a StatefulKnowledgeSession

StatefulKnowledgeSession ksession =

  JPAKnowledgeService.loadStatefulKnowledgeSession( sessionId, kbase, null, env );

To enable persistence several classes must be added to your persistence.xml, as in the example below:

Example 3.65. Configuring JPA


<persistence-unit name="org.drools.persistence.jpa" transaction-type="JTA">

   <provider>org.hibernate.ejb.HibernatePersistence</provider>

   <jta-data-source>jdbc/BitronixJTADataSource</jta-data-source>       

   <class>org.drools.persistence.session.SessionInfo</class>

   <class>org.drools.persistence.processinstance.ProcessInstanceInfo</class>

   <class>org.drools.persistence.processinstance.ProcessInstanceEventInfo</class>

   <class>org.drools.persistence.processinstance.WorkItemInfo</class>

   <properties>

         <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/>            

         <property name="hibernate.max_fetch_depth" value="3"/>

         <property name="hibernate.hbm2ddl.auto" value="update" />

         <property name="hibernate.show_sql" value="true" />

         <property name="hibernate.transaction.manager_lookup_class"

                      value="org.hibernate.transaction.BTMTransactionManagerLookup" />

   </properties>

</persistence-unit>

The jdbc JTA data source would have to be configured first. Bitronix provides a number of ways of doing this, and its documentation should be contsulted for details. For a quick start, here is the programmatic approach:

Example 3.66. Configuring JTA DataSource

PoolingDataSource ds = new PoolingDataSource();

ds.setUniqueName( "jdbc/BitronixJTADataSource" );

ds.setClassName( "org.h2.jdbcx.JdbcDataSource" );

ds.setMaxPoolSize( 3 );

ds.setAllowLocalTransactions( true );

ds.getDriverProperties().put( "user", "sa" );

ds.getDriverProperties().put( "password", "sasa" );

ds.getDriverProperties().put( "URL", "jdbc:h2:mem:mydb" );

ds.init();

Bitronix also provides a simple embedded JNDI service, ideal for testing. To use it add a jndi.properties file to your META-INF and add the following line to it:

Example 3.67. JNDI properties

java.naming.factory.initial=bitronix.tm.jndi.BitronixInitialContextFactory

3.3.11. Drools Clips

Drools Clips is an alpha level research project to provide a Clips like front end ot Drools. As of Drools 5.2 this module as stopped working, we will try and fix for 5.3.

Deftemplates are working, the knowledge base handles multiple name spaces and you can attach the knoweldge base to the session for interative building, to provide a more "shell" like environment suitable for Clips.

deftemplate
defrule
deffunction
and/or/not/exists/test conditional elements
Literal, Variable, Return Value and Predicate field constarints

This project is very early stages and in need of love. If you want to help, open up eclipse import api, core, compiler and clips and you should be good to go. The unit tests should be self explanatory.

Chapter 4. Rule Language Reference

4.1. Overview

4.1.1. A rule file
4.1.2. What makes a rule

4.2. Keywords

4.3. Comments

4.3.1. Single line comment
4.3.2. Multi-line comment

4.4. Error Messages

4.4.1. Message format
4.4.2. Error Messages Description
4.4.3. Other Messages

4.5. Package

4.5.1. import
4.5.2. global

4.6. Function

4.7. Type Declaration

4.7.1. Declaring New Types
4.7.2. Declaring Metadata
4.7.3. Declaring Metadata for Existing Types
4.7.4. Parametrized constructors for declared types
4.7.5. Non Typesafe Classes
4.7.6. Accessing Declared Types from the Application Code
4.7.7. Type Declaration 'extends'
4.7.8. Traits

4.8. Rule

4.8.1. Rule Attributes
4.8.2. Timers and Calendars
4.8.3. Left Hand Side (when) syntax
4.8.4. The Right Hand Side (then)
4.8.5. A Note on Auto-boxing and Primitive Types

4.9. Query

4.10. Domain Specific Languages

4.10.1. When to Use a DSL
4.10.2. DSL Basics
4.10.3. Adding Constraints to Facts
4.10.4. Developing a DSL
4.10.5. DSL and DSLR Reference

4.11. XML Rule Language

4.11.1. Legacy Drools 2.x XML rule format
4.11.2. When to use XML
4.11.3. The XML format
4.11.4. Legacy Drools 2.x XML rule format
4.11.5. Automatic transforming between formats (XML and DRL)

4.1. Overview

Drools has a "native" rule language. This format is very light in terms of punctuation, and supports natural and domain specific languages via "expanders" that allow the language to morph to your problem domain. This chapter is mostly concerted with this native rule format. The diagrams used to present the syntax are known as "railroad" diagrams, and they are basically flow charts for the language terms. The technically very keen may also refer to DRL.g which is the Antlr3 grammar for the rule language. If you use the Rule Workbench, a lot of the rule structure is done for you with content assistance, for example, type "ru" and press ctrl+space, and it will build the rule structure for you.

4.1.1. A rule file

A rule file is typically a file with a .drl extension. In a DRL file you can have multiple rules, queries and functions, as well as some resource declarations like imports, globals and attributes that are assigned and used by your rules and queries. However, you are also able to spread your rules across multiple rule files (in that case, the extension .rule is suggested, but not required) - spreading rules across files can help with managing large numbers of rules. A DRL file is simply a text file.

The overall structure of a rule file is:

Example 4.1. Rules file

package package-name

imports

globals

functions

queries

rules

The order in which the elements are declared is not important, except for the package name that, if declared, must be the first element in the rules file. All elements are optional, so you will use only those you need. We will discuss each of them in the following sections.

4.1.2. What makes a rule

For the inpatients, just as an early view, a rule has the following rough structure:

rule "name"
    attributes
    when
        LHS
    then
        RHS
end

It's really that simple. Mostly punctuation is not needed, even the double quotes for "name" are optional, as are newlines. Attributes are simple (always optional) hints to how the rule should behave. LHS is the conditional parts of the rule, which follows a certain syntax which is covered below. RHS is basically a block that allows dialect specific semantic code to be executed.

It is important to note that white space is not important, except in the case of domain specific languages, where lines are processed one by one and spaces may be significant to the domain language.

4.2. Keywords

Drools 5 introduces the concept of hard and soft keywords.

Hard keywords are reserved, you cannot use any hard keyword when naming your domain objects, properties, methods, functions and other elements that are used in the rule text.

Here is the list of hard keywords that must be avoided as identifiers when writing rules:

true
false
null

Soft keywords are just recognized in their context, enabling you to use these words in any other place if you wish, although, it is still recommended to avoid them, to avoid confusions, if possible. Here is a list of the soft keywords:

Of course, you can have these (hard and soft) words as part of a method name in camel case, like notSomething() or accumulateSomething() - there are no issues with that scenario.

Although the 3 hard keywords above are unlikely to be used in your existing domain models, if you absolutely need to use them as identifiers instead of keywords, the DRL language provides the ability to escape hard keywords on rule text. To escape a word, simply enclose it in grave accents, like this:

Holiday( `true` == "yes" ) // please note that Drools will resolve that reference to the method Holiday.isTrue()

4.3. Comments

Comments are sections of text that are ignored by the rule engine. They are stripped out when they are encountered, except inside semantic code blocks, like the RHS of a rule.

4.3.1. Single line comment

To create single line comments, you can use '//'. The parser will ignore anything in the line after the comment symbol. Example:

rule "Testing Comments"
when
    // this is a single line comment
    eval( true ) // this is a comment in the same line of a pattern
then
    // this is a comment inside a semantic code block
end

Warning

Drools also supports for backward compatibility the use of '#' for single line comments, but this is deprecated and will be removed in future releases.

4.3.2. Multi-line comment

Figure 4.1. Multi-line comment

Multi-line comments are used to comment blocks of text, both in and outside semantic code blocks. Example:

rule "Test Multi-line Comments"
when
    /* this is a multi-line comment
       in the left hand side of a rule */
    eval( true )
then
    /* and this is a multi-line comment
       in the right hand side of a rule */
end

4.4. Error Messages

Drools 5 introduces standardized error messages. This standardization aims to help users to find and resolve problems in a easier and faster way. In this section you will learn how to identify and interpret those error messages, and you will also receive some tips on how to solve the problems associated with them.

4.4.1. Message format

The standardization includes the error message format and to better explain this format, let's use the following example:

Figure 4.2. Error Message Format

1st Block: This area identifies the error code.

2nd Block: Line and column information.

3rd Block: Some text describing the problem.

4th Block: This is the first context. Usually indicates the rule, function, template or query where the error occurred. This block is not mandatory.

5th Block: Identifies the pattern where the error occurred. This block is not mandatory.

4.4.2. Error Messages Description

4.4.2.1. 101: No viable alternative

Indicates the most common errors, where the parser came to a decision point but couldn't identify an alternative. Here are some examples:

Example 4.2.

1: rule one
2:   when
3:     exists Foo()
4:     exits Bar()
5:   then
6: end

The above example generates this message:

[ERR 101] Line 4:4 no viable alternative at input 'exits' in rule one

At first glance this seems to be valid syntax, but it is not (exits != exists). Let's take a look at next example:

Example 4.3.

1: package org.drools.examples;
2: rule
3:   when
4:     Object()
5:   then
6:     System.out.println("A RHS");
7: end

Now the above code generates this message:

[ERR 101] Line 3:2 no viable alternative at input 'WHEN'

This message means that the parser encountered the token WHEN, actually a hard keyword, but it's in the wrong place since the the rule name is missing.

The error "no viable alternative" also occurs when you make a simple lexical mistake. Here is a sample of a lexical problem:

Example 4.4.

1: rule simple_rule
2:   when
3:     Student( name == "Andy )
4:   then
5: end

The above code misses to close the quotes and because of this the parser generates this error message:

[ERR 101] Line 0:-1 no viable alternative at input '<eof>' in rule simple_rule in pattern Student

Note

Usually the Line and Column information are accurate, but in some cases (like unclosed quotes), the parser generates a 0:-1 position. In this case you should check whether you didn't forget to close quotes, apostrophes or parentheses.

4.4.2.2. 102: Mismatched input

This error indicates that the parser was looking for a particular symbol that it didn't ﬁnd at the current input position. Here are some samples:

Example 4.5.

1: rule simple_rule
2:   when
3:     foo3 : Bar(

The above example generates this message:

[ERR 102] Line 0:-1 mismatched input '<eof>' expecting ')' in rule simple_rule in pattern Bar

To fix this problem, it is necessary to complete the rule statement.

Note

Usually when you get a 0:-1 position, it means that parser reached the end of source.

The following code generates more than one error message:

Example 4.6.

1: package org.drools.examples;
2:
3: rule "Avoid NPE on wrong syntax"
4:   when
5:     not( Cheese( ( type == "stilton", price == 10 ) || ( type == "brie", price == 15 ) ) from $cheeseList )
6:   then
7:     System.out.println("OK");
8: end

These are the errors associated with this source:

[ERR 102] Line 5:36 mismatched input ',' expecting ')' in rule "Avoid NPE on wrong syntax" in pattern Cheese
[ERR 101] Line 5:57 no viable alternative at input 'type' in rule "Avoid NPE on wrong syntax"
[ERR 102] Line 5:106 mismatched input ')' expecting 'then' in rule "Avoid NPE on wrong syntax"

Note that the second problem is related to the first. To fix it, just replace the commas (',') by AND operator ('&&').

Note

In some situations you can get more than one error message. Try to fix one by one, starting at the first one. Some error messages are generated merely as consequences of other errors.

4.4.2.3. 103: Failed predicate

A validating semantic predicate evaluated to false. Usually these semantic predicates are used to identify soft keywords. This sample shows exactly this situation:

Example 4.7.

 1: package nesting;
 2: dialect "mvel"
 3:
 4: import org.drools.compiler.Person
 5: import org.drools.compiler.Address
 6: 
 7: fdsfdsfds
 8: 
 9: rule "test something"
10:   when
11:     p: Person( name=="Michael" )
12:   then
13:     p.name = "other";
14:     System.out.println(p.name);
15: end

With this sample, we get this error message:

[ERR 103] Line 7:0 rule 'rule_key' failed predicate: {(validateIdentifierKey(DroolsSoftKeywords.RULE))}? in rule

The fdsfdsfds text is invalid and the parser couldn't identify it as the soft keyword rule.

Note

This error is very similar to 102: Mismatched input, but usually involves soft keywords.

4.4.2.4. 104: Trailing semi-colon not allowed

This error is associated with the eval clause, where its expression may not be terminated with a semicolon. Check this example:

Example 4.8.

1: rule simple_rule
2:   when
3:     eval(abc();)
4:   then
5: end

Due to the trailing semicolon within eval, we get this error message:

[ERR 104] Line 3:4 trailing semi-colon not allowed in rule simple_rule

This problem is simple to fix: just remove the semi-colon.

4.4.2.5. 105: Early Exit

The recognizer came to a subrule in the grammar that must match an alternative at least once, but the subrule did not match anything. Simply put: the parser has entered a branch from where there is no way out. This example illustrates it:

Example 4.9.

1: template test_error
2:   aa s  11;
3: end

This is the message associated to the above sample:

[ERR 105] Line 2:2 required (...)+ loop did not match anything at input 'aa' in template test_error

To fix this problem it is necessary to remove the numeric value as it is neither a valid data type which might begin a new template slot nor a possible start for any other rule file construct.

4.4.3. Other Messages

Any other message means that something bad has happened, so please contact the development team.

4.5. Package

A package is a collection of rules and other related constructs, such as imports and globals. The package members are typically related to each other - perhaps HR rules, for instance. A package represents a namespace, which ideally is kept unique for a given grouping of rules. The package name itself is the namespace, and is not related to files or folders in any way.

It is possible to assemble rules from multiple rule sources, and have one top level package configuration that all the rules are kept under (when the rules are assembled). Although, it is not possible to merge into the same package resources declared under different names. A single Rulebase may, however, contain multiple packages built on it. A common structure is to have all the rules for a package in the same file as the package declaration (so that is it entirely self-contained).

The following railroad diagram shows all the components that may make up a package. Note that a package must have a namespace and be declared using standard Java conventions for package names; i.e., no spaces, unlike rule names which allow spaces. In terms of the order of elements, they can appear in any order in the rule file, with the exception of the package statement, which must be at the top of the file. In all cases, the semicolons are optional.

Figure 4.3. package

Notice that any rule attribute (as described the section Rule Attributes) may also be written at package level, superseding the attribute's default value. The modified default may still be replaced by an attribute setting within a rule.

4.5.1. import

Figure 4.4. import

Import statements work like import statements in Java. You need to specify the fully qualified paths and type names for any objects you want to use in the rules. Drools automatically imports classes from the Java package of the same name, and also from the package java.lang.

4.5.2. global

Figure 4.5. global

With global you define global variables. They are used to make application objects available to the rules. Typically, they are used to provide data or services that the rules use, especially application services used in rule consequences, and to return data from the rules, like logs or values added in rule consequences, or for the rules to interact with the application, doing callbacks. Globals are not inserted into the Working Memory, and therefore a global should never be used to establish conditions in rules except when it has a constant immutable value. The engine cannot be notified about value changes of globals and does not track their changes. Incorrect use of globals in constraints may yield surprising results - surprising in a bad way.

If multiple packages declare globals with the same identifier they must be of the same type and all of them will reference the same global value.

In order to use globals you must:

Declare your global variable in your rules file and use it in rules. Example:

global java.util.List myGlobalList;

rule "Using a global"
when
    eval( true )
then
    myGlobalList.add( "Hello World" );
end

Set the global value on your working memory. It is a best practice to set all global values before asserting any fact to the working memory. Example:
```
List list = new ArrayList();

WorkingMemory wm = rulebase.newStatefulSession();

wm.setGlobal( "myGlobalList", list );
```

Note that these are just named instances of objects that you pass in from your application to the working memory. This means you can pass in any object you want: you could pass in a service locator, or perhaps a service itself. With the new from element it is now common to pass a Hibernate session as a global, to allow from to pull data from a named Hibernate query.

One example may be an instance of a Email service. In your integration code that is calling the rule engine, you obtain your emailService object, and then set it in the working memory. In the DRL, you declare that you have a global of type EmailService, and give it the name "email". Then in your rule consequences, you can use things like email.sendSMS(number, message).

Globals are not designed to share data between rules and they should never be used for that purpose. Rules always reason and react to the working memory state, so if you want to pass data from rule to rule, assert the data as facts into the working memory.

It is strongly discouraged to set or change a global value from inside your rules. We recommend to you always set the value from your application using the working memory interface.

4.6. Function

Figure 4.6. function

Functions are a way to put semantic code in your rule source file, as opposed to in normal Java classes. They can't do anything more than what you can do with helper classes. (In fact, the compiler generates the helper class for you behind the scenes.) The main advantage of using functions in a rule is that you can keep the logic all in one place, and you can change the functions as needed (which can be a good or a bad thing). Functions are most useful for invoking actions on the consequence (then) part of a rule, especially if that particular action is used over and over again, perhaps with only differing parameters for each rule.

A typical function declaration looks like:

function String hello(String name) {
    return "Hello "+name+"!";
}

Note that the function keyword is used, even though its not really part of Java. Parameters to the function are defined as for a method, and you don't have to have parameters if they are not needed. The return type is defined just like in a regular method.

Alternatively, you could use a static method in a helper class, e.g., Foo.hello(). Drools supports the use of function imports, so all you would need to do is:

import function my.package.Foo.hello

Irrespective of the way the function is defined or imported, you use a function by calling it by its name, in the consequence or inside a semantic code block. Example:

rule "using a static function"
when 
    eval( true )
then
    System.out.println( hello( "Bob" ) );
end

4.7. Type Declaration

Figure 4.7. meta_data

Figure 4.8. type_declaration

Type declarations have two main goals in the rules engine: to allow the declaration of new types, and to allow the declaration of metadata for types.

Declaring new types: Drools works out of the box with plain Java objects as facts. Sometimes, however, users may want to define the model directly to the rules engine, without worrying about creating models in a lower level language like Java. At other times, there is a domain model already built, but eventually the user wants or needs to complement this model with additional entities that are used mainly during the reasoning process.
Declaring metadata: facts may have meta information associated to them. Examples of meta information include any kind of data that is not represented by the fact attributes and is consistent among all instances of that fact type. This meta information may be queried at runtime by the engine and used in the reasoning process.

4.7.1. Declaring New Types

To declare a new type, all you need to do is use the keyword declare, followed by the list of fields, and the keyword end. A new fact must have a list of fields, otherwise the engine will look for an existing fact class in the classpath and raise an error if not found.

Example 4.10. Declaring a new fact type: Address

declare Address
   number : int
   streetName : String
   city : String
end

The previous example declares a new fact type called Address. This fact type will have three attributes: number, streetName and city. Each attribute has a type that can be any valid Java type, including any other class created by the user or even other fact types previously declared.

For instance, we may want to declare another fact type Person:

Example 4.11. declaring a new fact type: Person

declare Person
    name : String
    dateOfBirth : java.util.Date
    address : Address
end

As we can see on the previous example, dateOfBirth is of type java.util.Date, from the Java API, while address is of the previously defined fact type Address.

You may avoid having to write the fully qualified name of a class every time you write it by using the import clause, as previously discussed.

Example 4.12. Avoiding the need to use fully qualified class names by using import

import java.util.Date

declare Person
    name : String
    dateOfBirth : Date
    address : Address
end

When you declare a new fact type, Drools will, at compile time, generate bytecode that implements a Java class representing the fact type. The generated Java class will be a one-to-one Java Bean mapping of the type definition. So, for the previous example, the generated Java class would be:

Example 4.13. generated Java class for the previous Person fact type declaration

public class Person implements Serializable {

    private String name;

    private java.util.Date dateOfBirth;

    private Address address;


    // empty constructor

    public Person() {...}


    // constructor with all fields 

    public Person( String name, Date dateOfBirth, Address address ) {...}


    // if keys are defined, constructor with keys

    public Person( ...keys... ) {...}


    // getters and setters

    // equals/hashCode

    // toString

}

Since the generated class is a simple Java class, it can be used transparently in the rules, like any other fact.

Example 4.14. Using the declared types in rules

rule "Using a declared Type"
when 
    $p : Person( name == "Bob" )
then
    // Insert Mark, who is Bob's mate.
    Person mark = new Person();
    mark.setName("Mark");
    insert( mark );
end

4.7.2. Declaring Metadata

Metadata may be assigned to several different constructions in Drools: fact types, fact attributes and rules. Drools uses the at sign ('@') to introduce metadata, and it always uses the form:

@metadata_key( metadata_value )

The parenthesized metadata_value is optional.

For instance, if you want to declare a metadata attribute like author, whose value is Bob, you could simply write:

Example 4.15. Declaring a metadata attribute

@author( Bob )

Drools allows the declaration of any arbitrary metadata attribute, but some will have special meaning to the engine, while others are simply available for querying at runtime. Drools allows the declaration of metadata both for fact types and for fact attributes. Any metadata that is declared before the attributes of a fact type are assigned to the fact type, while metadata declared after an attribute are assigned to that particular attribute.

Example 4.16. Declaring metadata attributes for fact types and attributes

import java.util.Date

declare Person
    @author( Bob )
    @dateOfCreation( 01-Feb-2009 )

    name : String @key @maxLength( 30 )
    dateOfBirth : Date 
    address : Address
end

In the previous example, there are two metadata items declared for the fact type (@author and @dateOfCreation) and two more defined for the name attribute (@key and @maxLength). Please note that the @key metadata has no required value, and so the parentheses and the value were omitted.:

@position can be used to declare the position of a field, overriding the default declared order. This is used for positional constraints in patterns.

declare Cheese
    name : String @position(1)
    shop : String @position(2)
    price : int @position(0)
end

4.7.2.1. Predefined class level annotations

Some annotations have predefined semantics that are interpreted by the engine. The following is a list of some of these predefined annotations and their meaning.

4.7.2.1.1. @role( <fact | event> )

See Drools Fusion documentation for details.

4.7.2.1.2. @typesafe( <boolean> )

By default all type declarations are compiled with type safety enabled; @typesafe( false ) provides a means to override this behaviour by permitting a fall-back, to type unsafe evaluation where all constraints are generated as MVEL constraints and executed dynamically. This can be important when dealing with collections that do not have any generics or mixed type collections.

4.7.2.1.3. @timestamp( <attribute name> )

See Drools Fusion documentation for details.

4.7.2.1.4. @duration( <attribute name> )

See Drools Fusion documentation for details.

4.7.2.1.5. @expires( <time interval> )

See Drools Fusion documentation for details.

4.7.2.1.6. @propertyChangeSupport

Facts that implement support for property changes as defined in the Javabean(tm) spec, now can be annotated so that the engine register itself to listen for changes on fact properties. The boolean parameter that was used in the insert() method in the Drools 4 API is deprecated and does not exist in the drools-api module.

Example 4.17. @propertyChangeSupport

declare Person
  @propertyChangeSupport
end

4.7.2.1.7. @propertyReactive

Make this type property reactive. See Fine grained property change listeners section for details.

4.7.2.2. Predefined attribute level annotations

As noted before, Drools also supports annotations in type attributes. Here is a list of predefined attribute annotations.

4.7.2.2.1. @key

Declaring an attribute as a key attribute has 2 major effects on generated types:

The attribute will be used as a key identifier for the type, and as so, the generated class will implement the equals() and hashCode() methods taking the attribute into account when comparing instances of this type.
Drools will generate a constructor using all the key attributes as parameters.

For instance:

Example 4.18. example of @key declarations for a type

declare Person
    firstName : String @key
    lastName : String @key
    age : int
end

For the previous example, Drools will generate equals() and hashCode() methods that will check the firstName and lastName attributes to determine if two instances of Person are equal to each other, but will not check the age attribute. It will also generate a constructor taking firstName and lastName as parameters, allowing one to create instances with a code like this:

Example 4.19. creating an instance using the key constructor

Person person = new Person( "John", "Doe" );

4.7.2.2.2. @position

Patterns support positional arguments on type declarations.

Positional arguments are ones where you don't need to specify the field name, as the position maps to a known named field. i.e. Person( name == "mark" ) can be rewritten as Person( "mark"; ). The semicolon ';' is important so that the engine knows that everything before it is a positional argument. Otherwise we might assume it was a boolean expression, which is how it could be interpreted after the semicolon. You can mix positional and named arguments on a pattern by using the semicolon ';' to separate them. Any variables used in a positional that have not yet been bound will be bound to the field that maps to that position.

declare Cheese
    name : String
    shop : String
    price : int
end

The default order is the declared order, but this can be overridden using @position

declare Cheese
    name : String @position(1)
    shop : String @position(2)
    price : int @position(0)
end

The @Position annotation, in the org.drools.definition.type package, can be used to annotate original pojos on the classpath. Currently only fields on classes can be annotated. Inheritance of classes is supported, but not interfaces of methods yet.

Example patterns, with two constraints and a binding. Remember semicolon ';' is used to differentiate the positional section from the named argument section. Variables and literals and expressions using just literals are supported in positional arguments, but not variables.

Cheese( "stilton", "Cheese Shop", p; )
Cheese( "stilton", "Cheese Shop"; p : price )
Cheese( "stilton"; shop == "Cheese Shop", p : price )
Cheese( name == "stilton"; shop == "Cheese Shop", p : price )

4.7.3. Declaring Metadata for Existing Types

Drools allows the declaration of metadata attributes for existing types in the same way as when declaring metadata attributes for new fact types. The only difference is that there are no fields in that declaration.

For instance, if there is a class org.drools.examples.Person, and one wants to declare metadata for it, it's possible to write the following code:

Example 4.20. Declaring metadata for an existing type

import org.drools.examples.Person

declare Person
    @author( Bob )
    @dateOfCreation( 01-Feb-2009 )
end

Instead of using the import, it is also possible to reference the class by its fully qualified name, but since the class will also be referenced in the rules, it is usually shorter to add the import and use the short class name everywhere.

Example 4.21. Declaring metadata using the fully qualified class name

declare org.drools.examples.Person
    @author( Bob )
    @dateOfCreation( 01-Feb-2009 )
end

4.7.4. Parametrized constructors for declared types

Generate constructors with parameters for declared types.

Example: for a declared type like the following:

declare Person
    firstName : String @key
    lastName : String @key
    age : int
end

The compiler will implicitly generate 3 constructors: one without parameters, one with the @key fields, and one with all fields.

Person() // parameterless constructor
Person( String firstName, String lastName )
Person( String firstName, String lastName, int age )

4.7.5. Non Typesafe Classes

@typesafe( <boolean>) has been added to type declarations. By default all type declarations are compiled with type safety enabled; @typesafe( false ) provides a means to override this behaviour by permitting a fall-back, to type unsafe evaluation where all constraints are generated as MVEL constraints and executed dynamically. This can be important when dealing with collections that do not have any generics or mixed type collections.

4.7.6. Accessing Declared Types from the Application Code

Declared types are usually used inside rules files, while Java models are used when sharing the model between rules and applications. Although, sometimes, the application may need to access and handle facts from the declared types, especially when the application is wrapping the rules engine and providing higher level, domain specific user interfaces for rules management.

In such cases, the generated classes can be handled as usual with the Java Reflection API, but, as we know, that usually requires a lot of work for small results. Therefore, Drools provides a simplified API for the most common fact handling the application may want to do.

The first important thing to realize is that a declared fact will belong to the package where it was declared. So, for instance, in the example below, Person will belong to the org.drools.examples package, and so the fully qualified name of the generated class will be org.drools.examples.Person.

Example 4.22. Declaring a type in the org.drools.examples package

package org.drools.examples

import java.util.Date

declare Person
    name : String
    dateOfBirth : Date
    address : Address
end

Declared types, as discussed previously, are generated at knowledge base compilation time, i.e., the application will only have access to them at application run time. Therefore, these classes are not available for direct reference from the application.

Drools then provides an interface through which users can handle declared types from the application code: org.drools.definition.type.FactType. Through this interface, the user can instantiate, read and write fields in the declared fact types.

Example 4.23. Handling declared fact types through the API

// get a reference to a knowledge base with a declared type:
KnowledgeBase kbase = ...

// get the declared FactType
FactType personType = kbase.getFactType( "org.drools.examples",
                                         "Person" );

// handle the type as necessary:
// create instances:
Object bob = personType.newInstance();

// set attributes values
personType.set( bob,
                "name",
                "Bob" );
personType.set( bob,
                "age",
                42 );

// insert fact into a session
StatefulKnowledgeSession ksession = ...
ksession.insert( bob );
ksession.fireAllRules();

// read attributes
String name = personType.get( bob, "name" );
int age = personType.get( bob, "age" );

The API also includes other helpful methods, like setting all the attributes at once, reading values from a Map, or reading all attributes at once, into a Map.

Although the API is similar to Java reflection (yet much simpler to use), it does not use reflection underneath, relying on much more performant accessors implemented with generated bytecode.

4.7.7. Type Declaration 'extends'

Type declarations now support 'extends' keyword for inheritance

In order to extend a type declared in Java by a DRL declared subtype, repeat the supertype in a declare statement without any fields.

import org.people.Person

declare Person
end

declare Student extends Person
    school : String
end

declare LongTermStudent extends Student
    years : int
    course : String
end

4.7.8. Traits

The same fact may have multiple dynamic types which do not fit naturally in a class hierarchy. Traits allow to model this very common scenario. A trait is an interface that can be applied (and eventually removed) to an individual object at runtime. To create a trait out of an interface, one has to add a @format(trait) annotation to its declaration in DRL as in the following example:

declare GoldenCustomer
    @format(trait)
    // fields will map to getters/setters
    code     : String
    balance  : long
    discount : int
    maxExpense : long
end

In order to apply a trait to an object, we provide the new don keyword, which can be used as simply as this:

when
    $c : Customer()
then
    GoldenCustomer gc = don( $c, Customer.class );
end

when a core object dons a trait, a proxy class is created on the fly (one such class will be generated lazily for each core/trait class combination). The proxy instance, which wraps the core object and implements the trait interface, is inserted automatically and will possibly activate other rules. An immediate advantage of declaring and using interfaces, getting the implementation proxy for free from the engine, is that multiple inheritance hierarchies can be exploited when writing rules. The core classes, however, need not implement any of those interfaces statically, also facilitating the use of legacy classes as cores. In fact, any object can don a trait. For efficiency reasons, however, one can add the @Traitable annotation to a declared bean class to reduce the amount of glue code that the compiler will have to generate. This is optional and will not change the behavior of the engine:

declare Customer
    @Traitable
    code    : String
    balance : long
end

The only connection between core classes and trait interfaces is at the proxy level, a trait is not specifically tied to a core class. This means that the same trait can be applied to totally different objects. For this reason, the trait does not transparently expose the fields of its core object. So, when writing a rule using a trait interface, only the fields of the interface will be available, as usual. However, any field in the interface that corresponds to a core object field, will be mapped by the proxy class:

when
    $o: OrderItem( $p : price, $code : custCode )
    $c: GoldenCustomer( code == $code, $a : balance, $d: discount )
then
    $c.setBalance( $a - $p*$d );
end

In this case, the code and balance would be read from the underlying Customer object. Likewise, the setAccount will modify the underlying object, preserving a strongly typed access to the data structures. Now, one might wonder what happens when a core class does NOT provide the implementation for a field defined in an interface. We call hard fields those trait fields which are also core fields and thus readily available, while we define soft those fields which are NOT provided by the core class. Hidden fields, instead, are fields in the core class not exposed by the interface.

So, while hard field management is intuitive, there remains the problem of soft and hidden fields. The solution we have adopted is to use a two-part proxy. Internally, proxies are formed by a proper proxy and a wrapper. The former implements the interface, while the latter manages the core object fields, implementing a name/value map to supports soft fields. The proxy, then, uses both the core object and the map wrapper to implement the interface, as needed. So, you can write:

when
    $sc : GoldenCustomer( $c : code, // hard getter
                          $maxExpense : maxExpense > 1000 // soft getter
    )
then
    $sc.setDiscount( ... ); // soft setter
end

The wrapper, then, provides a looser form of typing when writing rules. However, it has also other uses. The wrapper is specific to the object it wraps, regardless of how many traits have been attached to an object: all the proxies on the same object will share the same wrapper. Secondly, the wrapper also contains a back-reference to all proxies attached to the wrapped object, effectively allowing traits to see each other. To this end, we have provided the new isA operator:

$sc : GoldenCustomer( $maxExpense : maxExpense > 1000,
                      this isA "SeniorCustomer"
)

Eventually, the business logic may require that a trait is removed from a wrapped object. To this end, we provide two options. The first is a "logical don", which will result in a logical insertion of the proxy resulting from the traiting operation:

then
    don( $x, // core object
         Customer.class, // trait class
         true // optional flag for logical insertion
    )

The second is the use of the shed keyword, which causes the retraction of the proxy corresponding to the given argument type:

then
    Thing t = shed( $x, GoldenCustomer.class )

This operation returns another proxy implementing the org.drools.core.factmodel.traits.Thing interface, where the getFields() and getCore() methods are defined. Internally, in fact, all declared traits are generated to extend this interface (in addition to any others specified). This allows to preserve the wrapper with the soft fields which would otherwise be lost.

4.8. Rule

Figure 4.9. rule

A rule specifies that when a particular set of conditions occur, specified in the Left Hand Side (LHS), then do what queryis specified as a list of actions in the Right Hand Side (RHS). A common question from users is "Why use when instead of if?" "When" was chosen over "if" because "if" is normally part of a procedural execution flow, where, at a specific point in time, a condition is to be checked. In contrast, "when" indicates that the condition evaluation is not tied to a specific evaluation sequence or point in time, but that it happens continually, at any time during the life time of the engine; whenever the condition is met, the actions are executed.

A rule must have a name, unique within its rule package. If you define a rule twice in the same DRL it produces an error while loading. If you add a DRL that includes a rule name already in the package, it replaces the previous rule. If a rule name is to have spaces, then it will need to be enclosed in double quotes (it is best to always use double quotes).

Attributes - described below - are optional. They are best written one per line.

The LHS of the rule follows the when keyword (ideally on a new line), similarly the RHS follows the then keyword (again, ideally on a newline). The rule is terminated by the keyword end. Rules cannot be nested.

Example 4.24. Rule Syntax Overview

rule "<name>"
    <attribute>*
when
    <conditional element>*
then
    <action>*
end

Example 4.25. A simple rule

rule "Approve if not rejected"
  salience -100 
  agenda-group "approval"
    when
        not Rejection() 
        p : Policy(approved == false, policyState:status)
        exists Driver(age > 25)
        Process(status == policyState)
    then
        log("APPROVED: due to no objections."); 
        p.setApproved(true);
end

4.8.1. Rule Attributes

Rule attributes provide a declarative way to influence the behavior of the rule. Some are quite simple, while others are part of complex subsystems such as ruleflow. To get the most from Drools you should make sure you have a proper understanding of each attribute.

Figure 4.10. rule attributes

no-loop

default value: false

type: Boolean

When a rule's consequence modifies a fact it may cause the rule to activate again, causing an infinite loop. Setting no-loop to true will skip the creation of another Activation for the rule with the current set of facts.

ruleflow-group

default value: N/A

type: String

Ruleflow is a Drools feature that lets you exercise control over the firing of rules. Rules that are assembled by the same ruleflow-group identifier fire only when their group is active.

lock-on-active

default value: false

type: Boolean

Whenever a ruleflow-group becomes active or an agenda-group receives the focus, any rule within that group that has lock-on-active set to true will not be activated any more; irrespective of the origin of the update, the activation of a matching rule is discarded. This is a stronger version of no-loop, because the change could now be caused not only by the rule itself. It's ideal for calculation rules where you have a number of rules that modify a fact and you don't want any rule re-matching and firing again. Only when the ruleflow-group is no longer active or the agenda-group loses the focus those rules with lock-on-active set to true become eligible again for their activations to be placed onto the agenda.

salience

default value: 0

type: integer

Each rule has an integer salience attribute which defaults to zero and can be negative or positive. Salience is a form of priority where rules with higher salience values are given higher priority when ordered in the Activation queue.

Drools also supports dynamic salience where you can use an expression involving bound variables.

Example 4.26. Dynamic Salience

rule "Fire in rank order 1,2,.."
        salience( -$rank )
    when
        Element( $rank : rank,... )
    then
        ...
end

agenda-group

default value: MAIN

type: String

Agenda groups allow the user to partition the Agenda providing more execution control. Only rules in the agenda group that has acquired the focus are allowed to fire.

auto-focus

default value: false

type: Boolean

When a rule is activated where the auto-focus value is true and the rule's agenda group does not have focus yet, then it is given focus, allowing the rule to potentially fire.

activation-group

default value: N/A

type: String

Rules that belong to the same activation-group, identified by this attribute's string value, will only fire exclusively. More precisely, the first rule in an activation-group to fire will cancel all pending activations of all rules in the group, i.e., stop them from firing.

Note: This used to be called Xor group, but technically it's not quite an Xor. You may still hear people mention Xor group; just swap that term in your mind with activation-group.

dialect

default value: as specified by the package

type: String

possible values: "java" or "mvel"

The dialect species the language to be used for any code expressions in the LHS or the RHS code block. Currently two dialects are available, Java and MVEL. While the dialect can be specified at the package level, this attribute allows the package definition to be overridden for a rule.

date-effective

default value: N/A

type: String, containing a date and time definition

A rule can only activate if the current date and time is after date-effective attribute.

date-expires

default value: N/A

type: String, containing a date and time definition

A rule cannot activate if the current date and time is after the date-expires attribute.

duration

default value: no default value

type: long

The duration dictates that the rule will fire after a specified duration, if it is still true.

Example 4.27. Some attribute examples

rule "my rule"
  salience 42
  agenda-group "number 1"
    when ...

4.8.2. Timers and Calendars

Rules now support both interval and cron based timers, which replace the now deprecated duration attribute.

Example 4.28. Sample timer attribute uses

timer ( int: <initial delay> <repeat interval>? )

timer ( int: 30s )

timer ( int: 30s 5m )


timer ( cron: <cron expression> )

timer ( cron:* 0/15 * * * ? )

Interval (indicated by "int:") timers follow the semantics of java.util.Timer objects, with an initial delay and an optional repeat interval. Cron (indicated by "cron:") timers follow standard Unix cron expressions:

Example 4.29. A Cron Example

rule "Send SMS every 15 minutes"

    timer (cron:* 0/15 * * * ?)

when

    $a : Alarm( on == true )

then

    channels[ "sms" ].insert( new Sms( $a.mobileNumber, "The alarm is still on" );

end

Calendars are used to control when rules can fire. The Calendar API is modelled on Quartz:

Example 4.30. Adapting a Quartz Calendar

Calendar weekDayCal = QuartzHelper.quartzCalendarAdapter(org.quartz.Calendar quartzCal)

Calendars are registered with the StatefulKnowledgeSession:

Example 4.31. Registering a Calendar

ksession.getCalendars().set( "weekday", weekDayCal );

They can be used in conjunction with normal rules and rules including timers. The rule attribute "calendars" may contain one or more comma-separated calendar names written as string literals.

Example 4.32. Using Calendars and Timers together

rule "weekdays are high priority"

   calendars "weekday"

   timer (int:0 1h)

when 

    Alarm()

then

    send( "priority high - we have an alarm );

end 


rule "weekend are low priority"

   calendars "weekend"

   timer (int:0 4h)

when 

    Alarm()

then

    send( "priority low - we have an alarm );

end

4.8.3. Left Hand Side (when) syntax

4.8.3.1. What is the Left Hand Side?

The Left Hand Side (LHS) is a common name for the conditional part of the rule. It consists of zero or more Conditional Elements. If the LHS is empty, it will be considered as a condition element that is always true and it will be activated once, when a new WorkingMemory session is created.

Figure 4.11. Left Hand Side

Example 4.33. Rule without a Conditional Element

rule "no CEs"
when
    // empty
then
    ... // actions (executed once)
end

// The above rule is internally rewritten as:

rule "eval(true)"
when
    eval( true )
then
    ... // actions (executed once)
end

Conditional elements work on one or more patterns (which are described below). The most common conditional element is "and". Therefore it is implicit when you have multiple patterns in the LHS of a rule that are not connected in any way:

Example 4.34. Implicit and

rule "2 unconnected patterns"
when
    Pattern1()
    Pattern2()
then
    ... // actions
end

// The above rule is internally rewritten as:

rule "2 and connected patterns"
when
    Pattern1()
    and Pattern2()
then
    ... // actions
end

Note

An "and" cannot have a leading declaration binding (unlike for example or). This is obvious, since a declaration can only reference a single fact at a time, and when the "and" is satisfied it matches both facts - so which fact would the declaration bind to?

// Compile error
$person : (Person( name == "Romeo" ) and Person( name == "Juliet"))

4.8.3.2. Pattern (conditional element)

4.8.3.2.1. What is a pattern?

A pattern element is the most important Conditional Element. It can potentially match on each fact that is inserted in the working memory.

A pattern contains of zero or more constraints and has an optional pattern binding. The railroad diagram below shows the syntax for this.

Figure 4.12. Pattern

In its simplest form, with no constraints, a pattern matches against a fact of the given type. In the following case the type is Cheese, which means that the pattern will match against all Person objects in the Working Memory:

Person()

The type need not be the actual class of some fact object. Patterns may refer to superclasses or even interfaces, thereby potentially matching facts from many different classes.

Object() // matches all objects in the working memory

Inside of the pattern parenthesis is where all the action happens: it defines the constraints for that pattern. For example, with a age related constraint:

Person( age == 100 )

Note

For backwards compatibility reasons it's allowed to suffix patterns with the ; character. But it is not recommended to do that.

4.8.3.2.2. Pattern binding

For referring to the matched object, use a pattern binding variable such as $p.

Example 4.35. Pattern with a binding variable

rule ...
when
    $p : Person()
then
    System.out.println( "Person " + $p );
end

The prefixed dollar symbol ($) is just a convention; it can be useful in complex rules where it helps to easily differentiate between variables and fields, but it is not mandatory.

4.8.3.3. Constraint (part of a pattern)

4.8.3.3.1. What is a constraint?

A constraint is an expression that returns true or false. This example has a constraint that states 5 is smaller than 6:

Person( 5 < 6 )  // just an example, as constraints like this would be useless in a real pattern

In essence, it's a Java expression with some enhancements (such as property access) and a few differences (such as equals() semantics for ==). Let's take a deeper look.

4.8.3.3.2. Property access on Java Beans (POJO's)

Any bean property can be used directly. A bean property is exposed using a standard Java bean getter: a method getMyProperty() (or isMyProperty() for a primitive boolean) which takes no arguments and return something. For example: the age property is written as age in DRL instead of the getter getAge():

Person( age == 50 )

// this is the same as:
Person( getAge() == 50 )

Drools uses the standard JDK Introspector class to do this mapping, so it follows the standard Java bean specification.

Note

We recommend using property access (age) over using getters explicitly (getAge()) because of performance enhancements through field indexing.

Warning

Property accessors must not change the state of the object in a way that may effect the rules. Remember that the rule engine effectively caches the results of its matching in between invocations to make it faster.

public int getAge() {
    age++; // Do NOT do this
    return age;
}

public int getAge() {
    Date now = DateUtil.now(); // Do NOT do this
    return DateUtil.differenceInYears(now, birthday);
}

To solve this latter case, insert a fact that wraps the current date into working memory and update that fact between fireAllRules as needed.

Note

The following fallback applies: if the getter of a property cannot be found, the compiler will resort to using the property name as a method name and without arguments:

Person( age == 50 )

// If Person.getAge() does not exists, this falls back to:
Person( age() == 50 )

Nested property access is also supported:

Person( address.houseNumber == 50 )

// this is the same as:
Person( getAddress().getHouseNumber() == 50 )

Nested properties are also indexed.

Warning

In a stateful session, care should be taken when using nested accessors as the Working Memory is not aware of any of the nested values, and does not know when they change. Either consider them immutable while any of their parent references are inserted into the Working Memory. Or, instead, if you wish to modify a nested value you should mark all of the outer facts as updated. In the above example, when the houseNumber changes, any Person with that Address must be marked as updated.

4.8.3.3.3. Java expression

You can use any Java expression that returns a boolean as a constraint inside the parentheses of a pattern. Java expressions can be mixed with other expression enhancements, such as property access:

Person( age == 50 )

It is possible to change the evaluation priority by using parentheses, as in any logic or mathematical expression:

Person( age > 100 && ( age % 10 == 0 ) )

It is possible to reuse Java methods:

Person( Math.round( weight / ( height * height ) ) < 25.0 )

Warning

As for property accessors, methods must not change the state of the object in a way that may affect the rules. Any method executed on a fact in the LHS should be a read only method.

Person( incrementAndGetAge() == 10 ) // Do NOT do this

Warning

The state of a fact should not change between rule invocations (unless those facts are marked as updated to the working memory on every change):

Person( System.currentTimeMillis() % 1000 == 0 ) // Do NOT do this

Normal Java operator precedence applies, see the operator precedence list below.

Important

All operators have normal Java semantics except for == and !=.

The == operator has null-safe equals() semantics:

// Similar to: java.util.Objects.equals(person.getFirstName(), "John")
// so (because "John" is not null) similar to:
// "John".equals(person.getFirstName())
Person( firstName == "John" )

The != operator has null-safe !equals() semantics:

// Similar to: !java.util.Objects.equals(person.getFirstName(), "John")
Person( firstName != "John" )

Type coercion is always attempted if the field and the value are of different types; exceptions will be thrown if a bad coercion is attempted. For instance, if "ten" is provided as a string in a numeric evaluator, an exception is thrown, whereas "10" would coerce to a numeric 10. Coercion is always in favor of the field type and not the value type:

Person( age == "10" ) // "10" is coerced to 10

4.8.3.3.4. Comma separated AND

The comma character (',') is used to separate constraint groups. It has implicit AND connective semantics.

// Person is at least 50 and weighs at least 80 kg
Person( age > 50, weight > 80 )

// Person is at least 50, weighs at least 80 kg and is taller than 2 meter.
Person( age > 50, weight > 80, height > 2 )

Note

Although the && and , operators have the same semantics, they are resolved with different priorities: The && operator precedes the || operator. Both the && and || operator precede the , operator. See the operator precedence list below.

The comma operator should be preferred at the top level constraint, as it makes constraints easier to read and the engine will often be able to optimize them better.

The comma (,) operator cannot be embedded in a composite constraint expression, such as parentheses:

Person( ( age > 50, weight > 80 ) || height > 2 ) // Do NOT do this: compile error

// Use this instead
Person( ( age > 50 && weight > 80 ) || height > 2 )

4.8.3.3.5. Binding variables

A property can be bound to a variable:

// 2 persons of the same age
Person( $firstAge : age ) // binding
Person( age == $firstAge ) // constraint expression

The prefixed dollar symbol ($) is just a convention; it can be useful in complex rules where it helps to easily differentiate between variables and fields.

Note

For backwards compatibility reasons, It's allowed (but not recommended) to mix a constraint binding and constraint expressions as such:

// Not recommended
Person( $age : age * 2 < 100 )

// Recommended (separates bindings and constraint expressions)
Person( age * 2 < 100, $age : age )

Bound variable restrictions using the operator == provide for very fast execution as it use hash indexing to improve performance.

4.8.3.3.6. Unification

Drools does not allow bindings to the same declaration. However this is an important aspect to derivation query unification. While positional arguments are always processed with unification a special unification symbol, ':=', was introduced for named arguments named arguments. The following "unifies" the age argument across two people.

Person( $age := age ) 
Person( $age := age)

In essence unification will declare a binding for the first occurrence and constrain to the same value of the bound field for sequence occurrences.

4.8.3.3.7. Special literal support

Besides normal Java literals (including Java 5 enums), this literal is also supported:

4.8.3.3.7.1. Date literal

The date format dd-mmm-yyyy is supported by default. You can customize this by providing an alternative date format mask as the System property named drools.dateformat. If more control is required, use a restriction.

Example 4.36. Date Literal Restriction

Cheese( bestBefore < "27-Oct-2009" )

4.8.3.3.8. List and Map access

It's possible to directly access a List value by index:

// Same as childList(0).getAge() == 18
Person( childList[0].age == 18 )

It's also possible to directly access a Map value by key:

// Same as credentialMap.get("jsmith").isValid()
Person( credentialMap["jsmith"].valid )

4.8.3.3.9. Abbreviated combined relation condition

This allows you to place more than one restriction on a field using the restriction connectives && or ||. Grouping via parentheses is permitted, resulting in a recursive syntax pattern.

Figure 4.13. Abbreviated combined relation condition

Figure 4.14. Abbreviated combined relation condition with parentheses

// Simple abbreviated combined relation condition using a single &&
Person( age > 30 && < 40 )

// Complex abbreviated combined relation using groupings
Person( age ( (> 30 && < 40) ||
              (> 20 && < 25) ) )

// Mixing abbreviated combined relation with constraint connectives
Person( age > 30 && < 40 || location == "london" )

4.8.3.3.10. Special DRL operators

Figure 4.15. Operators

Coercion to the correct value for the evaluator and the field will be attempted.

4.8.3.3.10.1. The operators `<` `<=` `>` `>=`

These operators can be used on properties with natural ordering. For example, for Date fields, < means before, for String fields, it means alphabetically lower.

Person( firstName < $otherFirstName )

Person( birthDate < $otherBirthDate )

Only applies on Comparable properties.

4.8.3.3.10.2. The operator `matches`

Matches a field against any valid Java Regular Expression. Typically that regexp is a string literal, but variables that resolve to a valid regexp are also allowed.

Example 4.37. Regular Expression Constraint

Cheese( type matches "(Buffalo)?\\S*Mozarella" )

Note

Like in Java, regular expressions written as string literals need to escape '\'.

Only applies on String properties. Using matches against a null value always evaluates to false.

4.8.3.3.10.3. The operator `not matches`

The operator returns true if the String does not match the regular expression. The same rules apply as for the matches operator. Example:

Example 4.38. Regular Expression Constraint

Cheese( type not matches "(Buffulo)?\\S*Mozarella" )

Only applies on String properties. Using not matches against a null value always evaluates to true.

4.8.3.3.10.4. The operator `contains`

The operator contains is used to check whether a field that is a Collection or elements contains the specified value.

Example 4.39. Contains with Collections

CheeseCounter( cheeses contains "stilton" ) // contains with a String literal
CheeseCounter( cheeses contains $var ) // contains with a variable

Only applies on Collection properties.

4.8.3.3.10.5. The operator `not contains`

The operator not contains is used to check whether a field that is a Collection or elements does not contain the specified value.

Example 4.40. Literal Constraint with Collections

CheeseCounter( cheeses not contains "cheddar" ) // not contains with a String literal
CheeseCounter( cheeses not contains $var ) // not contains with a variable

Only applies on Collection properties.

Note
For backward compatibility, the excludes operator is supported as a synonym for not contains.

4.8.3.3.10.6. The operator `memberOf`

The operator memberOf is used to check whether a field is a member of a collection or elements; that collection must be a variable.

Example 4.41. Literal Constraint with Collections

CheeseCounter( cheese memberOf $matureCheeses )

4.8.3.3.10.7. The operator `not memberOf`

The operator not memberOf is used to check whether a field is not a member of a collection or elements; that collection must be a variable.

Example 4.42. Literal Constraint with Collections

CheeseCounter( cheese not memberOf $matureCheeses )

4.8.3.3.10.8. The operator `soundslike`

This operator is similar to matches, but it checks whether a word has almost the same sound (using English pronunciation) as the given value. This is based on the Soundex algorithm (see http://en.wikipedia.org/wiki/Soundex).

Example 4.43. Test with soundslike

// match cheese "fubar" or "foobar"
Cheese( name soundslike 'foobar' )

4.8.3.3.10.9. The operator `str`

This operator str is used to check whether a field that is a String starts with or ends with a certain value. It can also be used to check the length of the String.

Message( routingValue str[startsWith] "R1" )

Message( routingValue str[endsWith] "R2" )

Message( routingValue str[length] 17 )

4.8.3.3.10.10. The operators `in` and `not in` (compound value restriction)

The compound value restriction is used where there is more than one possible value to match. Currently only the in and not in evaluators support this. The second operand of this operator must be a comma-separated list of values, enclosed in parentheses. Values may be given as variables, literals, return values or qualified identifiers. Both evaluators are actually syntactic sugar, internally rewritten as a list of multiple restrictions using the operators != and ==.

Figure 4.16. compoundValueRestriction

Example 4.44. Compound Restriction using "in"

Person( $cheese : favouriteCheese )
Cheese( type in ( "stilton", "cheddar", $cheese ) )

4.8.3.3.11. Inline eval operator (deprecated)

Figure 4.17. Inline Eval Expression

An inline eval constraint can use any valid dialect expression as long as it results to a primitive boolean. The expression must be constant over time. Any previously bound variable, from the current or previous pattern, can be used; autovivification is also used to auto-create field binding variables. When an identifier is found that is not a current variable, the builder looks to see if the identifier is a field on the current object type, if it is, the field binding is auto-created as a variable of the same name. This is called autovivification of field variables inside of inline eval's.

This example will find all male-female pairs where the male is 2 years older than the female; the variable age is auto-created in the second pattern by the autovivification process.

Example 4.45. Return Value operator

Person( girlAge : age, sex = "F" )
Person( eval( age == girlAge + 2 ), sex = 'M' ) // eval() is actually obsolete in this example

Note

Inline eval's are effectively obsolete as their inner syntax is now directly supported. It's recommended not to use them. Simply write the expression without wrapping eval() around it.

4.8.3.3.12. Operator precedence

The operators are evaluated in this precedence:

Table 4.1. Operator precedence

Operator type	Operators	Notes
(nested) property access	`.`	Not normal Java semantics
List/Map access	`[ ]`	Not normal Java semantics
constraint binding	`:`	Not normal Java semantics
multiplicative	`*` `/` `%`
additive	`+` `-`
shift	`<<` `>>` `>>>`
relational	`<` `>` `<=` `>=` `instanceof`
equality	`==` `!=`	Does not use normal Java (not) same semantics: uses (not) equals semantics instead.
non-short circuiting AND	`&`
non-short circuiting exclusive OR	`^`
non-short circuiting inclusive OR	`\|`
logical AND	`&&`
logical OR	`\|\|`
ternary	`? :`
Comma separated AND	`,`	Not normal Java semantics

4.8.3.4. Positional Arguments

Patterns now support positional arguments on type declarations.

declare Cheese
    name : String
    shop : String
    price : int
end

Cheese( "stilton", "Cheese Shop", p; )
Cheese( "stilton", "Cheese Shop"; p : price )
Cheese( "stilton"; shop == "Cheese Shop", p : price )
Cheese( name == "stilton"; shop == "Cheese Shop", p : price )

Positional arguments that are given a previously declared binding will constrain against that using unification; these are referred to as input arguments. If the binding does not yet exist, it will create the declaration binding it to the field represented by the position argument; these are referred to as output arguments.

4.8.3.5. Fine grained property change listeners

When you call modify() (see the modify statement section) on a given object it will trigger a revaluation of all patterns of the matching object type in the knowledge base. This can can lead to unwanted and useless evaluations and in the worst cases to infinite recursions. The only workaround to avoid it was to split up your objects into smaller ones having a 1 to 1 relationship with the original object.

This feature allows the pattern matching to only react to modification of properties actually constrained or bound inside of a given pattern. That will help with performance and recursion and avoid artificial object splitting.

By default this feature is off in order to make the behavior of the rule engine backward compatible with the former releases. When you want to activate it on a specific bean you have to annotate it with @propertyReactive. This annotation works both on drl type declarations:

declare Person
          @propertyReactive
          firstName : String
          lastName : String
          end

and on Java classes:

@PropertyReactive
          public static class Person {
          private String firstName;
          private String lastName;
          }

In this way, for instance, if you have a rule like the following:

rule "Every person named Mario is a male" when
          $person : Person( firstName == "Mario" )
          then
          modify ( $person )  { setMale( true ) }
          end

you won't have to add the no-loop attribute to it in order to avoid an infinite recursion because the engine recognizes that the pattern matching is done on the 'firstName' property while the RHS of the rule modifies the 'male' one. Note that this feature does not work for update(), and this is one of the reasons why we promote modify() since it encapsulates the field changes within the statement. Moreover, on Java classes, you can also annotate any method to say that its invocation actually modifies other properties. For instance in the former Person class you could have a method like:

@Modifies( { "firstName", "lastName" } )
          public void setName(String name) {
          String[] names = name.split("\\s");
          this.firstName = names[0];
          this.lastName = names[1];
          }

That means that if a rule has a RHS like the following:

modify($person) { setName("Mario Fusco") }

it will correctly recognize that the values of both properties 'firstName' and 'lastName' could have potentially been modified and act accordingly, not missing of reevaluating the patterns constrained on them. At the moment the usage of @Modifies is not allowed on fields but only on methods. This is coherent with the most common scenario where the @Modifies will be used for methods that are not related with a class field as in the Person.setName() in the former example. Also note that @Modifies is not transitive, meaning that if another method internally invokes the Person.setName() one it won't be enough to annotate it with @Modifies( { "name" } ), but it is necessary to use @Modifies( { "firstName", "lastName" } ) even on it. Very likely @Modifies transitivity will be implemented in the next release.

For what regards nested accessors, the engine will be notified only for top level fields. In other words a pattern matching like:

Person ( address.city.name == "London )

will be revaluated only for modification of the 'address' property of a Person object. In the same way the constraints analysis is currently strictly limited to what there is inside a pattern. Another example could help to clarify this. An LHS like the following:

$p : Person( )
          Car( owner = $p.name )

will not listen on modifications of the person's name, while this one will do:

Person( $name : name )
          Car( owner = $name )

To overcome this problem it is possible to annotate a pattern with @watch as it follows:

$p : Person( ) @watch ( name )
          Car( owner = $p.name )

Indeed, annotating a pattern with @watch allows you to modify the inferred set of properties for which that pattern will react. Note that the properties named in the @watch annotation are actually added to the ones automatically inferred, but it is also possible to explicitly exclude one or more of them prepending their name with a ! and to make the pattern to listen for all or none of the properties of the type used in the pattern respectively with the wildcrds * and !*. So, for example, you can annotate a pattern in the LHS of a rule like:

// listens for changes on both firstName (inferred) and lastName
          Person( firstName == $expectedFirstName ) @watch( lastName )

          // listens for all the properties of the Person bean
          Person( firstName == $expectedFirstName ) @watch( * )

          // listens for changes on lastName and explicitly exclude firstName
          Person( firstName == $expectedFirstName ) @watch( lastName, !firstName )

          // listens for changes on all the properties except the age one
          Person( firstName == $expectedFirstName ) @watch( *, !age )

Since doesn't make sense to use this annotation on a pattern using a type not annotated with @PropertyReactive the rule compiler will raise a compilation error if you try to do so. Also the duplicated usage of the same property in @watch (for example like in: @watch( firstName, ! firstName ) ) will end up in a compilation error. In a next release we will make the automatic detection of the properties to be listened smarter by doing analysis even outside of the pattern.

It also possible to enable this feature by default on all the types of your model or to completely disallow it by using on option of the KnowledgeBuilderConfiguration. In particular this new PropertySpecificOption can have one of the following 3 values:

- DISABLED => the feature is turned off and all the other related annotations are just ignored
          - ALLOWED => this is the default behavior: types are not property reactive unless they are not annotated with @PropertySpecific
          - ALWAYS => all types are property reactive by default

So, for example, to have a KnowledgeBuilder generating property reactive types by default you could do:

KnowledgeBuilderConfiguration config = KnowledgeBuilderFactory.newKnowledgeBuilderConfiguration();
          config.setOption(PropertySpecificOption.ALWAYS);
          KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder(config);

In this last case it will be possible to disable the property reactivity feature on a specific type by annotating it with @ClassReactive.

4.8.3.6. Basic conditional elements

4.8.3.6.1. Conditional Element `and`

The Conditional Element "and" is used to group other Conditional Elements into a logical conjunction. Drools supports both prefix and and infix and.

Figure 4.18. infixAnd

Traditional infix and is supported:

//infixAnd
Cheese( cheeseType : type ) and Person( favouriteCheese == cheeseType )

Explicit grouping with parentheses is also supported:

//infixAnd with grouping
( Cheese( cheeseType : type ) and
  ( Person( favouriteCheese == cheeseType ) or 
    Person( favouriteCheese == cheeseType ) )

Note

The symbol && (as an alternative to and) is deprecated. But it is still supported in the syntax for backwards compatibility.

Figure 4.19. prefixAnd

Prefix and is also supported:

(and Cheese( cheeseType : type )
     Person( favouriteCheese == cheeseType ) )

The root element of the LHS is an implicit prefix and and doesn't need to be specified:

Example 4.46. implicit root prefixAnd

when
    Cheese( cheeseType : type )
    Person( favouriteCheese == cheeseType )
then
    ...

4.8.3.6.2. Conditional Element `or`

The Conditional Element or is used to group other Conditional Elements into a logical disjunction. Drools supports both prefix or and infix or.

Figure 4.20. infixOr

Traditional infix or is supported:

//infixOr
Cheese( cheeseType : type ) or Person( favouriteCheese == cheeseType )

Explicit grouping with parentheses is also supported:

//infixOr with grouping
( Cheese( cheeseType : type ) or
  ( Person( favouriteCheese == cheeseType ) and
    Person( favouriteCheese == cheeseType ) )

Note

The symbol || (as an alternative to or) is deprecated. But it is still supported in the syntax for backwards compatibility.

Figure 4.21. prefixOr

Prefix or is also supported:

(or Person( sex == "f", age > 60 )
    Person( sex == "m", age > 65 )

Note

The behavior of the Conditional Element or is different from the connective || for constraints and restrictions in field constraints. The engine actually has no understanding of the Conditional Element or. Instead, via a number of different logic transformations, a rule with or is rewritten as a number of subrules. This process ultimately results in a rule that has a single or as the root node and one subrule for each of its CEs. Each subrule can activate and fire like any normal rule; there is no special behavior or interaction between these subrules. - This can be most confusing to new rule authors.

The Conditional Element or also allows for optional pattern binding. This means that each resulting subrule will bind its pattern to the pattern binding. Each pattern must be bound separately, using eponymous variables:

pensioner : ( Person( sex == "f", age > 60 ) or Person( sex == "m", age > 65 ) )

(or pensioner : Person( sex == "f", age > 60 ) 
    pensioner : Person( sex == "m", age > 65 ) )

Since the conditional element or results in multiple subrule generation, one for each possible logically outcome, the example above would result in the internal generation of two rules. These two rules work independently within the Working Memory, which means both can match, activate and fire - there is no shortcutting.

The best way to think of the conditional element or is as a shortcut for generating two or more similar rules. When you think of it that way, it's clear that for a single rule there could be multiple activations if two or more terms of the disjunction are true.

4.8.3.6.3. Conditional Element `not`

Figure 4.22. not

The CE not is first order logic's non-existential quantifier and checks for the non-existence of something in the Working Memory. Think of "not" as meaning "there must be none of...".

The keyword not may be followed by parentheses around the CEs that it applies to. In the simplest case of a single pattern (like below) you may optionally omit the parentheses.

Example 4.47. No Busses

not Bus()

Example 4.48. No red Busses

// Brackets are optional:
not Bus(color == "red")
// Brackets are optional:
not ( Bus(color == "red", number == 42) )
// "not" with nested infix and - two patterns,
// brackets are requires:
not ( Bus(color == "red") and
      Bus(color == "blue") )

4.8.3.6.4. Conditional Element `exists`

Figure 4.23. exists

The CE exists is first order logic's existential quantifier and checks for the existence of something in the Working Memory. Think of "exists" as meaning "there is at least one..". It is different from just having the pattern on its own, which is more like saying "for each one of...". If you use exists with a pattern, the rule will only activate at most once, regardless of how much data there is in working memory that matches the condition inside of the exists pattern. Since only the existence matters, no bindings will be established.

The keyword exists must be followed by parentheses around the CEs that it applies to. In the simplest case of a single pattern (like below) you may omit the parentheses.

Example 4.49. At least one Bus

exists Bus()

Example 4.50. At least one red Bus

exists Bus(color == "red")
// brackets are optional:
exists ( Bus(color == "red", number == 42) )
// "exists" with nested infix and,
// brackets are required:
exists ( Bus(color == "red") and
         Bus(color == "blue") )

4.8.3.7. Advanced conditional elements

4.8.3.7.1. Conditional Element `forall`

Figure 4.24. forall

The Conditional Element forall completes the First Order Logic support in Drools. The Conditional Element forall evaluates to true when all facts that match the first pattern match all the remaining patterns. Example:

rule "All English buses are red"
when
    forall( $bus : Bus( type == 'english') 
                   Bus( this == $bus, color = 'red' ) )
then
    // all English buses are red
end

In the above rule, we "select" all Bus objects whose type is "english". Then, for each fact that matches this pattern we evaluate the following patterns and if they match, the forall CE will evaluate to true.

To state that all facts of a given type in the working memory must match a set of constraints, forall can be written with a single pattern for simplicity. Example:

Example 4.51. Single Pattern Forall

rule "All Buses are Red"
when
    forall( Bus( color == 'red' ) )
then
    // all Bus facts are red
end

Another example shows multiple patterns inside the forall:

Example 4.52. Multi-Pattern Forall

rule "all employees have health and dental care programs"
when
    forall( $emp : Employee()
            HealthCare( employee == $emp )
            DentalCare( employee == $emp )
          )
then
    // all employees have health and dental care
end

Forall can be nested inside other CEs. For instance, forall can be used inside a not CE. Note that only single patterns have optional parentheses, so that with a nested forall parentheses must be used:

Example 4.53. Combining Forall with Not CE

rule "not all employees have health and dental care"
when 
    not ( forall( $emp : Employee()
                  HealthCare( employee == $emp )
                  DentalCare( employee == $emp ) ) 
        )
then
    // not all employees have health and dental care
end

As a side note, forall( p1 p2 p3...) is equivalent to writing:

not(p1 and not(and p2 p3...))

Also, it is important to note that forall is a scope delimiter. Therefore, it can use any previously bound variable, but no variable bound inside it will be available for use outside of it.

4.8.3.7.2. Conditional Element `from`

Figure 4.25. from

The Conditional Element from enables users to specify an arbitrary source for data to be matched by LHS patterns. This allows the engine to reason over data not in the Working Memory. The data source could be a sub-field on a bound variable or the results of a method call. It is a powerful construction that allows out of the box integration with other application components and frameworks. One common example is the integration with data retrieved on-demand from databases using hibernate named queries.

The expression used to define the object source is any expression that follows regular MVEL syntax. Therefore, it allows you to easily use object property navigation, execute method calls and access maps and collections elements.

Here is a simple example of reasoning and binding on another pattern sub-field:

rule "validate zipcode"
when
    Person( $personAddress : address ) 
    Address( zipcode == "23920W") from $personAddress 
then
    // zip code is ok
end

With all the flexibility from the new expressiveness in the Drools engine you can slice and dice this problem many ways. This is the same but shows how you can use a graph notation with the 'from':

rule "validate zipcode"
when
    $p : Person( ) 
    $a : Address( zipcode == "23920W") from $p.address 
then
    // zip code is ok
end

Previous examples were evaluations using a single pattern. The CE from also support object sources that return a collection of objects. In that case, from will iterate over all objects in the collection and try to match each of them individually. For instance, if we want a rule that applies 10% discount to each item in an order, we could do:

rule "apply 10% discount to all items over US$ 100,00 in an order"
when
    $order : Order()
    $item  : OrderItem( value > 100 ) from $order.items
then
    // apply discount to $item
end

The above example will cause the rule to fire once for each item whose value is greater than 100 for each given order.

You must take caution, however, when using from, especially in conjunction with the lock-on-active rule attribute as it may produce unexpected results. Consider the example provided earlier, but now slightly modified as follows:

rule "Assign people in North Carolina (NC) to sales region 1"
ruleflow-group "test"
lock-on-active true
when
    $p : Person( ) 
    $a : Address( state == "NC") from $p.address 
then
    modify ($p) {} // Assign person to sales region 1 in a modify block
end

rule "Apply a discount to people in the city of Raleigh"
ruleflow-group "test"
lock-on-active true
when
    $p : Person( ) 
    $a : Address( city == "Raleigh") from $p.address 
then
    modify ($p) {} // Apply discount to person in a modify block
end

In the above example, persons in Raleigh, NC should be assigned to sales region 1 and receive a discount; i.e., you would expect both rules to activate and fire. Instead you will find that only the second rule fires.

If you were to turn on the audit log, you would also see that when the second rule fires, it deactivates the first rule. Since the rule attribute lock-on-active prevents a rule from creating new activations when a set of facts change, the first rule fails to reactivate. Though the set of facts have not changed, the use of from returns a new fact for all intents and purposes each time it is evaluated.

First, it's important to review why you would use the above pattern. You may have many rules across different rule-flow groups. When rules modify working memory and other rules downstream of your RuleFlow (in different rule-flow groups) need to be reevaluated, the use of modify is critical. You don't, however, want other rules in the same rule-flow group to place activations on one another recursively. In this case, the no-loop attribute is ineffective, as it would only prevent a rule from activating itself recursively. Hence, you resort to lock-on-active.

There are several ways to address this issue:

Avoid the use of from when you can assert all facts into working memory or use nested object references in your constraint expressions (shown below).
Place the variable assigned used in the modify block as the last sentence in your condition (LHS).
Avoid the use of lock-on-active when you can explicitly manage how rules within the same rule-flow group place activations on one another (explained below).

The preferred solution is to minimize use of from when you can assert all your facts into working memory directly. In the example above, both the Person and Address instance can be asserted into working memory. In this case, because the graph is fairly simple, an even easier solution is to modify your rules as follows:

rule "Assign people in North Carolina (NC) to sales region 1"
ruleflow-group "test"
lock-on-active true
when
    $p : Person(address.state == "NC" )  
then
    modify ($p) {} // Assign person to sales region 1 in a modify block
end

rule "Apply a discount to people in the city of Raleigh"
ruleflow-group "test"
lock-on-active true
when
    $p : Person(address.city == "Raleigh" )  
then
    modify ($p) {} //Apply discount to person in a modify block
end

Now, you will find that both rules fire as expected. However, it is not always possible to access nested facts as above. Consider an example where a Person holds one or more Addresses and you wish to use an existential quantifier to match people with at least one address that meets certain conditions. In this case, you would have to resort to the use of from to reason over the collection.

There are several ways to use from to achieve this and not all of them exhibit an issue with the use of lock-on-active. For example, the following use of from causes both rules to fire as expected:

rule "Assign people in North Carolina (NC) to sales region 1"
ruleflow-group "test"
lock-on-active true
when
    $p : Person($addresses : addresses)
    exists (Address(state == "NC") from $addresses)  
then
    modify ($p) {} // Assign person to sales region 1 in a modify block
end

rule "Apply a discount to people in the city of Raleigh"
ruleflow-group "test"
lock-on-active true
when
    $p : Person($addresses : addresses)
    exists (Address(city == "Raleigh") from $addresses)  
then
    modify ($p) {} // Apply discount to person in a modify block
end

However, the following slightly different approach does exhibit the problem:

rule "Assign people in North Carolina (NC) to sales region 1"
ruleflow-group "test"
lock-on-active true
when
    $assessment : Assessment()
    $p : Person()
    $addresses : List() from $p.addresses
    exists (Address( state == "NC") from $addresses) 
then
    modify ($assessment) {} // Modify assessment in a modify block
end

rule "Apply a discount to people in the city of Raleigh"
ruleflow-group "test"
lock-on-active true
when
    $assessment : Assessment()
    $p : Person()
    $addresses : List() from $p.addresses 
    exists (Address( city == "Raleigh") from $addresses)
then
    modify ($assessment) {} // Modify assessment in a modify block
end

In the above example, the $addresses variable is returned from the use of from. The example also introduces a new object, assessment, to highlight one possible solution in this case. If the $assessment variable assigned in the condition (LHS) is moved to the last condition in each rule, both rules fire as expected.

Though the above examples demonstrate how to combine the use of from with lock-on-active where no loss of rule activations occurs, they carry the drawback of placing a dependency on the order of conditions on the LHS. In addition, the solutions present greater complexity for the rule author in terms of keeping track of which conditions may create issues.

A better alternative is to assert more facts into working memory. In this case, a person's addresses may be asserted into working memory and the use of from would not be necessary.

There are cases, however, where asserting all data into working memory is not practical and we need to find other solutions. Another option is to reevaluate the need for lock-on-active. An alternative to lock-on-active is to directly manage how rules within the same rule-flow group activate one another by including conditions in each rule that prevent rules from activating each other recursively when working memory is modified. For example, in the case above where a discount is applied to citizens of Raleigh, a condition may be added to the rule that checks whether the discount has already been applied. If so, the rule does not activate.

4.8.3.7.3. Conditional Element `collect`

Figure 4.26. collect

The Conditional Element collect allows rules to reason over a collection of objects obtained from the given source or from the working memory. In First Oder Logic terms this is the cardinality quantifier. A simple example:

import java.util.ArrayList

rule "Raise priority if system has more than 3 pending alarms"
when
    $system : System()
    $alarms : ArrayList( size >= 3 )
              from collect( Alarm( system == $system, status == 'pending' ) )
then
    // Raise priority, because system $system has
    // 3 or more alarms pending. The pending alarms
    // are $alarms.
end

In the above example, the rule will look for all pending alarms in the working memory for each given system and group them in ArrayLists. If 3 or more alarms are found for a given system, the rule will fire.

The result pattern of collect can be any concrete class that implements the java.util.Collection interface and provides a default no-arg public constructor. This means that you can use Java collections like ArrayList, LinkedList, HashSet, etc., or your own class, as long as it implements the java.util.Collection interface and provide a default no-arg public constructor.

Both source and result patterns can be constrained as any other pattern.

Variables bound before the collect CE are in the scope of both source and result patterns and therefore you can use them to constrain both your source and result patterns. But note that collect is a scope delimiter for bindings, so that any binding made inside of it is not available for use outside of it.

Collect accepts nested from CEs. The following example is a valid use of "collect":

import java.util.LinkedList;

rule "Send a message to all mothers"
when
    $town : Town( name == 'Paris' )
    $mothers : LinkedList() 
               from collect( Person( gender == 'F', children > 0 ) 
                             from $town.getPeople() 
                           )
then
    // send a message to all mothers
end

4.8.3.7.4. Conditional Element `accumulate`

Figure 4.27. accumulate

The Conditional Element accumulate is a more flexible and powerful form of collect, in the sense that it can be used to do what collect does and also achieve results that the CE collect is not capable of doing. Basically, what it does is that it allows a rule to iterate over a collection of objects, executing custom actions for each of the elements, and at the end it returns a result object.

Accumulate supports both the use of pre-defined accumulate functions, or the use of inline custom code. Inline custom code should be avoided though, as it is harder for rule authors to maintain, and frequently leads to code duplication. Accumulate functions are easier to test and reuse.

The Accumulate CE also supports multiple different syntaxes. The preferred syntax is the top level accumulate, as noted bellow, but all other syntaxes are supported for backward compatibility.

4.8.3.7.4.1. Accumulate CE (preferred syntax)

The top level accumulate syntax is the most compact and flexible syntax. The simplified syntax is as follows:

accumulate( <source pattern>; <functions> [;<constraints>] )

For instance, a rule to calculate the minimum, maximum and average temperature reading for a given sensor and that raises an alarm if the minimum temperature is under 20C degrees and the average is over 70C degrees could be written in the following way, using Accumulate:

rule "Raise alarm"
when
    $s : Sensor()
    accumulate( Reading( sensor == $s, $temp : temperature );
                $min : min( $temp ),
                $max : max( $temp ),
                $avg : average( $temp );
                $min < 20, $avg > 70 )
then
    // raise the alarm
end

In the above example, min, max and average are Accumulate Functions and will calculate the minimum, maximum and average temperature values over all the readings for each sensor.

Drools ships with several built-in accumulate functions, including:

average
min
max
count
sum
collectList
collectSet

These common functions accept any expression as input. For instance, if someone wants to calculate the average profit on all items of an order, a rule could be written using the average function:

rule "Average profit"
when
    $order : Order()
    accumulate( OrderItem( order == $order, $cost : cost, $price : price );
                $avgProfit : average( 1 - $cost / $price ) )
then
    // average profit for $order is $avgProfit
end

Accumulate Functions are all pluggable. That means that if needed, custom, domain specific functions can easily be added to the engine and rules can start to use them without any restrictions. To implement a new Accumulate Function all one needs to do is to create a Java class that implements the org.drools.core.runtime.rule.TypedAccumulateFunction interface and add a line to the configuration file or set a system property to let the engine know about the new function. As an example of an Accumulate Function implementation, the following is the implementation of the average function:

/**

 * An implementation of an accumulator capable of calculating average values

 */

public class AverageAccumulateFunction implements org.drools.core.runtime.rule.TypedAccumulateFunction {


    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {


    }


    public void writeExternal(ObjectOutput out) throws IOException {


    }


    public static class AverageData implements Externalizable {

        public int    count = 0;

        public double total = 0;


        public AverageData() {}


        public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {

            count   = in.readInt();

            total   = in.readDouble();

        }


        public void writeExternal(ObjectOutput out) throws IOException {

            out.writeInt(count);

            out.writeDouble(total);

        }


    }


    /* (non-Javadoc)

     * @see org.drools.base.accumulators.AccumulateFunction#createContext()

     */

    public Serializable createContext() {

        return new AverageData();

    }


    /* (non-Javadoc)

     * @see org.drools.core.base.accumulators.AccumulateFunction#init(java.lang.Object)

     */

    public void init(Serializable context) throws Exception {

        AverageData data = (AverageData) context;

        data.count = 0;

        data.total = 0;

    }


    /* (non-Javadoc)

     * @see org.drools.core.base.accumulators.AccumulateFunction#accumulate(java.lang.Object, java.lang.Object)

     */

    public void accumulate(Serializable context,

                           Object value) {

        AverageData data = (AverageData) context;

        data.count++;

        data.total += ((Number) value).doubleValue();

    }


    /* (non-Javadoc)

     * @see org.drools.core.base.accumulators.AccumulateFunction#reverse(java.lang.Object, java.lang.Object)

     */

    public void reverse(Serializable context,

                        Object value) throws Exception {

        AverageData data = (AverageData) context;

        data.count--;

        data.total -= ((Number) value).doubleValue();

    }


    /* (non-Javadoc)

     * @see org.drools.core.base.accumulators.AccumulateFunction#getResult(java.lang.Object)

     */

    public Object getResult(Serializable context) throws Exception {

        AverageData data = (AverageData) context;

        return new Double( data.count == 0 ? 0 : data.total / data.count );

    }


    /* (non-Javadoc)

     * @see org.drools.core.base.accumulators.AccumulateFunction#supportsReverse()

     */

    public boolean supportsReverse() {

        return true;

    }


    /**

     * {@inheritDoc}

     */

    public Class< ? > getResultType() {

        return Number.class;

    }


}

The code for the function is very simple, as we could expect, as all the "dirty" integration work is done by the engine. Finally, to plug the function into the engine, we added it to the configuration file:

drools.accumulate.function.average = org.drools.core.base.accumulators.AverageAccumulateFunction

Here, "drools.accumulate.function." is a prefix that must always be used, "average" is how the function will be used in the rule file, and "org.drools.core.base.accumulators.AverageAccumulateFunction" is the fully qualified name of the class that implements the function behavior.

4.8.3.7.4.2. Alternate Syntax: single function with return type

The accumulate syntax evolved over time with the goal of becoming more compact and expressive. Nevertheless, Drools still supports previous syntaxes for backward compatibility purposes.

In case the rule is using a single accumulate function on a given accumulate, the author may add a pattern for the result object and use the "from" keyword to link it to the accumulate result. Example: a rule to apply a 10% discount on orders over $100 could be written in the following way:

rule "Apply 10% discount to orders over US$ 100,00"
when
    $order : Order()
    $total : Number( doubleValue > 100 ) 
             from accumulate( OrderItem( order == $order, $value : value ),
                              sum( $value ) )
then
    # apply discount to $order
end

In the above example, the accumulate element is using only one function (sum), and so, the rules author opted to explicitly write a pattern for the result type of the accumulate function (Number) and write the constraints inside it. There are no problems in using this syntax over the compact syntax presented before, except that is is a bit more verbose. Also note that it is not allowed to use both the return type and the functions binding in the same accumulate statement.

4.8.3.7.4.3. Accumulate with inline custom code

Warning

The use of accumulate with inline custom code is not a good practice for several reasons, including difficulties on maintaining and testing rules that use them, as well as the inability of reusing that code. Implementing your own accumulate functions is very simple and straightforward, they are easy to unit test and to use. This form of accumulate is supported for backward compatibility only.

Another possible syntax for the accumulate is to define inline custom code, instead of using accumulate functions. As noted on the previous warned, this is discouraged though for the stated reasons.

The general syntax of the accumulate CE with inline custom code is:

<result pattern> from accumulate( <source pattern>,
                                  init( <init code> ),
                                  action( <action code> ),
                                  reverse( <reverse code> ),
                                  result( <result expression> ) )

The meaning of each of the elements is the following:

<source pattern>: the source pattern is a regular pattern that the engine will try to match against each of the source objects.
<init code>: this is a semantic block of code in the selected dialect that will be executed once for each tuple, before iterating over the source objects.
<action code>: this is a semantic block of code in the selected dialect that will be executed for each of the source objects.
<reverse code>: this is an optional semantic block of code in the selected dialect that if present will be executed for each source object that no longer matches the source pattern. The objective of this code block is to undo any calculation done in the <action code> block, so that the engine can do decremental calculation when a source object is modified or retracted, hugely improving performance of these operations.
<result expression>: this is a semantic expression in the selected dialect that is executed after all source objects are iterated.
<result pattern>: this is a regular pattern that the engine tries to match against the object returned from the <result expression>. If it matches, the accumulate conditional element evaluates to true and the engine proceeds with the evaluation of the next CE in the rule. If it does not matches, the accumulate CE evaluates to false and the engine stops evaluating CEs for that rule.

It is easier to understand if we look at an example:

rule "Apply 10% discount to orders over US$ 100,00"
when
    $order : Order()
    $total : Number( doubleValue > 100 ) 
             from accumulate( OrderItem( order == $order, $value : value ),
                              init( double total = 0; ),
                              action( total += $value; ),
                              reverse( total -= $value; ),
                              result( total ) )
then
    # apply discount to $order
end

In the above example, for each Order in the Working Memory, the engine will execute the init code initializing the total variable to zero. Then it will iterate over all OrderItem objects for that order, executing the action for each one (in the example, it will sum the value of all items into the total variable). After iterating over all OrderItem objects, it will return the value corresponding to the result expression (in the above example, the value of variable total). Finally, the engine will try to match the result with the Number pattern, and if the double value is greater than 100, the rule will fire.

The example used Java as the semantic dialect, and as such, note that the usage of the semicolon as statement delimiter is mandatory in the init, action and reverse code blocks. The result is an expression and, as such, it does not admit ';'. If the user uses any other dialect, he must comply to that dialect's specific syntax.

As mentioned before, the reverse code is optional, but it is strongly recommended that the user writes it in order to benefit from the improved performance on update and retract.

The accumulate CE can be used to execute any action on source objects. The following example instantiates and populates a custom object:

rule "Accumulate using custom objects"
when
    $person   : Person( $likes : likes )
    $cheesery : Cheesery( totalAmount > 100 )
                from accumulate( $cheese : Cheese( type == $likes ),
                                 init( Cheesery cheesery = new Cheesery(); ),
                                 action( cheesery.addCheese( $cheese ); ),
                                 reverse( cheesery.removeCheese( $cheese ); ),
                                 result( cheesery ) );
then
    // do something
end

4.8.3.8. Conditional Element `eval`

Figure 4.28. eval

The conditional element eval is essentially a catch-all which allows any semantic code (that returns a primitive boolean) to be executed. This code can refer to variables that were bound in the LHS of the rule, and functions in the rule package. Overuse of eval reduces the declarativeness of your rules and can result in a poorly performing engine. While eval can be used anywhere in the patterns, the best practice is to add it as the last conditional element in the LHS of a rule.

Evals cannot be indexed and thus are not as efficient as Field Constraints. However this makes them ideal for being used when functions return values that change over time, which is not allowed within Field Constraints.

For folks who are familiar with Drools 2.x lineage, the old Drools parameter and condition tags are equivalent to binding a variable to an appropriate type, and then using it in an eval node.

p1 : Parameter()
p2 : Parameter()
eval( p1.getList().containsKey( p2.getItem() ) )

p1 : Parameter()
p2 : Parameter()
// call function isValid in the LHS
eval( isValid( p1, p2 ) )

Name	Description
uc	Converts all letters to upper case.
lc	Converts all letters to lower case.
ucfirst	Converts the first letter to upper case, and all other letters to lower case.
num	Extracts all digits and "-" from the string. If the last two digits in the original string are preceded by "." or ",", a decimal period is inserted in the corresponding position.
a?b/c	Compares the string with string a, and if they are equal, replaces it with b, otherwise with c. But c can be another triplet a, b, c, so that the entire structure is, in fact, a translation table.

Word	Description
result	Prints the resulting DRL text, with line numbers.
steps	Prints each expansion step of condition and consequence lines.
keyword	Dumps the internal representation of all DSL entries with scope "keyword".
when	Dumps the internal representation of all DSL entries with scope "when" or "*".
then	Dumps the internal representation of all DSL entries with scope "then" or "*".
usage	Displays a usage statistic of all DSL entries.

	Every fact and instance is marked internally with a "time tag" to indicate its relative recency with respect to every other fact and instance in the system. The pattern entities associated with each rule activation are sorted in descending order for determining placement. An activation with a more recent pattern entity is placed before activations with less recent pattern entities. To determine the placement order of two activations, compare the sorted time tags of the two activations one by one starting with the largest time tags. The comparison should continue until one activation’s time tag is greater than the other activation’s corresponding time tag. The activation with the greater time tag is placed before the other activation on the agenda. If one activation has more pattern entities than the other activation and the compared time tags are all identical, then the activation with more time tags is placed before the other activation on the agenda.
	--CLIPS Reference Manual

	Newly activated rules are placed above all rules of the same salience. For example, given that fact-a activates rule-1 and rule-2 and fact-b activates rule-3 and rule-4, then if fact-a is asserted before fact-b, rule-3 and rule-4 will be above rule-1 and rule-2 on the agenda. However, the position of rule-1 relative to rule-2 and rule-3 relative to rule-4 will be arbitrary.
	--CLIPS Reference Manual

	The default conflict resolution strategy for CLIPS, Depth, is different than the default conflict resolution strategy used by OPS5. Therefore if you directly translate an OPS5 program to CLIPS, but use the default depth conflict resolution strategy, you're only likely to get the correct behavior by coincidence. The LEX and MEA conflict resolution strategies are provided in CLIPS to allow you to quickly convert and correctly run an OPS5 program in CLIPS.
	--Clips Support Forum