Drools excel sheet with multiple rules, hash map and list example

JBoss Drools is rule engine

Rule engine provides a standard way to handle your business logic.

Drools is split into two main parts: Authoring and Runtime.

Authoring:

The Authoring process involves the creation Rules files (.DRL or excel file) which contain the rules which are fed into a parser.

The parser checks for correctly syntax of the rules and produces an intermediate structure that "describes" the rules.

This is then passed to the Package Builder which produces Packagesand undertakes any code generation and compilation that is necessary for the creation of the Package.

RuleBase is a runtime component made up of one or more Packages.

A RuleBase can instantiate one or more WorkingMemories at any time.
The Working Memory consists of a number of sub components, including Working Memory Event Support, Truth Maintenance System, Agenda and Agenda Event Support.

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S/W installation:

copy the plugin .jar file in the "plugin" directory of your Eclipse. Last thing to do, copy the directory org.drools.eclipse.feature_5.0.0 (containing the feature.xml file) in the features directory.

decision tables

final KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();

// this will parse and compile in one step

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

        HelloWorldExample.class), ResourceType.DRL);

// Check the builder for errors

if (kbuilder.hasErrors()) {

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

    throw new RuntimeException("Unable to compile \"HelloWorld.drl\".");

}

// get the compiled packages (which are serializable)

final Collection<KnowledgePackage> pkgs = kbuilder.getKnowledgePackages();

// add the packages to a knowledgebase (deploy the knowledge packages).

final KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();

kbase.addKnowledgePackages(pkgs);

final StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();

By default, all rules are in the agenda group "MAIN".

The "agenda-group" attribute lets you specify a different agenda group for the rule.

salience

The execution of rule "A to B" changes the state of B to FINISHED, which activates both, rules "B to C" and "B to D", placing their Activations onto the Agenda. From this moment on, both rules may fire and, therefore, they are said to be "in conflict". The conflict resolution strategy allows the engine's Agenda to decide which rule to fire. As rule "B to C" has the higher salience value (10 versus the default salience value of 0), it fires first, modifying object C to state FINISHED. The Audit view shown above reflects the modification of the State object in the rule "A to B", which results in two activations being in conflict.

Example:

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5.4. 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.

5.4.1. Message format

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

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Figure 5.3. 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.

5.8.1. 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.

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