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Drools

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Drools is a business rule management system BRMS with a forward and backward chaining inference based rules engine, more correctly known as a production rule system, using an enhanced implementation of the Rete algorithm.

KIE Knowledge Is Everything is the new umbrella name to drools, optaPlanner, jBPM, Guvnor, uberFire and related technologies.

Drools supports the Java Rules Engine API Java Specification Request 94 standard for its business rule engine and enterprise framework for the construction, maintenance, and enforcement of business policies in an organization, application, or service.

Red Hat Decision Manager

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Red Hat Decision Manager formerly Red Hat JBoss BRMS is a business rule management system and reasoning engine for business policy and rules development, access, and change management. JBoss Enterprise BRMS is a productized version of Drools with enterprise-level support available. JBoss Rules is also a productized version of Drools, but JBoss Enterprise BRMS is the flagship product.

Components of the enterprise version:

Drools and Guvnor are JBoss Community open source projects. As they are mature, they are brought into the enterprise-ready product JBoss Enterprise BRMS.

Components of the JBoss Community version:

Example

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This example illustrates a simple rule to print out information about a holiday in July. It checks a condition on an instance of the Holiday class, and executes Java code if that condition is true.

rule "validate holiday" 
dialect "mvel"
dialect "java"
when
    $h1 : Holiday month == "july" 
then
    System.out.println$h1.name + ":" + $h1.month;
end

The purpose of dialect "mvel" is to point the getter and setters of the variables of your Plain Old Java Object POJO classes. Consider the above example, in which a Holiday class is used and inside the circular brackets parentheses "month" is used. So with the help dialect "mvel" the getter and setters of the variable "month" can be accessed.

Dialect "java" is used to help us write our Java code in our rules. There is one restriction or characteristic on this. We cannot use Java code inside "when" part of the rule but we can use Java code in "then" part.

We can also declare a Reference variable $h1 without the $ symbol. There is no restriction on this. The main purpose of putting the $ symbol before the variable is to mark the difference between variables of POJO classes and Rules.