2.9. Rule actions in DRL (THEN)

The then part of the rule (also known as the Right Hand Side (RHS) of the rule) contains the actions to be performed when the conditional part of the rule has been met. Actions consist of one or more methods that execute consequences based on the rule conditions and on available data objects in the package. For example, if a bank requires loan applicants to have over 21 years of age (with a rule condition Applicant( age < 21 )) and a loan applicant is under 21 years old, the then action of an "Underage" rule would be setApproved( false ), declining the loan because the applicant is under age.

The main purpose of rule actions is to to insert, delete, or modify data in the working memory of the decision engine. Effective rule actions are small, declarative, and readable. If you need to use imperative or conditional code in rule actions, then divide the rule into multiple smaller and more declarative rules.

Example rule for loan application age limit

rule "Underage"
  when
    application : LoanApplication()
    Applicant( age < 21 )
  then
    application.setApproved( false );
    application.setExplanation( "Underage" );
end

2.9.1. Supported rule action methods in DRL

DRL supports the following rule action methods that you can use in DRL rule actions. You can use these methods to modify the working memory of the decision engine without having to first reference a working memory instance. These methods act as shortcuts to the methods provided by the KnowledgeHelper class in your Red Hat Decision Manager distribution.

For all rule action methods, download the Red Hat Decision Manager 7.5.1 Source Distribution ZIP file from the Red Hat Customer Portal and navigate to ~/rhdm-7.5.1-sources/src/drools-$VERSION/drools-core/src/main/java/org/drools/core/spi/KnowledgeHelper.java.

set

Use this to set the value of a field. 

set<field> ( <value> )

Example rule action to set the values of a loan application approval

$application.setApproved ( false );
$application.setExplanation( "has been bankrupt" );

modify

Use this to specify fields to be modified for a fact and to notify the decision engine of the change. This method provides a structured approach to fact updates. It combines the update operation with setter calls to change object fields. 

modify ( <fact-expression> ) {
    <expression>,
    <expression>,
    ...
}

Example rule action to modify a loan application amount and approval

modify( LoanApplication ) {
        setAmount( 100 ),
        setApproved ( true )
}

update

Use this to specify fields and the entire related fact to be updated and to notify the decision engine of the change. After a fact has changed, you must call update before changing another fact that might be affected by the updated values. To avoid this added step, use the modify method instead. 

update ( <object, <handle> )  // Informs the decision engine that an object has changed

update ( <object> )  // Causes `KieSession` to search for a fact handle of the object

Example rule action to update a loan application amount and approval

LoanApplication.setAmount( 100 );
update( LoanApplication );

注記

If you provide property-change listeners, you do not need to call this method when an object changes. For more information about property-change listeners, see Decision engine in Red Hat Decision Manager.

insert

Use this to insert a new fact into the working memory of the decision engine and to define resulting fields and values as needed for the fact. 

insert( new <object> );

Example rule action to insert a new loan applicant object

insert( new Applicant() );

insertLogical

Use this to insert a new fact logically into the decision engine. The decision engine is responsible for logical decisions on insertions and retractions of facts. After regular or stated insertions, facts must be retracted explicitly. After logical insertions, the facts that were inserted are automatically retracted when the conditions in the rules that inserted the facts are no longer true. 

insertLogical( new <object> );

Example rule action to logically insert a new loan applicant object

insertLogical( new Applicant() );

delete

Use this to remove an object from the decision engine. The keyword retract is also supported in DRL and executes the same action, but delete is typically preferred in DRL code for consistency with the keyword insert. 

delete( <object> );

Example rule action to delete a loan applicant object

delete( Applicant );