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82.7.2. Rule unit execution control

Rule units are helpful when you want to coordinate rule execution so that the execution of one rule unit triggers the start of another rule unit and so on.

To facilitate rule unit execution control, the decision engine supports the following rule unit methods that you can use in DRL rule actions to coordinate the execution of rule units:

  • drools.run(): Triggers the execution of a specified rule unit class. This method imperatively interrupts the execution of the rule unit and activates the other specified rule unit.
  • drools.guard(): Prevents (guards) a specified rule unit class from being executed until the associated rule condition is met. This method declaratively schedules the execution of the other specified rule unit. When the decision engine produces at least one match for the condition in the guarding rule, the guarded rule unit is considered active. A rule unit can contain multiple guarding rules.

As an example of the drools.run() method, consider the following DRL rules that each belong to a specified rule unit. The NotAdult rule uses the drools.run( AdultUnit.class ) method to trigger the execution of the AdultUnit rule unit:

Example DRL rules with controlled execution using drools.run()

package org.mypackage.myunit
unit AdultUnit

rule Adult
  when
    Person(age >= 18, $name : name) from persons
  then
    System.out.println($name + " is adult");
end
 

package org.mypackage.myunit
unit NotAdultUnit

rule NotAdult
  when
    $p : Person(age < 18, $name : name) from persons
  then
    System.out.println($name + " is NOT adult");
    modify($p) { setAge(18); }
    drools.run( AdultUnit.class );
end

The example also uses a RuleUnitExecutor class created from the KIE base that was built from these rules and a DataSource definition of persons bound to it:

Example rule executor and data source definitions

RuleUnitExecutor executor = RuleUnitExecutor.create().bind( kbase );
DataSource<Person> persons = executor.newDataSource( "persons",
                                                     new Person( "John", 42 ),
                                                     new Person( "Jane", 44 ),
                                                     new Person( "Sally", 4 ) );

In this case, the example creates the DataSource definition directly from the RuleUnitExecutor class and binds it to the "persons" variable in a single statement.

The example execution code produces the following output when the relevant Person facts are inserted in the persons data source:

Example rule unit execution output

Sally is NOT adult
John is adult
Jane is adult
Sally is adult

The NotAdult rule detects a match when evaluating the person "Sally", who is under 18 years old. The rule then modifies her age to 18 and uses the drools.run( AdultUnit.class ) method to trigger the execution of the AdultUnit rule unit. The AdultUnit rule unit contains a rule that can now be executed for all of the 3 persons in the DataSource definition.

As an example of the drools.guard() method, consider the following BoxOffice class and BoxOfficeUnit rule unit class:

Example BoxOffice class

public class BoxOffice {
    private boolean open;

    public BoxOffice( boolean open ) {
        this.open = open;
    }

    public boolean isOpen() {
        return open;
    }

    public void setOpen( boolean open ) {
        this.open = open;
    }
}

Example BoxOfficeUnit rule unit class

public class BoxOfficeUnit implements RuleUnit {
    private DataSource<BoxOffice> boxOffices;

    public DataSource<BoxOffice> getBoxOffices() {
        return boxOffices;
    }
}

The example also uses the following TicketIssuerUnit rule unit class to keep selling box office tickets for the event as long as at least one box office is open. This rule unit uses DataSource definitions of persons and tickets:

Example TicketIssuerUnit rule unit class

public class TicketIssuerUnit implements RuleUnit {
    private DataSource<Person> persons;
    private DataSource<AdultTicket> tickets;

    private List<String> results;

    public TicketIssuerUnit() { }

    public TicketIssuerUnit( DataSource<Person> persons, DataSource<AdultTicket> tickets ) {
        this.persons = persons;
        this.tickets = tickets;
    }

    public DataSource<Person> getPersons() {
        return persons;
    }

    public DataSource<AdultTicket> getTickets() {
        return tickets;
    }

    public List<String> getResults() {
        return results;
    }
}

The BoxOfficeUnit rule unit contains a BoxOfficeIsOpen DRL rule that uses the drools.guard( TicketIssuerUnit.class ) method to guard the execution of the TicketIssuerUnit rule unit that distributes the event tickets, as shown in the following DRL rule examples:

Example DRL rules with controlled execution using drools.guard()

package org.mypackage.myunit;
unit TicketIssuerUnit;

rule IssueAdultTicket when
    $p: /persons[ age >= 18 ]
then
    tickets.insert(new AdultTicket($p));
end
rule RegisterAdultTicket when
    $t: /tickets
then
    results.add( $t.getPerson().getName() );
end
 

package org.mypackage.myunit;
unit BoxOfficeUnit;

rule BoxOfficeIsOpen
  when
    $box: /boxOffices[ open ]
  then
    drools.guard( TicketIssuerUnit.class );
end

In this example, so long as at least one box office is open, the guarded TicketIssuerUnit rule unit is active and distributes event tickets. When no more box offices are in open state, the guarded TicketIssuerUnit rule unit is prevented from being executed.

The following example class illustrates a more complete box office scenario:

Example class for the box office scenario

DataSource<Person> persons = executor.newDataSource( "persons" );
DataSource<BoxOffice> boxOffices = executor.newDataSource( "boxOffices" );
DataSource<AdultTicket> tickets = executor.newDataSource( "tickets" );

List<String> list = new ArrayList<>();
executor.bindVariable( "results", list );

// Two box offices are open:
BoxOffice office1 = new BoxOffice(true);
FactHandle officeFH1 = boxOffices.insert( office1 );
BoxOffice office2 = new BoxOffice(true);
FactHandle officeFH2 = boxOffices.insert( office2 );

persons.insert(new Person("John", 40));

// Execute `BoxOfficeIsOpen` rule, run `TicketIssuerUnit` rule unit, and execute `RegisterAdultTicket` rule:
executor.run(BoxOfficeUnit.class);

assertEquals( 1, list.size() );
assertEquals( "John", list.get(0) );
list.clear();

persons.insert(new Person("Matteo", 30));

// Execute `RegisterAdultTicket` rule:
executor.run(BoxOfficeUnit.class);

assertEquals( 1, list.size() );
assertEquals( "Matteo", list.get(0) );
list.clear();

// One box office is closed, the other is open:
office1.setOpen(false);
boxOffices.update(officeFH1, office1);
persons.insert(new Person("Mark", 35));
executor.run(BoxOfficeUnit.class);

assertEquals( 1, list.size() );
assertEquals( "Mark", list.get(0) );
list.clear();

// All box offices are closed:
office2.setOpen(false);
boxOffices.update(officeFH2, office2); // Guarding rule is no longer true.
persons.insert(new Person("Edson", 35));
executor.run(BoxOfficeUnit.class); // No execution

assertEquals( 0, list.size() );

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