4.5. Fact propagation modes in the decision engine

The decision engine supports the following fact propagation modes that determine how the decision engine progresses inserted facts through the engine network in preparation for rule execution:

By default, the Phreak rule algorithm in the decision engine uses lazy fact propagation for improved rule evaluation overall. However, in few cases, this lazy propagation behavior can alter the expected result of certain rule executions that may require immediate or eager propagation.

For example, the following rule uses a specified query with a ? prefix to invoke the query in pull-only or passive fashion:

query Q (Integer i)
    String( this == i.toString() )
end

rule "Rule"
  when
    $i : Integer()
    ?Q( $i; )
  then
    System.out.println( $i );
end

For this example, the rule should be executed only when a String that satisfies the query is inserted before the Integer, such as in the following example commands:

KieSession ksession = ...
ksession.insert("1");
ksession.insert(1);
ksession.fireAllRules();

However, due to the default lazy propagation behavior in Phreak, the decision engine does not detect the insertion sequence of the two facts in this case, so this rule is executed regardless of String and Integer insertion order. For this example, immediate propagation is required for the expected rule evaluation.

To alter the decision engine propagation mode to achieve the expected rule evaluation in this case, you can add the @Propagation(<type>) tag to your rule and set <type> to LAZY, IMMEDIATE, or EAGER.

In the same example rule, the immediate propagation annotation enables the rule to be evaluated only when a String that satisfies the query is inserted before the Integer, as expected:

query Q (Integer i)
    String( this == i.toString() )
end

rule "Rule" @Propagation(IMMEDIATE)
  when
    $i : Integer()
    ?Q( $i; )
  then
    System.out.println( $i );
end