I'm working on writing a rule set in Drools and the following situation causes an infinite loop by retriggering the rule:
rule "My Rule"
when
a1:ObjectA()
b1:ObjectB(field1 > 0 || a1.field1 in (1,2,3))
ObjectB(field2 > 10)
then
modify( b1 ) { setField3(5) };
end
The following rule change doesn't result in an infinite loop, ie, when a1 is no longer referenced within ObjectB:
rule "My Rule"
when
b1:ObjectB(field1 > 0 || field4 in (1,2,3))
ObjectB(field2 > 10)
then
modify( b1 ) { setField3(5) };
end
Another scenario which doesn't cause an infinite loop is when I change the ||
to an &&
in the second when
line:
rule "My Rule"
when
a1:ObjectA()
b1:ObjectB(field1 > 0 && a1.field1 in (1,2,3))
ObjectB(field2 > 10)
then
modify( b1 ) { setField3(5) };
end
From the Drools docs I understand that calling modify(){}
will "trigger a revaluation of all patterns of the matching object type in the knowledge base," but since the field I'm modifying, field3
, isn't used in the LHS conditions, I didn't think it should reevaluate. However, I am faily certain it has to do with referencing a1.field1
within ObjectB
, but I can't find a concrete reason why. Thanks in advance!
It matters at the object level, not the field level. Since you modify a1
, the rule is re-evaluated because it relies on the ObjectA objects in working memory. Note that the docs indicate it will "trigger a reevaluation of all patterns of the matching object type in the knowledge base." Not the parameter values.
One way you could avoid this would be to add a constraint like field3 != 5
on the left hand side. That is:
rule "My Rule"
when
a1: ObjectA( field3 != 5 ) // <-- if field3 is already 5, don't fire the rule
b1: ObjectB(field1 > 0 || a1.field1 in (1,2,3))
exists(ObjectA(field2 > 10))
then
modify( a1 ) { setField3(5) };
end
Basically you need to make the rule no longer eligible for re-fire.
Alternatively, depending on your structure you could try to use no-loop
, but that only keeps the rule from refiring when immediately triggered by the right hand side. If you have multiple modifications/updates/etc going on, you could get into a "broader" looping scenario where multiple rules cause reevaluation. (Example: Rule A does a re-evaluate, no-loop keeps A from firing again; then B causes re-eval, so A can trigger because no-loop does not apply.)
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