Drools, How to disable forward chaining? - drools

Question

I am using Drools rule engine and I was wondering if it was possible to define when a rule can be executed. For example:
If executing a rule A leads to executing 10 rules B1 to B10, is it possible to choose to execute only one of the Bi rules as a result of rule A??
An example would be :
rule "Rule A"
When
$var : Data(value>10)
then
doSmthg();
Event e = new Event();
insert(e);
end;
rule "Rule B"
When
$var : Data(range >100)
then
doSmthg();
Event e = new Event();
insert(e);
end;
rule "Rule C"
When
$e : Event()
then
doSmthg();
end;
Firing Rule A would lead to the execution of rule C.
Firing Rule B would lead to the execution of rule C.
Is there a way for me to make it so that rule C won't be fired even after rule A is executed? At the same time, firing B should still lead to the execution of rule C.
EDIT:
I have other rules that I need to be fired if Rule A is executed for example :
rule "Rule D"
When
$e : Event()
then
doSmthgElse();
end;
so in this case, I just want to disable Rule C from being fired without altering my other rules.

Answer 1

You are triggering subsequent rules by inserting data into working memory. If you don't want a rule to trigger a 'downstream' rule, don't insert the data. Or, alternatively, update the downstream rule to not trigger on the condition.
Consider these three rules, which are the same as yours but with syntax cleaned up and indentations fixed.
rule "Rule A"
when
Data( value>10 )
then
doSmthg();
insert(new Event());
end
rule "Rule B"
when
Data(range > 100)
then
doSmthg();
insert(new Event());
end
rule "Rule C"
when
$e : Event()
then
doSmthg();
end
Let's assume your input is Data( value = 100, range = 500 ). This is what will happen:
Rule A will hit because the condition value > 10 is met.
Rule B will hit because the condition range > 100 is met.
Rule C will hit twice because there are two instances of Event in working memory (one added by Rule A, one added by Rule B.)
Per your question, we don't want Rule C to fire because of Rule A in this condition. To do this, we need to change Rule A to not trigger Rule C. This is simple: remove the insert(new Event()) from the right hand side of Rule A.
rule "New Rule A"
when
Data( value > 10 )
then
doSmthg();
// No insert!
end
Rule B will still trigger because its condition (range > 100) is still met and it still inserts the Event instance into working memory.
Alternatively if what you're actually trying to do is have both A and B trigger C, but only to trigger C once, you can do that by inserting a semaphor into working memory to indicate that C is fired.
Here is an example:
rule "Rule A"
when
Data( value>10 )
then
doSmthg();
insert(new Event());
end
rule "Rule B"
when
Data(range > 100)
then
doSmthg();
insert(new Event());
end
rule "Rule C"
when
not( EventTriggered() )
$e : Event()
then
doSmthg();
insert( new EventTriggered() )
end
For the same input ( Data( value = 50, range = 500 ) ) this is what will happen:
Rule A will insert Event into working memory
Rule B will insert Event into working memory
Rule C will trigger from one of these Events. It will insert an instance of EventTriggered into working memory.
Rule C will not trigger a second time because the not() condition is not satisfied.
Using this setup, an input of Data( value = 5, range = 500 ) will still trigger Rule C via B:
Rule A will NOT fire (condition not met, value <= 10)
Rule B will fire, Event inserted into working memory
Rule C will fire.
And further, an input of Data( value = 50, range = 0) will also trigger Rule C via A:
Rule A will fire, Event inserted into working memory
Rule B will NOT fire (condition not met, range <= 100)
Rule C will fire.
Which solution you choose depends on what your actual requirements are.
If your requirement is that Rule A should never trigger Rule C, then the first solution (removing the insert statement) is the way to go.
If your requirement is that both Rule A and Rule B should trigger Rule C, but only once, then the second solution is the way to go.