Re: [rules-users] Fire rule only upon change of particular event (or fact)

Hi all, after using drools for some time now, I have repeatedly stumbled upon situations where it would have been useful to be able to tell Drools to fire a rule only upon the insertion of a particular event, but not other events/facts on which the rule depends. It might be that I am hopelessly mis-guided, but I would like to float an idea and see what you think. To illustrate what I am talking about, please consider the following rules: ------------------------------------------------------------------------------------------------------------ package test declare SomeEvent @role(event) name: String end declare SomeFact @role(fact) name: String end rule "Trigger on some event when there is some fact" when e: SomeEvent() f: SomeFact() then System.out.println("=> Got Event '" + e.getName() + "' and Fact '" + f.getName() + "'"); end rule "Discard 'used' events" salience -99999 when e: SomeEvent() then retract(e); end ------------------------------------------------------------------------------------------------------------ Now we insert facts and events, for example like this ------------------------------------------------------------------------------------------------------------ rule "Insert fact" when then System.out.println("Inserting initial Fact"); insert(new SomeFact("initial fact")); end rule "Insert an event" timer(int: 5s) when then System.out.println("Inserting initial Event"); insert(new SomeEvent("initial event")); end rule "Insert yet another fact" timer(int: 10s) when then System.out.println("Inserting yet another fact"); insert(new SomeFact("yet another")); end rule "Insert yet another event" timer(int: 10s) when then System.out.println("Inserting yet another event"); insert(new SomeEvent("yet another")); end ------------------------------------------------------------------------------------------------------------ The result of running those rules is, of course: ------------------------------------------------------------------------------------------------------------ Inserting initial Fact Inserting initial Event => Got Event 'initial event' and Fact 'initial fact' Inserting yet another fact => Got Event 'initial event' and Fact 'yet another' Inserting yet another event => Got Event 'yet another' and Fact 'yet another' => Got Event 'yet another' and Fact 'initial fact' ------------------------------------------------------------------------------------------------------------ Please note the output does not match the name of the rule "Trigger on some event when there is some fact". In fact, this rule fires irrespective of whether an event is inserted or a fact. But what I really want is, that it just fires upon the 'arrival' of events. A possible syntax extension whould be something along the lines of: ------------------------------------------------------------------------------------------------------------ rule "Trigger on some event when there is some fact" when upon(e: SomeEvent()) f: SomeFact() then System.out.println("=> Got Event '" + e.getName() + "' and Fact '" + f.getName() + "'"); end ------------------------------------------------------------------------------------------------------------ The expected output of that would be: ------------------------------------------------------------------------------------------------------------ Inserting initial Fact Inserting initial Event => Got Event 'initial event' and Fact 'initial fact' Inserting yet another fact Inserting yet another event => Got Event 'yet another' and Fact 'yet another' => Got Event 'yet another' and Fact 'initial fact' ------------------------------------------------------------------------------------------------------------ There are work-arounds that let me achieve the desired behaviour, for example this one: ------------------------------------------------------------------------------------------------------------ rule "Focus on 'process SomeEvent'" agenda-group "process SomeEvent" auto-focus when e: SomeEvent() then // nothing to do but focus the agenda-group end rule "Trigger on some event when there is some fact" agenda-group "process SomeEvent" when e: SomeEvent() f: SomeFact() then System.out.println("=> Got Event '" + e.getName() + "' and Fact '" + f.getName() + "'"); end ------------------------------------------------------------------------------------------------------------ However, I think the hypothetical 'upon'-keyword would be a lot clearer. Is there any fundamental reason (e.g. rooted in the rete-algorithm or the way Drools implements it) that would preclude the implementation of 'upon'? And, more importantly, do you even agree with me that it would be a useful extension? Please note that while I always spoke of events in conjunction with 'upon' I don't see a reason why it should not behave this way with plain facts as well. Please not as well that the inverse of upon would also be conceivable: "don't fire this rule just because <some pattern> triggered", e.g. maybe simply "not(upon(Pattern()))". Thanks Joerg Henne -- View this message in context: http://drools.46999.n3.nabble.com/Fire-rule-only-upon-change-of-particula... Sent from the Drools: User forum mailing list archive at Nabble.com. _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users