[rules-users] Misunderstanding salience?

[David Faulkner]

Peter,

The EXACT way to accomplish the functionality that you are looking for is "activation-group"; if two rules are in the same activation group, only one of them will fire. Note that the rule with HIGHER salience will fire first; to accomplish what you are looking for you'd have to give the exception rule a higher salience.

I would also note that although there are specific instances where activation-group has a strong need, many in the community find that the most power and flexibility from the rule engine comes from "letting go" of trying to exactly order your rule execution, and instead letting the rule engine decide what would happen here. One way to accomplish this in your case would be to simply add (isZombie == false) to your constraint on the general rule.  Another way that involves salience but NOT agenda groups is to set a high salience on your exception rule, but only add advice if advice is null. The possibilities are endless.

With kind regards,
David Faulkner
david.faulkner at amentra.com

From: rules-users-bounces at lists.jboss.org [mailto:rules-users-bounces at lists.jboss.org] On Behalf Of Peter Ashford
Sent: Wednesday, March 09, 2011 7:24 AM
To: Rules Users List
Subject: Re: [rules-users] Misunderstanding salience?

Actually, I think I've figured this one out : in the Zombie case, it's firing both rules and it's just that with the negative salience,  the zombie exception rule is the last rule fired, therefore, the last thing written into advice.

So... what would be the correct way to do what I'm trying to do here?  The idea is that the Zombie exception rule should fire in preference to the general rule and that none of the general processing should occur at all (imaging that these rules had side-effects for the rest of the system they're attached to, we don't want all the general rule side effects to apply and then all the exception case side effects)

Thanks in advance!

Peter.

From: rules-users-bounces at lists.jboss.org [mailto:rules-users-bounces at lists.jboss.org] On Behalf Of Peter Ashford
Sent: Wednesday, 9 March 2011 1:31 p.m.
To: rules-users at lists.jboss.org
Subject: [rules-users] Misunderstanding salience?

Hi There

I'm new to drools.  I've just set up the Drools-Server and it is (finally!) working and serving my test rule-set.  The one thing that's not working as I expect it is the rule ordering via salience.  This is my simple test rule set:

rule "General brain eating advice"
       when
              p : Patient(eatsBrains == true)
       then
              p.setAdvice("Stop eating brains, or at least, try to cut down");
end

rule "Zombie exception to brain eating advice"
       salience -50
       when
              p : Patient(eatsBrains == true, isZombie == true)
       then
              p.setAdvice("Evidence suggests that the undead cannot contract Kuru or that the effects are irellevant given the " +
                              "patient's current zombified state.\nSuggest euthenasing patient lest he/she eat your (or someone " +
                              "else's) brains");
end


The idea is that the first rule fires all the time unless the patient happens to be a zombie, in which case the exception rule (the second rule) kicks in.  Now, as I have it here, with the exception at salience at -50 it actually works, which is the opposite of what I was expecting.  I'd thought that I would have had to have the exception at a higher salience to fire first.  That was what I tried first but that didn't work - everyone got the general advice, zombies included.

What am I misunderstanding here?

Thanks!

Peter.

---
"It is very difficult to get a man to understand something when his tribal identity depends on his not understanding it" - Michael Bérubé on Republican climate change denial.

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