[rules-users] Avoid evaluation of expensive LHS terms

[Michael Anstis]

Have you tried salience?

On 23 June 2011 20:20, Joe Ammann <joe at pyx.ch> wrote:

> Stumbled over this today in a specific case, but my question is about a
> general practice (I'm quite new to Drools, 5.2.0.M2 btw). I have a
> knowledge session with a fair amount of facts (0.5 Mio) and mostly
> simple rules. One rule has a LHS expression that I know will be
> expensive to execute, in the example that I encountered this it looks like:
>
> rule "Count facts"
>    when
>        all : Number() from accumulate (s : X(), count(s))
>        blacklisted : Number() from accumulate (s : X(status ==
> X.BLACKLISTED), count(s))
>        ....
>    then
>        ....
> end
>
> I want to execute this rule only after other rules have already executed
> (and changed the status of most of the X facts). There's quite a number
> of X facts (~200'000). Most of them have their status modified during
> execution of other rules. If I run the rule base without the above
> rules, performance is perfect, some tens of seconds. If I activate the
> rule, execution never finishes (I stop it after a few minutes, never saw
> it finished).
>
> I *THINK* (correct me if I'm wrong) what happens is that especially the
> second expression of the LHS gets executes over and over again whenever
> one of the facts is modified by one of the other rules. Makes sense, but
> I nevertheless want to try to avoid it.
>
> Until now I tried unsuccessfully:
> - "protect" the rule with a guard fact as the first expression, which is
> inserted only late in the processing
> - move the rule to another ruleflow-group (or agenda-group) and activate
> it only at the end of the processing
> - use the rule in a second independent knowledge base and session, and
> insert the facts once the first has finished
>
> I'm perfectly aware that I could solve this specific problem of counting
> facts with different techniques, but my question is more general (as the
> subject says). Because I have other cases where (as a last resort) I
> need to go to a database or online service to get the required
> information. It seems that others have similar problems
> (
> http://drools.46999.n3.nabble.com/Short-circuiting-evaluations-on-LHS-td56379.html
> ).
> But in my experience, that technique ("protecting" the expensive pattern
> in the LHS with a cheap one that evaluates to false until you really
> need the expensive one) does not work. I get the impression that
> (contrary to popular experience that a "false" value in an AND sequence
> terminates the whole sequence) the drools LHS are processed differently.
>
> Am I missing something really stupid?!
>
> --
> CU, Joe
>
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