[rules-users] drools quirks

[Wolfgang Laun]

On 18 September 2010 17:19, Matt Young <solid at youngdev.net> wrote:

>        ksession.execute(document);
>

I suppose this inserts a single EventRulesDocument object.


> Here is the rule:
>
> package sample1
>
> import sample1.bean.EventRulesDocument;
> import sample1.bean.Transaction;
> import java.util.Date;
>
> rule "Standard processing rule"
>
>    when
>          $doc : EventRulesDocument(
>            ruleRun == false &&  # <<< Wolfgang
>            event != null && event.eventType.eventCode == "testevent" )
>    then
>
>       Transaction t = new Transaction();
>        t.setEvent($doc.getEvent());
>        t.setCategory($doc.getEvent().getEventType().getEventCode());
>        t.setTransactionAmount($doc.getEvent().getEventAmount());
>         t.setCreateTs(new Date());
>         t.setUsername($doc.getEvent().getCreateUsername());
>
>        modify( $doc ) {setRuleRun(true)};   # Wolfgang <<<<<<
>        modify( $doc ) {addSaveTransaction(t)} ;
>

Here it is: You modify the EventRulesDocument, which causes another
activation of the same rule. One way of avoiding this is to add the rule
attribute no-loop true; another one is what you've done.

This isn't going to be your only rule?? This one alone is scant reason to
use an RBS.

fireAllRules( 1 ) isn't a good remedy, because (normally) a Production
System
relies on repeated firings, depending on rules and facts.


> end
>

> Additionally If any code in the world can be executed in the RHS, then
> why shouldn't I just use groovy as my rule language?  At least then, I
> can write math like such:
> def num1=95;
> def num2=90;
> def num3=num2/num1;
>
> and all of the math is done with big decimal objects.  IF there is no
> restriction on what can be done in the RHS what is the advantage to
> using a rules engine over a general purpose scripting language?


There is a good section in the Drools Expert manual: "Why use a rule
engine?".
Don't do it just to be trendy ;-)


>  The
> whole purpose for me choosing drools over Groovy eval() was that I
> didn't want to give my users the ability to spawn threads, allocate
> massive amounts of memory etc.
>

Seriously now, a Domain Specifiy Language is the maximum corset you can
force upon your users. But you'll have to know *all* they *may *do, and all
the
conditions.

-W


>
> On 09/18/2010 10:59 AM, Wolfgang Laun wrote:
> > On 18 September 2010 16:15, Matt Young <solid at youngdev.net
> > <mailto:solid at youngdev.net>> wrote:
> >
> > I just started using drools and this is my first time implementing
> > a rules engine.  Everything has been pretty smooth but I have
> > some quirks that I am not sure I can live with. 1) for some reason,
> > if I execute the Knowlege session against an object, the knowlege
> > session never returns.
> >
> >
> >> I don't understand the term "execute...against an object". Do you
> >> mean that you have just one fact inserted befor you call
> >> fireAllRules()?
> >
> >> But anyway, this call not returning is almost certainly due to a
> >> loop in your rules, or have you made sure that they don't?
> >
> >
> > The only way I can get the ksession to return is to make IF (obj ==
> > null) part of the LHS and make modify($input){setObj("complete")}
> > part of the RHS This seems like a deficiency since I have users
> > writing their own rules, I can see them forgetting this
> > requirement.
> >
> >
> >> Writing rules is programming, no holds barred :-)
> >
> >> Look into Domain Specific Languages (DSL) as a cushion for the
> >> unwary.
> >
> >
> >
> > IS their a way to get the ksession to firerules only once?  If so
> > what does that look like?
> >
> >
> >> kSession.fireUntilHalt( 1 );
> >
> >
> > 2) It seems that any code I want can be executed in the RHS.  I
> > could literally execute something like the following in the RHS.
> >
> > byte[] b= new byte[10000000000000]; // Really big memory waste
> >
> > I also could just start a bunch of threads.
> >
> > The point is that I am intending to let the users write their own
> > rules but I can't do that if there are no restrictions on how/what
> > can be done inside the rules.  Any suggestions?  Are there
> > sandboxes or filters I can activate to restrict the RHS?
> >
> >
> >> Again: DSL.
> >
> >> Also, Rule Templates might be the starting point; it'd depend on
> >> what the may be allowed to do.
> >
> >> And what about good old-fashioned training?!
> >
> >> -W
> >
> >
>
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>
>
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> - --
> Cheers,
> Matt Young
> solid at youngdev.net
> http://youngdev.net
>
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