[rules-users] populating global variable in Condition Section?

[Wolfgang Laun]

 <quote>
It is rather common for people to write something like "(not (exists (A))),"
but this is just a very inefficient way to write (not (A)).
</quote>

Kudos to Ernest Friedman-Hill to put this sentence into the Jess
documentation.

Maybe I should quote hiom in the Drools Expert doc ;-)

-W


On Fri, Aug 21, 2009 at 11:11 PM, Shah, Malay
<Malay.Shah at morganstanley.com>wrote:

> May be I have not put the question right. We have a drool rule that is
> running extremely slow for large sets of data. The rule is something like:
>
> For every object A, perform action if there does not exist an object B such
> that A.id = B.id.
> And the corresponding drool rule is as follows:
>
> when
>    A()
>    not(exists B(A.id = id))
> then
>    action......
>
> The performance is bad because of not exists clause here. We have got OOM
> exceptions with relatively large amounts of data for this rule. To improve
> performance, we basically hacked the rule/code into something like this:
>
> Create a global variable x that is a HashSet of all ids of object A, and
> the drool rule now is:
>
>  global java.util.HashSet x;
> when
>        B()
>        eval
>        (
>                ! (x.contains(b.id) )
>        )
> then
>        action.....
>
> This obviously is performing much better with hashes involved. But, I don't
> like the fact that we have to write the logic of this rule at two places. I
> would rather create this variable x (don't care whether it is global or
> temporarily bound) in the rule itself and use it in the condition to help
> performance and have all logic for the rule at one place.
>
> Is there a way to generate this hashset x in the condition part of the rule
> itself? Yes, I understand that I am trying to mix up the procedural part of
> code with drool code. But, I was just wondering if this is possible to keep
> the java code clean and have the intelligence of information that the rule
> needs to be evaluated in the rule itself.
>
> Hope this clears some doubts.
>
> Thanks
>
> Malay Shah
>
> -----Original Message-----
> From: rules-users-bounces at lists.jboss.org [mailto:
> rules-users-bounces at lists.jboss.org] On Behalf Of Greg Barton
> Sent: Friday, August 21, 2009 4:44 PM
> To: Rules Users List
> Subject: Re: [rules-users] populating global variable in Condition Section?
>
> This is a galactically bad idea.  Using a global in this way is inherently
> unsafe as there's no guarantee that ebtween the time you set the var and
> when it's used later in the condition that the value is the same.  What is
> your reason for wanting to use a global in this way?  You should use a
> temporary bound variable instead.
>
> --- On Fri, 8/21/09, Shah, Malay <Malay.Shah at morganstanley.com> wrote:
>
> > From: Shah, Malay <Malay.Shah at morganstanley.com>
> > Subject: [rules-users] populating global variable in Condition Section?
> > To: rules-users at lists.jboss.org
> > Date: Friday, August 21, 2009, 3:19 PM
> >
> >
> >
> >
> >
> >
> > Hi,
> >
> > Is it possible
> > to populate a global variable in
> > the condition section of the
> > drool rule, and use it later in the condition itself?
> > I currently have a
> > global HashSet variable that I construct before firing the rule, but I
> > would like this code of constructing this global variable to be with
> > the drool rule itself. Any help on this would be much appreciated.
> >
> > Thanks
> >
> > Malay
> >
> >
> >
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