Objects resulting from "from"+expression are not facts in Working Memory,
and thus they aren't events either.

You might investigate a similar approach where you have, for each rule
according to

rule "example"
  when
    Event1(..) from entry-point Event1Store
    Event2(..) from entry-point Event2Store
  then //...
end

an additional rule

rule "example sentinel"
  when
    Event1(..) from entry-point Event1Store
    $es: Event2Sentinel( evRef != null ) //  from entry-point Event2Store
  then
    Event2 ev2 = $event2Provider.evaluate();
    insert( new Event2() );
    modify( $es ){ setEvRef( ev2 ) }
end

You will have to update the sentinel when the Event2 is retracted.
This depends how you want this done, but the following rule should
work in any case_

rule "clear sentinel"
when
    $es: Event2Sentinel( $er: evRef != null )
    not Event2( this == $er )
then
   modify( $es ){ setEvRef( null ) }
end

I'm not sure why you need events in the first place, but you can use
explicit retract for events as well.

I'm not sure whether you need the "from"+entry-point at all.

-W


On 6 February 2011 00:07, James Willans <james.willans@ceteva.com> wrote:
Hello,

I'm a new user of Drools and have experimented with the basics.  I am
interested in processing events and have had success when defining rules
of the type:

rule "example"
  when
    Event1(..) from entry-point Event1Store
    Event2(..) from entry-point Event2Store
  then
    System.out.println("Fired!");
end

Where events of type Event1 and Event2 are continually injected into
entry-points within the working memory.

The events I have in mind are formed from data that sit in data stores
that are expensive to access and I would like to explore strategies for
minimising access to these.  In the above example, I assume that if
Drools does not match the pattern Event1(..) then it will not try and
match the pattern Event2(..), in which case it would not be necessary to
construct facts/events which may match the Event2(..) pattern.

I understand that the 'from' statement can be used to evaluate facts
that are not in working memory, on this basis I have defined proxy
objects that are inserted into working memory which are then evaluated
for the actual data from the database on demand:

1 rule "example"
2  when
3    $event1Provider : Event1Provider()
4    Event1(..) from $event1Provider.evaluate()
5    $event2Provider : Event2Provider()
6    Event2(..) from $event2Provider.evaluate()
7  then
8    System.out.println("Fired!");
9 end

On the basis that if line 4 fails to match, then line 5 and 6 will not
get evaluated therefore avoiding accessing the database describing the
Event2 fact/event.  I think that this works - but only once, since it
looks like the system is treating these as facts, that don't expire,
rather than events.  Is it the case that the 'from' statement cannot be
used for events in this way?  If not, what approach should be taken to
addressing the problem I am trying to solve?

Any direction would be much appreciated.

James
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