[rules-users] On demand events

[James Willans]

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