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Question

I am inserting data to drools rule engine but i can not understand how it processed inserted data. The code for inserting data is:

final StatefulKnowledgeSession session = getSession()
        new Thread() {
                    @Override public void run() {
                        Thread.currentThread().setName("RuleEngineThread")
                        println 'engine starting fire'+Thread.currentThread().getName()
                        session.fireUntilHalt();
                    }
                }.start();
        WorkingMemoryEntryPoint entrypoint=session.getWorkingMemoryEntryPoint("Multiple Stream")
        entrypoint.insert(new Categories([categoryid:120,name:"catN1"]))
        entrypoint.insert(new Test(100,120))

        entrypoint.insert(new Categories([categoryid:121,name:"catN2"]))
        entrypoint.insert(new Test(100,121))
        entrypoint.insert(new Categories([categoryid:1220,name:"catN3"]))
        entrypoint.insert(new Test(100,1220))
        entrypoint.insert(new Categories([categoryid:1202,name:"catN4"]))
        entrypoint.insert(new Test(100,1202))
        println "Thread sleeeping for 3 secs"
        Thread.currentThread().sleep(3000)

don't worry for syntax this is groovy file. And the rule being:

 rule "multiple-opt"
   //duration  120
    no-loop true
 when
 $c: Categories() from entry-point "Multiple Stream"
 $t: Test()  from entry-point "Multiple Stream"
 then
 System.out.println("@@Multiple "+$c.getName()+":"+$t.getPrice());
 end

The output i am getting is quite strange so I think i have less understanding of drools runtime. The output is :

engine starting fireRuleEngineThread
Thread sleeeping for 3 secs
@@Multiple catN1:100
@@Multiple catN4:100
@@Multiple catN3:100
@@Multiple catN2:100
@@Multiple catN1:100
@@Multiple catN4:100
@@Multiple catN3:100
@@Multiple catN2:100
@@Multiple catN1:100
@@Multiple catN3:100
@@Multiple catN2:100
@@Multiple catN1:100
@@Multiple catN2:100

I can not understand how rule got fired so many times while i inserted objects less times than the number of outputs i receive.
Please help if i am missing some knowledge about drools.Thanks in advance

Answer 1

This is a very basic feature of production rule systems: the exhaustive search of all possible combinations as defined by the patterns of a rule.

Categories()    // <= match with any object of class Categories
Test()          // <= match with any object of class Test

You have inserted 4 of each, so the rule will fire for each possible pairing.

Answer 2

Just to add to @laune, depending on your scenario, you can eg retract each pair of facts after a match:

then
 System.out.println("@@Multiple "+$c.getName()+":"+$t.getPrice());
 retract($c);
 retract($t);

Edit

Yes, in order to correlate the pairs, you can use a binding variable and then filter on this in the pattern match:

when
 $c: Categories($catid : categoryid ) from entry-point "Multiple Stream"
 $t: Test(categoryid == $catid)  from entry-point "Multiple Stream"
then
 System.out.println("@@Multiple "+$c.getName()+":"+$t.getPrice());

(Assuming that there is a getCategoryId() getter on class Test . Also, correlating in this way will reduce the number of rule match permutations). You might not need to retract the facts if they are matched in correlated pairs this way.