A condition based on the negated existence quantifier is true when no such object is in the WM. Once recognized as true, the rule fires, and that's it until it isn't true any more, which is a sort of "rewind" for the condition after which the begin of another period of absence is celebrated with another firing. Repeated stop-and-go of the rule engine does not influence this monitoring of truth.

-W


2011/8/20 Chris Richmond <crichmond@referentia.com>
Well..I insert some objects, fire the rules and this rule will trigger the first time (when it finds no object with those characterstis) but every time after than when I insert more objects and fire the rules, the rule never fires again. I have no idea why.  Here is my simple test case.

Two clasess: TestMain and TestObject and rule file Test.drl I have included below.

It insterts a group of facts at one time, fires the rules, and retracts all those facts from the stream.  I have an event listener on the session, as you see to verify injections and retractions are occuring. 
So the rule fires on the first batch, but on no other batches after that???? What gives???? 

Among the Inserttion and retraction events I only see:

A proper object does not exist

One time, during firing rules on the first batch.  Why does this rule never fire again, even though every single batch of objects I insert/retract does not contain the proper rule values, and so should fire the rule.

What is going on???





TestMain.java *************************************

package com.sample;

import java.util.ArrayList;
import java.util.List;

import org.drools.KnowledgeBase;
import org.drools.KnowledgeBaseConfiguration;
import org.drools.KnowledgeBaseFactory;
import org.drools.builder.KnowledgeBuilder;
import org.drools.builder.KnowledgeBuilderError;
import org.drools.builder.KnowledgeBuilderErrors;
import org.drools.builder.KnowledgeBuilderFactory;
import org.drools.builder.ResourceType;
import org.drools.conf.EventProcessingOption;
import org.drools.event.rule.ObjectInsertedEvent;
import org.drools.event.rule.ObjectRetractedEvent;
import org.drools.event.rule.ObjectUpdatedEvent;
import org.drools.event.rule.WorkingMemoryEventListener;
import org.drools.io.ResourceFactory;
import org.drools.runtime.KnowledgeSessionConfiguration;
import org.drools.runtime.StatefulKnowledgeSession;
import org.drools.runtime.conf.ClockTypeOption;
import org.drools.runtime.rule.FactHandle;
import org.drools.runtime.rule.WorkingMemoryEntryPoint;

public class TestMain {

  @SuppressWarnings("restriction")
  public static void main(String[] args) {
   

    try {
     
      KnowledgeSessionConfiguration config = KnowledgeBaseFactory.newKnowledgeSessionConfiguration();
      config.setOption( ClockTypeOption.get("realtime") );
     

      KnowledgeBase kbase;
      kbase = readKnowledgeBase();
      final StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
     
      WorkingMemoryEntryPoint myStream = ksession.getWorkingMemoryEntryPoint("My Stream");
     
      ksession.addEventListener(new WorkingMemoryEventListener(){

        @Override
        public void objectInserted(ObjectInsertedEvent oie) {
          System.err.println("Inserted: " + oie.toString());
         
        }

        @Override
        public void objectRetracted(ObjectRetractedEvent arg0) {
          System.err.println("Retracted: " + arg0.toString());
         
        }

        @Override
        public void objectUpdated(ObjectUpdatedEvent arg0) {
          // TODO Auto-generated method stub
         
        }
       
      });
     

      for (int a = 0; a < 1000; a++){
        List<FactHandle> factHandles = new ArrayList<FactHandle>();
        for (int x = 0; x < 6; x++){
         
          double reading = 11.3;
          float f = (float)reading;
          TestObject dr = new TestObject("Reading " + x, f);
          FactHandle fh = myStream.insert(dr);
          factHandles.add(fh);

   
         
        }
        ksession.fireAllRules();
        for(FactHandle fh : factHandles){
          myStream.retract(fh);
        }
        Thread.sleep(4000);
      }
     
    } catch (Exception e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }

  }

  @SuppressWarnings("restriction")
  private static KnowledgeBase readKnowledgeBase() throws Exception {
    KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
    kbuilder.add(ResourceFactory.newClassPathResource("Test.drl"), ResourceType.DRL);
    KnowledgeBuilderErrors errors = kbuilder.getErrors();
    if (errors.size() > 0) {
      for (KnowledgeBuilderError error: errors) {
        System.err.println(error);
      }
      throw new IllegalArgumentException("Could not parse knowledge.");
    }
   
    KnowledgeBaseConfiguration kbConfig = KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
    kbConfig.setOption( EventProcessingOption.STREAM );
    KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase(kbConfig);
    kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());
   
   


    return kbase;
  }

 


}

TestObject.java *************************************
package com.sample;

public class TestObject {

 

  private String name;
  private float reading;
 
 
  public TestObject(String name, float reading){
    this.name = name;
    this.reading = reading;
  }


  public String getName() {
    return name;
  }


  public void setName(String name) {
    this.name = name;
  }


  public float getReading() {
    return reading;
  }


  public void setReading(float reading) {
    this.reading = reading;
  }
}


Test.drl*************************************************
#created on: Aug 19, 2011
package com.sample



rule "Test For Lack of Objects with criteria"
   
when
        not(TestObject(name=="blah")  from entry-point "My Stream")
then
        System.err.println("A proper object does not exist");
end




On 8/18/2011 8:46 PM, Wolfgang Laun wrote:
On 19 August 2011 15:17, Chris Richmond <crichmond@referentia.com> wrote:
How do I fire a rule if an object with certain characterstics does not
exists.

For example my class Foo, if I have a rule:

rule "Identify Foos with values"

when
        Foo(stringProp=="blah", intProp==5)
then
        System.err.println("A Foo was found!");
end


So how do I check for lack of existence of an object with certain
characteristics

I tried:
rule "Flag missing Foos with values"

when
        not(Foo(stringProp=="blah", intProp==5))
then
        System.err.println("A proper foo does not exist");
end

That's the way to do it.
 

I also tried:
rule "Flag missing Foos with values"

when
        not(exists(Foo(stringProp=="blah", intProp==5)))
then
        System.err.println("A proper foo does not exist");
end

Since not( Foo(...)) means "if no Foo(...) exists", the addition of "exists" is superfluous and generally considered bad style (even though some systems accept it to mean just "not".

-W
 
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