[rules-users] StatefulKnowledgeSession synchronization problem

[heldeen]

Hello,

Using Drools 5.1.0.M1.
I am running into a blocking problem when I concurrently try to update a
fact in my knowledge session, which is fireUntilHalt() in its own thread,
from another thread, an updator thread. I know that only these 2 threads are
ever accessing each knowledge session. 

Has anyone else run into this problem? 

I am including the relavant details and can provide a working example if
need be. The block happens randomly anywhere from 20 - 1000's of sessions.

I consistantly get this stack trace from the blocked thread:
org.drools.common.AbstractWorkingMemory.executeQueuedActions(AbstractWorkingMemory.java:1540) 
org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:1529)
org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:1403)
org.drools.impl.StatefulKnowledgeSessionImpl.update(StatefulKnowledgeSessionImpl.java:248)
...

I have a simple drools knowledge base that is a basic rule flow [Start -> 
Rule Flow Group -> Event Wait -> Rule Flow Group -> End]

I build a knowledge base like this:

      KnowledgeBaseConfiguration knowledgeBaseConfiguration =
KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
      knowledgeBaseConfiguration.setOption(AssertBehaviorOption.EQUALITY);
      knowledgeBaseConfiguration.setOption(MBeansOption.ENABLED);

      KnowledgeBase knowledgeBase =
KnowledgeBaseFactory.newKnowledgeBase(knowledgeBaseConfiguration);

      KnowledgeBuilder kbuilder =
KnowledgeBuilderFactory.newKnowledgeBuilder();
      kbuilder.add(ResourceFactory.newClassPathResource(CHANGE_SET_XML,
getClass()), ResourceType.CHANGE_SET);
      if (kbuilder.hasErrors()) {
        throw new IllegalArgumentException("Unable to build knowledge");
      }
      knowledgeBase.addKnowledgePackages(kbuilder.getKnowledgePackages());


My basic test fact is:

public class Fact() {

  String name;
  boolean isReady;

  //getters and setters
}

I have a fixed thread pool (Sun's ExecutorService.newFixedThreadPool() with
20 threads). Each session is submitted from a for loop of 0 to 100000 where
it generates a simple fact with Fact.name = "fact"+i and Fact.isReady =
false, then puts it in an array like so:
final Object[] facts...
fixedPoolExecutor.submit(new Runnable() {
  @Override
  public void run() {
    StatefulKnowledgeSession knowledgeSession = null;
    try {  
      knowledgeSession = knowledgeBase.newStatefulKnowledgeSession();
      //put it in a ConcurrentHashMap keyed by session id.
      ...
      for (Object fact : facts) {
        knowledgeSession.insert(fact);
      }

      //add session completion halt event - omiting other unused methods
      knowledgeSession.addEventListener(new ProcessEventListener() {

        @Override
        public void afterProcessCompleted(ProcessCompletedEvent event) {
          knowledgeSession.halt();
        }
      });

      knowledgeSession.startProcess(PROCESS_ID);

      knowledgeSession.fireUntilHalt();
    } finally {
      if (knowledgeSession != null) {
        knowledgeSession.dispose();
      }
    }
});

I have a single thread executor (Sun's
ExecutorService.newSingleThreadExecutor()) that is used to update these
sessions (basically set the Fact.isReady from false to true for the
EventWait in my rule flow):

final int sessionId...
final Object[] facts...
singleThreadExecutor.submit(new Runnable() {
  @Override
  public void run() {
    StatefulKnowledgeSession knowledgeSession = null;
    try {  
      knowledgeSession = ... from ConcurrentHashMap based on sessionId

      FactHandle factHandle = knowledgeSession.getFactHandle(fact);
      if (null != factHandle) {
        knowledgeSession.update(factHandle, fact);//This is where it blocks
and never returns
      } else {
        knowledgeSession.insert(fact);
      }
});

-Heath
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