Byteman plays with Drools

Due to my recent suspicion that there is a bug in Drools I needed to monitor its guts in detail. To be more specific, I needed to see what is in the working memory after execution of all rules.

Debbuger was not a good option since there were hundruds of invocations of the problematic part and I needed to get overview of quite a large amount of data. If I inly could store all the data for each invocation in a file and then grep through it.

But wait! Of course I can. Byteman will help me! The identified point in source code was execution of fireAllRules() in org.drools.common.AbstractWorkingMemory. Ideally, just before return from the method. So here is the corresponding rule:

RULE Debug fireAllRules
HELPER org.drools.planner.examples.tournaments.helper.BytemanHelper
CLASS ^org.drools.common.AbstractWorkingMemory
METHOD fireAllRules
AT EXIT
IF TRUE
DO
  printWorkingMemory($0.getKnowledgeRuntime());
ENDRULE

Just a few notes to it:

• ^ means to apply to child classes as well
• $0 refers to this

As you can see there is a custom helper class to print the working memory content since Byteman rules are little bit limited in the supported Java syntax.

import org.drools.impl.StatefulKnowledgeSessionImpl;
import org.jboss.byteman.rule.Rule;
import org.jboss.byteman.rule.helper.Helper;

public class BytemanHelper extends Helper {
   protected BytemanHelper(Rule rule) {
      super(rule);
   }
   
   public void printWorkingMemory(Object obj) {
      StatefulKnowledgeSessionImpl session = (StatefulKnowledgeSessionImpl) obj;
      debug("vvvvvvvvvvvvvvvvvvvvvvvvvvv");
      debug(session.toString());
      for (Object o: session.getObjects()) {
         debug(o.toString());
      }
      debug("^^^^^^^^^^^^^^^^^^^^^^^^^^^");
   }
}

And now I was able to get some nice output of individual working memories! So the resulting bug is JBRULES-3337.