[jboss-user] [JBoss jBPM] - Re: CommandExecutor in aclustered environment

[mgommeringer]

Hi,

I would also like to use the CommandExecutor in an cluster (starting it multiple times on different machines). It sounds as if the DBMessageService will not be cluster-ready in the future. Am i right?

If I am not fully wrong it is no big effort to make the DBMessageService cluster-ready.
I looked at the sources and think that the right place (at least in the current CVS) to ensure that a single message is only processed by one CommandExecutor is org.jbpm.db.MessagingSession. The method "nextMessage(String destination)" should return only Message objects that are not locked by another transaction.

Here my -untested- proposal:


  |   public Message nextMessage(String destination) {
  |     Message message = null;
  |     if (nextMessage!=null) {
  |       message = nextMessage;
  |       nextMessage = null;
  |     } else {
  |       Iterator messageIterator = getMessageIterator(destination);
  |       if (messageIterator.hasNext()) {
  |           // Get the next message that was not locked on DB and lock it
  |           message = getNextUnlockedMessage(messageIterator);
  |       }
  |       if (messageIterator.hasNext()) {
  |           // Get the next message that was not locked on DB and lock it
  |           nextMessage = getNextUnlockedMessage(messageIterator);
  |       }
  |     }
  |     return message;
  |   }
  | 
  |   private Message getNextUnlockedMessage(Iterator messageIterator) {
  |       Message message = (Message) messageIterator.next();
  |       try {
  |     	  // try to lock the message
  |     	  session.lock(message, LockMode.WRITE);
  |     	  // Successfully locked
  |     	  return message;
  |       }
  |       catch(HibernateException e) {
  |     	  if(e.getCause() instanceof LockAcquisitionException) {
  |     		  // Failed to acquire the lock - try to get next
  | 			  return getNextUnlockedMessage(messageIterator);
  |     	  }
  |     	  else {
  |     		  throw e;
  |     	  }
  |       }
  | }
  | 

Is this code okay for this purpose?
I only recognized that several JBPM classes (also the PersistenceService which creates the MessagingSession) must be modified/overridden in order to plug in a custom implementation here and I would dislike to do this within my project.

Thanks,
Matthias

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