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

[mgommeringer]

Thanks for the reply, t-dome. Your solution has the advantage, that i only need to re-implement three classes: DBMessageService, DBMessageServiceFactory (for jbpm.cfg) and CommandExecutor. I my eyes, the only thing that is missing for clustering is the exception handling for the line


  | jbpmContext.getSession().lock(message, LockMode.UPGRADE);
  | 

When a LockAcquisitionException is thrown here, we must try to get the next message and skip the one we could not lock.

CommandExecutor.executeCommand():

  | ...
  | message = getAndLockNextMessage(dbMessageService, destination, jbpmContext.getSession());
  | // message = dbMessageService.receiveNoWait(destination);
  | 
  | // If we got a message here, we own the lock
  | if (message != null) {
  | 	checkForMoreMessages = true;
  | 	Command command = (Command) message;
  | 	log.trace("executing command '" + command + "'");
  | 	command.execute();
  | 	// Because our DBMessageService does not delete the
  | 	// message, we must delete it here
  | 	jbpmContext.getSession().delete(message);
  | }
  | ...
  | 


  | private Message getAndLockNextMessage(DbMessageService dbMessageService, String destination, Session session) {
  |     Message message = dbMessageService.receiveNoWait(destination);
  |     if (message != null) {
  | 	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 getAndLockNextMessage(dbMessageService, destination, session);
  | 		} else {
  | 			throw e;
  | 		}
  | 	}
  |     } else {
  | 	return null;
  |     }
  | }
  | 

>From the OO-Point-of-view, this is not the best solution (for me). I think that the MessageService itself, regardless of which implementation is used, should ensure, that the method "nextMessage()" or "receiveNoWait()" only returns Message objects, that have successfully been locked.

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