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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