Tuesday, July 27, 2010

Deploying JBPM Enterprise in OC4J, part 2: OC4J all other the place

To continue where I left:

1. Before deploying in OC4J I decided to get rid of semi-manual repackaging: I suspected I would need to do it a lot because of OC4J specific deployment descriptor. I succeeded. It is not an interesting subject, it is not the topic of this post, and I probably would not make many friends in maven or m2eclipse community if I say aloud what I think, so I say little: it is definitely not the primary task for maven.

2. Honestly, I do not know how many more times I had to change ejb-jar.xml before OC4J stopped complaining about JMS and started complaining about other things. Even more: some of those changes broke unit tests so I had to do even more changes to compensate for that. Eventually OC4J gave up: no more JMS errors during deployment.

3. Now I have TopLink errors (TopLink classes are not on the classpath). Remember I said the project was using Hibernate native API? The team had some problems making it work in OC4J with TopLink enabled, so they switched TopLink off for their application. I do not know how real the problems were, but this was quite a reasonable thing to do: your application is not using JPA, so you do not need support libraries for that, so you instruct OC4J to remove them from classpath.

But it turns out that OC4J is using TopLink for their entity bean support. And jbpm-enterprise has an entity bean. WTF? You are using Hibernate for all your persistency, "the best ORM in the world" (YMMV), and all of the sudden you need an entity bean?! OK, I have seen that bean, the code, and the deployment descriptor entries while I was busy with JMS configuration, so I knew it is there, and it did not give any troubles in openejb, so I did not worry about it...

Payback time. And more things to learn: I have never used the EJB timer service before, and the sole purpose of this entity bean is to work with the EJB timer service. It took me some reading to understand why jbpm-enterprise has two beans (an SLSB and this entity bean) to work with the EJB timer service, why the SLSB is deprecated, and what the reason for that deprecation is. I should say it is a clever trick, completely spec compliant and that I would probably never think of something like this. To introduce an entity bean, the only entity bean in the whole project, as a solution to a performance problem... Of course, if it is the only solution I would not say a word.

But within 5 minutes after I understood the reason why the entity bean is there I came up with another solution to the same problem that did not require any entity beans. Theoretically that should work, I did not have time to try it, but my guess I will get a chance in the near future. Anyway, at that moment the application did not use jbpm timers, so I went back to the deployment descriptor and now I had one bean less to deploy. Full build cycle, no unit test failures, deploy, no errors, success.

4. Almost all the time while working on this jbpm-enterprise I was thinking: drop it, it will not work. Do you ever get a feeling "one last step, and you are there"? Probably because of this feeling I carried on. And the feeling paid off, except I did not believe it. Successful deployment does not mean no errors at run time.

So I fired manual test scenario and got a big hello from OC4J JMS Database Persistence again. Very big. Stacktraces dozens of lines long. The exception was coming from JMS session.close() in org.jbpm.msg.jms.JmsMessageService. It said basically "rollback is not allowed in this context". Huh? Sequel "WTF comes back and strikes again". It is the same WTF I have mentioned before: javax.jms.Connection.createSession() parameters, nicely commented in class org.jbpm.msg.jms.JmsMessageService:

* If the connection supports XA, the session will always take part in the global transaction.
* Otherwise the first parameter specifies whether message productions and consumptions
* are part of a single transaction (TRUE) or performed immediately (FALSE).
* Messages are never meant to be received before the database transaction commits,
* hence the transacted is preferable.
session = connection.createSession(true,

When I was running my test application with JMS Database Persistence implementation, I ignored all the exceptions from xyz.close() calls. Well, it was a testing application, so why bother. Big mistake. I revisited my test application and put some logging in catch, and sure, closing a session created with createSession(true, ...) would throw the same exception while closing a session created with createSession(false, ...) would not.

The strange thing: even if the exception was thrown, everything would work without problems if the exception was just ignored. Messages were sent and successfully received, or not, depending on what was going on in the sending method (nothing/EJBContext.setRollbackOnly()/throw (un-)checked exception).

5. I went as far as looked at Oracle classes under class disassembler. And sure enough closing a transacted session would trigger a rollback. This was also the explanation why having a transacted session with In-Memory JMS resulted in messages being lost: rollback attempt. Except that with In-Memory JMS session.close() would actually rollback the JMS transaction and succeed, and with JMS Database Persistence rollback would fail, and if that exception is caught, the transaction would be committed by the container. But the exception is reported and rethrown in jbpm-enterprise.

6. This was the last straw. I have actually built a modified jbpm-enterprise with changed parameters to createSession(), and satisfied my feeling of 'being there'. Everything worked. And then I sent an email to the team members saying "forget it, it is just not worth the trouble".

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