Well, last time I used this title I got some useful comments. Meanwhile I fixed my problems and in fact it was a race condition that I couldn't fix by using locks.
The problem I was facing had to do with the fact that I have a graph of objects that I maintain in a database. Adding a new object to the graph, means that certain objects would point to it as being a new neighbor. Which objects in the graph would get a new neighbor is determined by an algorithm that ensures that this process is 'fair' to the other objects in the graph.
When two or more objects are added more or less simultaneously to the graph, my algorithm would do the same thing and select the same objects that would receive a neighbor. One might argue that the system on this topic is truly stateless.
Anyway, I tried to fix my algorithm to not always pick the same neighbors, that didn't work. I tried to change my datamodel such that the system would be a little more lightweight, that obviously didn't work either. I tried to work with locks which ensured consistency in my model, but still no scalability on this end. I did some tweaking with transaction-scope and that fixed some issues. But finally I realized that I needed the EJB 3.1 Singleton which is not available in 3.0 so I mimicked part of its functionality by introducing a synchronized block on the smallest piece of code that suffered from the concurrency. This together with a smaller transaction scope fixed the problem.
But since that doesn't give me that warm and fuzzy feeling. Especially not since I know there's a solution in EJB 3.1, I'm now considering moving to GF 3 and use EJB 3.1 features. Of course all thru NetBeans' support for GF3 and EJB 3.1.
I'll keep you posted.