> What pattern  are you using for synchronization  in middle  tier?

What do you mean by this? Are you going to deploy your application on a cluster
of application
servers and trying to keep all your nodes in-sync? Or are you caching your
entities and need to
keep your cache and backend RDBMS in-sync?

> There is a class  registration system, every class has a limited
> capacity, system  should not register students in any class more than
> its capacity. We want to handle  this in middle tier  (EJB).

I'm still not sure what are you asking for but you can simply use a stateless
session bean for
this. Sure the client might access the SLSB via a Session Facade or a Business
Delegate but this
check for capacity is better not be done in one of your Entity Beans' setters,
etc.