[jboss-dev-forums] [Design of JBoss ESB] - Re: Multithreaded services and their EPR(s)

[mark.little@jboss.com]

"kurt.stam at jboss.com" wrote : 1. I think we agree that the registration of the EPR should *not* be done in the thread. 
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  | 2. However as Esteban pointed out that if we register multiple listeners in different address spaces we still have a problem. And I think it is actually useful to know from a registry position that we have mutliple physical listeners, for load balancing and failover scenarios. So should we have these listeners register as 'different' EPRs of the same service?
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  | --Kurt

First, I've mentioned this several times, but this paper http://http://www.orablogs.com/pavlik/archives/The-Session-Concept-in-Web-Services.pdf is extremely important to the general debate on EPRs and Services (don't let the title fool you - it is relevant to our architecture]. Although an EPR is an address, it should NOT be an object-level address (c.f., CORBA IORs). That is the way to close-coupling of clients to services. We need to maintain a loose-coupling. Things like multiple threads in the same "service" are back-end implementation issues. If I have multiple objects within my service and they are able to be dispatched too separately then that information should be encoded within the body of the message and not in the EPR. WS-Addressing lets you do the latter, but then you end up with something like WS-RF which is essentially the CORBA object-key approach. This leads to brittle applications in that if we change the way the back-end implementation works, the EPRs that all clients have then need to change.

Everyone should read the paper.

As for clustering support, that's an interesting question. Logically we want a single service name that can map to multiple EPRs. (Let's ignore the issue of stateful versus stateless services at this point, because that really does complicate the discussion - we'll assume stateless). It could be supported with multiple EPRs registered with the same service name within the EPR (c.f., DNS). Alternatively, you could have a single EPR and use CBR to dispatch across different physical endpoints. Obviously the CBR would then need to maintain the mapping information. There are trade-offs to both approaches. For now, we don't support clustering so we can leave this as an excercise for the informed user of JBossESB 4.0 GA.

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