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In MDB's concurrency refers to the processing of more than one message at a time. If MDB's could process only a single message at a time, then they would practically be useless in a real-world application because they could not handle heavy message loads.
They show a diagram where in three message arrives and three instances of MDB's are pulled out from the pool to process these messages. My question here is doesn"t concurrency mean processing of the all three messages by a single MDB instance? Please advice guys!
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Generally speaking for queues, you could have a pool of MDBs each receiving a subset of the messages coming in but there are two things to be worried about:
- You can't control order of messages in queues. You never can. You may process messages that are newer than some old messageswaiting to be processed (even in the case of a single MDB in the pool this can happen).
- You can't control which MDB in the pool gets messages so you need some kind of persistent storage device like a database coordinating multi-step communications such as negotiating a customer transaction.
And if you follow the EJB spec, everything is thread safe. The only time you violate thread safe is if you spawn your own threads. [ September 28, 2008: Message edited by: Scott Selikoff ]