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Recently a colleague of mine said this:
When using different sessions for producing and consuming messages, there’s no guarantee that messages are delivered in the same order in which they are produced.
Now I know that JMS does not guarantee message delivery order in general, but this ‘multiple sessions’ angle is new to me. Is there any truth to this?
According to the JMS Spec (section 4.4.10 Message Order) only the order of messages sent from a session to a destination is guaranteed (and even then with some caveats).
This means:
Only the order of two messages sent from one session to the same destination is defined.
Note that it’s possible that a given JMS implementation will usually deliver items in the (global) order in which they are produced. But you should not rely on this in any way, since it’s basically a “best effort” thing: if they can manage it, it’s usually better. If they can’t, they are still well within the specification and your application must be able to handle that.
tl;dr Yes, your colleague is correct. Note again that even the guarantee in the one-session/one-destination case is not very strong, particularly if different message options are involved (see the linked PDF for details).