Question

I am struggling to understand Durable subscription. I understand that when a Listener registers itself as a Durable Subscriber to a Topic, it tells JMS – “Hey, I am durable subscriber, from now onwards you need to store all the messages in Topic if I am not there and pass me those messages when I come back”

Now, if that’s the case, why can’t two subscribers ask for this durable subscription?

Am I missing something?

Answer 1

Quoting from the Java EE tutorial

A durable subscriber registers a durable subscription by specifying a
unique identity that is retained by the JMS provider. Subsequent
subscriber objects that have the same identity resume the subscription
in the state in which it was left by the preceding subscriber. If a
durable subscription has no active subscriber, the JMS provider
retains the subscription’s messages until they are received by the
subscription or until they expire.

To make durable subscriptions work for multiple subscribers on 1 durable subscription the broker would have to store each individual message from the creation of the topic (by the first-ever subscriber) until its expiry, ie potentially forever if no message TTL is specified, because at any point in time a new subscriber can pop in and claim all the messages it “missed” (that is, all messages since the subscription was created). That’s just not feasible.

I may be missing the point here, but I can’t see how having multiple simultaneous subscribers sharing a subscription would be more practical than defining two separate subscriptions?