Java Message Service, 2nd Edition

Durable Versus Nondurable Subscribers

If you were to run the TBorrower class and then publish several rates, the TBorrower class would pick up the new rate and make a determination as to whether it was a good rate or not. However, if you were to terminate the TBorrower class, publish some new rates, then restart the TBorrower class, you would not have picked up the rates that were published to the topic while the TBorrower class was not running. Why? Because the TBorrower class was created as a nondurable subscriber:

TopicSubscriber subscriber = tSession.createSubscriber(topic);

Nondurable subscribers receive messages only when they are actively listening on that topic. Otherwise, the message is gone. In the pub/sub model, there is no real concept of a “topic” holding all of the messages; rather, when a message is received by the JMS provider, the provider makes a copy of that message for each subscriber. If the subscriber is not active, it does not receive a copy of that message. This concept is illustrated in Figure 5-3.

Figure 5-3. Nondurable subscribers

Durable subscribers, on the other hand, will receive all messages sent to that topic (depending on the message selectors applied to that subscriber), regardless of whether that subscriber is active or not. This is commonly referred to as “store-and-forward” messaging. Email is a good example of this concept, even though email is usually ...