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Java Message Service
publish( ) method returns, and allows client execution to continue. More on how and why
this works reliably can be found in Chapter 6.
4.4.1 Persistent Messages and Temporary Topics
When you are using a temporary topic as a way of posting a reply to a message, you
should realize that the total round trip (the initial message and the reply) isn't guaranteed to
survive a certain failure condition, even if you use persistent messages. The problem is that
temporary topics cannot be used for durable subscriptions. Consider the following
scenario:
1. A JMS client (producer) creates a temporary topic, puts it in the JMSReplyTo header
of a message, marks the message as persistent, and publishes it.
2. The subscriber gets the message and publishes a response on the temporary topic
using a persistent message.
3. The original producer expects a reply on the temporary topic, but disconnects or
crashes before it is received.
4. The original producer restarts, and is no longer able to subscribe to the original
temporary topic that it had established in its previous life. It can't resubscribe
because the temporary topic was only valid for the duration of the previous
connection. Calling createTemporaryTopic( ) in the new session returns a new
temporary topic, not the previous one.
This is a subtle point, since any client with a nondurable subscription will not get messages
during a failure. In other scenarios it may be acceptable to lose messages for a time, yet
still be able to start receiving newly published "responses" when the original producer of
the message starts up again. In the B2B example, a failure of the Wholesaler means that
the reply messages sent to the temporary topic will be lost. An alternative and superior
design would use the JMSReplyTo header, with an established topic instead of a temporary
one. Chapter 6 provides more detail on message delivery semantics, Quality of Service,
and failure conditions.
4.5 JMSCorrelationID
In the B2B example, we are using the JMSCorrelationID as a way for the Retailer to
associate its identity with its reply message, as illustrated by the following code in
Retailer.autoBuy( ) :
private void autoBuy (javax.jms.Message message){
...
publisher = session.createPublisher(buytopic);
textMsg.setJMSCorrelationID("DurableRetailer");
}
publisher.publish(
textMsg,
javax.jms.DeliveryMode.PERSISTENT,
javax.jms.Message.DEFAULT_PRIORITY,
1800000);
...
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