Asynchronous messaging means that the sender doesn't receive an immediate reply or confirmation of receipt. After sending a message for processing by a remote service, most applications require at least confirmation that the message was received and acted upon, and often require the results of the remote process. Remember—in a normal application, even if a method doesn't return a value, the system at least knows that the method has had a chance to execute.
A reply message is a message received in response to another message. Reply messages are generally document or event messages, but are distinguished by being associated with a particular message. Typically, this association is accomplished by assigning each outgoing message a unique identifier (MOM systems do this automatically). If an incoming message is a reply, it includes a reference to the message the remote application is replying to.
JMS includes built-in functionality for associating a reply message with the original. Each JMS message, upon creation, is assigned a message ID, which is accessible via the JMSMessageID property of the Message class. Messages also have a JMSCorrelationID field, not set by default. When composing a reply message, it's standard practice to take the JMSMessageID and store it in the JMSCorrelationID field. Assuming that message and reply are valid JMS messages, and that reply is a response to message, this is how we'd set the correlation:
