O'Reilly - Java Message Service

Chapter 5. Point-to-Point Messaging
Java Message Service
This chapter focuses on the point-to-point (p2p) messaging domain. Many of the concepts
of p2p messaging are similar to those we learned in Chapter 4. To avoid redundancy, this
chapter highlights the areas where the two models are the same, and focuses on the areas
where the two models differ.
In the p2p model, the producer is called a sender and the consumer is called a receiver.
The most important characteristics of the point-to-point model are:
• Messages are exchanged through a virtual channel called a queue. A queue is a
destination to which producers send messages, and a source from which receivers
consume messages.
• Each message is delivered only to one receiver. Multiple receivers may connect to
a queue, but each message in the queue may only be consumed by one of the
queue's receivers.
• Messages are ordered. A queue delivers messages to consumers in the order they
were placed in the queue by the message server. As messages are consumed they
are removed from the head of the queue.
• There is no coupling of the producers to the consumers. Receivers and senders can
be added dynamically at runtime, allowing the system to grow or shrink in
complexity over time. (This is a characteristic of messaging systems in general.)
In this chapter, we introduce new versions of our Wholesaler and Retailer classes, called
QWholesaler and QRetailer. QWholesaler still uses pub/sub to broadcast price quotes, while
QRetailer uses a p2p queue to respond with "buy" orders instead of publishing to a
temporary topic.
The rest of the chapter focuses on the unique capabilities offered by p2p: examining a
queue using the QueueBrowser interface, and load balancing among multiple recipients of a
queue.
5.1 Point-to-Point and Publish-and-Subscribe
Like publish/subscribe messaging, point-to-point messaging is based on the concept of
sending a message to a named destination. The actual network location of the destination is
transparent to the sender, because the p2p client works with a Queue identifier obtained
from a JNDI namespace, the same way that a pub/sub client uses a Topic identifier.
The pub/sub model is based on a push model, which means that consumers are delivered
messages without having to request them. Messages are exchanged through a virtual
channel called a topic. From the viewpoint of the receiver, a p2p queue can either push or
pull messages, depending on whether it uses the asynchronous onMessage( ) callback, or a
synchronous receive( ) method. Both of these methods are explained in more detail later.
In the p2p model, as in the pub/sub model, there is no direct coupling of the producers to
the consumers. The destination queue provides a virtual channel that decouples consumers
from producers. In the pub/sub model, multiple consumers that subscribe to the same topic
each receive their own copy of every message addressed to that topic. In the p2p model,
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