O'Reilly - Java Message Service

Chapter 4. Publish-and-Subscribe Messaging
Java Message Service
This chapter focuses on the publish-and-subscribe (pub/sub) messaging model that was
introduced in Chapter 2. The pub/sub messaging model allows a message producer (also
called a publisher) to broadcast a message to one or more consumers (called subscribers).
There are three important aspects of the pub/sub model:
• Messages are pushed to consumers, which means that consumers are delivered
messages without having to request them. Messages are exchanged through a
virtual channel called a topic. A topic is a destination where producers can publish,
and subscribers can consume, messages. Messages delivered to a topic are
automatically pushed to all qualified consumers.
• As in enterprise messaging in general, there is no coupling of the producers to the
consumers. Subscribers and publishers can be added dynamically at runtime,
which allows the system to grow or shrink in complexity over time.
• Every client that subscribes to a topic receives its own copy of messages published
to that topic. A single message produced by one publisher may be copied and
distributed to hundreds, or even thousands of subscribers.
In Chapter 2 you learned the basics of the pub/sub model by developing a simple chat
client. In this chapter we will build on those lessons and examine more advanced features
of this model, including guaranteed messaging, topic-based addressing, durable
subscriptions, request-reply, and temporary topics.
4.1 Getting Started with the B2B Application
In this chapter we abandon the simple chat example for a more complex and real-world
Business-to-Business (B2B) scenario. In our new example, a wholesaler wants to distribute
price information to retailers, and the retailers want to respond by generating orders. We'll
implement this scenario using the publish-and-subscribe model: the wholesaler will
publish messages containing new prices and hot deals, and the retailers will respond by
creating their own messages to order stock.
This scenario is typical of many Business-to-Business operations. We call the clients
retailers and wholesalers, but these names are really only for convenience. There's little
difference between our wholesaler/retailer scenario and a stock broker broadcasting stock
prices to investors, or a manufacturer broadcasting bid requests to multiple suppliers. The
fact that we use a retailer and a wholesaler to illustrate our example is much less important
than the way we apply JMS.
Our simple trading system is implemented by two classes, both of which are JMS clients:
Wholesaler and Retailer . In the interest of keeping the code simple, we won't implement
a fancy user interface; our application has a rudimentary command-line user interface.
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