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if the Turtle server is down at any given time the shipping request is not lost and is simply delivered later. Another significant advantage of messaging is loosely coupled system integration. We could, if we wanted to, easily switch from the Turtle Shipping Company to O Hare Logistics once our current contract runs out. Note how different this is from having to know the exact interface details of the Turtle servers for synchronous communication technologies like RMI or even remote session beans. So far we ve described a particular form of messaging known as point-to-point to explain basic messaging concepts. This is a good time to move away from this simplification and fully discuss messaging models.

You have two or more entity beans that represent data in a single step of a transaction. For example, from the account transfer example in the chapter introduction,

A messaging model is simply a way of messaging when a number of senders and consumers are involved. It will be more obvious what this means as we describe each model. Two popular messaging models are standardized in Java EE: pointto-point (PTP) messaging and publish-subscribe messaging. We ll discuss each of these messaging models next. Point-to-point You can probably guess from the names of the messaging models how they function. In the PTP scheme, a single message travels from a single producer (point A)

to a single consumer (point B). PTP message destinations are called queues. Note that PTP doesn t guarantee that messages are delivered in any particular order the term queue is more symbolic than anything else. Also, if more than one potential receiver exists for a message, a random receiver is chosen, as figure 4.4 shows. The classic message-in-a-bottle story is a good analogy of PTP messaging. The message in a bottle is set afloat by the lonely castaway (the producer). The ocean (the queue) carries the message to an anonymous beach dweller (the consumer) and the message can only be found once. The ActionBazaar shipping request forwarding problem is an excellent candidate for the PTP model, as we want to be guaranteed that the message is received once and only once. Publish-subscribe (pub-sub) Publish-subscribe messaging is much like posting to an Internet newsgroup. As shown in figure 4.5, a single producer produces a message that is received by any number of consumers who happen to be connected to the destination at the time. Much like Internet postings, the message destination in this model is called a topic and a consumer is called a subscriber. Pub-sub messaging works particularly well in broadcasting information across systems. For example, it could be used to broadcast a system maintenance notification several hours before an outage to all premium sellers whose systems are directly integrated with ActionBazaar and who are listening at the moment.

J2EE design patterns advise the use of entity EJBs for the definition of persistent business objects. In theory, the use of an entity EJB allows the simple adding of transactional and persistence properties. However, several patterns or frameworks exist to deal with the same issues in a simpler and more efficient way. For example, using the Hibernate Open Source framework for persistence is an efficient replacement for entity EJBs. Most of the available frameworks do not require specific extensions, and business objects are defined as POJO or traditional JavaBeans. For further evolutions, it is interesting for the application to leave the business objects as independent as possible from EJBs.

Figure 4.5 The publish-subscribe messaging model with one producer and three consumers. Each topic subscriber receives a copy of the message.

you might have entity beans that represent each of the two accounts. In this case, you want to perform both updates in single transaction.

At this point, you should have a good conceptual foundation of messaging and are perhaps eager to get a taste of some code. Next, we take a brief look at JMS and implement the ActionBazaar message producer for sending the message.