Messaging concepts in Java

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Messaging concepts
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Figure 4.2 ActionBazaar ordering before MOM is introduced. Slow B2B processing is causing customer dissatisfaction.
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ActionBazaar started as a two-person Internet operation, Joe and John, the two founders, made a sweet deal with Turtle Shipping Company s founder Dave Turtle. As a part of the deal, Joe and John agreed to ship with Turtle for a few years. As soon as a user places an order for an ActionBazaar Special, a shipping request is sent to the Turtle system via a business-to-business (B2B) connection, as shown in figure 4.2. The order confirmation page loads only after Turtle confirms receipt. Now that the number of ActionBazaar customers has gone through the roof, the slow Turtle servers and B2B connection simply cannot keep up and completing a shipping order takes what seems like forever. To make matters worse, the Turtle server occasionally goes down, causing orders to fail altogether. Taking a closer look at things, we see that we could make the forwarding process of the shipping request asynchronous and solve this problem. Instead of communicating directly with the Turtle server, the ActionBazaar ordering process could send a message containing the shipping request to MOM, as depicted in figure 4.3. As soon as the message is stored in MOM, the order can be confirmed without making the user wait. At a later point in time, the Turtle server could request pending shipping request messages from MOM and process them at its own pace. In this case, the most obvious advantage MOM is offering is an increase in reliability. The reliability stems from not insisting that both the ActionBazaar and Turtle servers be up and running at the same time. Also, the servers are not expected to function at the same processing rate. In the most extreme case, even
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Messaging and developing MDBs
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Figure 4.3 ActionBazaar ordering after MOM is introduced. Messaging enables both fast customer response times and reliable processing.
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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.
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4.1.3 Messaging models
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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)
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Messaging concepts
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The PTP messaging model with one producer and two consumers
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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.
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