Timing Dependencies and JMS Publishing Models in Java

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Timing Dependencies and JMS Publishing Models
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The two JMS distribution models have different affects on the reliability of message consumption PTP messaging enqueues new messages as they're produced and only dequeues each message once (no matter how many consumers there are) Thus, a producer can be assured that a consumer has the opportunity to receive all messages However, to ensure that multiple clients receive that message it has to create a destination for each client In contrast, pub/sub messaging allows a producer to broadcast a message to multiple clients, but normally ensures only that a client has the opportunity to receive these messages during periods when the subscription is active Under the default scheme, any messages sent before the subscription is created or during client downtime aren't received Often this makes sense when old messages aren't useful and new messages aren't critical For example, if you're broadcasting noncritical stock quotes (for general use, not day-trading), a pub/sub model can be a natural fit However, many times it's desirable to have reliable message delivery to multiple consumers To achieve this, JMS offers durable subscriptions These are subscriptions where the provider assures the producer that a consumer will receive all of the messages for a given subscription until either the messages expire or the subscription is unsubscribed Each subscription is associated with a single subscriber To ensure that messages created before the subscription starts can be consumed, the delivery mode should be persistent
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A Sample JMS Pub/Sub Application
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In this section, we'll get a better feel for the details involved in writing a messaging application Instead of discussing how to do this under the different models (PTP or pub/sub) and reliability configurations (persistent delivery, nonpersistent delivery, durable subscriptions, and nondurable subscriptions), we will stick to our Red Planet example, which involves distributing price updates to resellers This means that we'll commit to specific set of JMS deployment options Coding under an alternative messaging model with alternative reliability configurations is very similar to what will be presented here In thinking about our specific Red Planet example, a few immediate observations and corresponding decisions can be made Clearly, the role of message producer is played by the main pricing application at headquarters and the consumer roles are played by the applications used by the resellers Since this means that price updates are being distributed in a one-to-many fashion, the natural delivery model for Red Planet is JMS publish/subscribe We should note that reseller applications may crash or be unavailable for reasons that Red Planet can't control Price updates are not tangential to business they are the business so failure to receive a message can impact a reseller (it will be charging the wrong price!) To avoid this, we'll design our application using durable subscriptions Also, since the host running the JMS provider may itself occasionally crash, message persistence is necessary
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Let's examine the code required to build the publisher and reseller subscribers
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Developing the Message Publisher
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To publish messages under a pub/sub model, it's necessary to do the following: 1 2 3 4 5 6 7 8 Create the topic (programmatically or with J2EE tools) Get the initial context (JNDI lookup) Obtain a connection factory from the messaging provider Obtain a connection using the factory Create a topic session using the connection Locate the topic Create a publisher object for the topic session Publish the message
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Creating the Topic The first step is to create a topic on the provider side (the provider owns the queue in this example) We can do this either through the JMS API or with tools that are commonly provided by the J2EE vendor To simplify our example, we'll choose the latter approach Specifically, we'll consider how a topic is created using the tools provided by Sun's J2EE reference implementation Sun provides a tool called j2eeadmin, which can be used to manage JMS queues and topics Here's an example of how we can use that tool to create a topic:
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