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Channels As mentioned previously, there are two basic types of channels. We discuss these in some detail in this section. In particular, we discuss how point-to-point channels can be used for synchronous messaging between two applications.
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The point-to-point messaging model allows messaging system clients to send and receive messages asynchronously via virtual channels known as queues. The point-to-point messaging model has traditionally been a pull- or polling-based model, where the messages are requested from queues, instead of being pushed to the client automatically. The point-topoint messaging model is intended for one-to-one delivery of messages, as shown in Figure 6.3. As shown in this figure, a queue may have multiple receivers, but only one receiver may receive each message. The message system (sometimes called the JMS provider) will take care of doling out messages among the receivers, thus ensuring that each message is consumed by only one receiver. The point-to-point asynchronous messaging model can also used to simulate synchronous messaging or interaction between two applications. This is shown in Figure 6.4, where one queue (the request queue) is used to deliver the request while the return values are obtained through another queue. The request queue is the output queue for the requesting application (Application A), while at the same time it serves as the input queue for the receiving application (Application B). Similarly, the response queue is used as an output queue for Application B and as an input queue for the return value for Application A. In order to correlate the request with the response, a correlation ID is included in the header of the response. The value of this correlation ID is usually the request message ID.
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Application A
Application B
Response Queue
Simulating a synchronous exchange using the messaging system
Publish-and-Subscribe
In publish-and-subscribe messaging, one producer can send a message to any number of consumers through a virtual channel called a topic. Consumers can choose to subscribe to a topic. Any messages addressed to a topic are delivered to all the subscribers. Every subscriber receives a copy of each message, as shown in Figure 6.5. The publish-and-subscribe messaging model is mostly a push-based model, where messages are automatically broadcast to consumers without the topic being polled for new messages. In the publish-and-subscribe messaging model, the publisher may not care if everybody is listening, or even if nobody is listening. For example, consider a publisher that broadcasts stock quotes. If any particular subscriber is not listening and misses out on a great quote, the publisher is not concerned. Messages A message consists of a header and a body. The body contains the data to be processed by the receiving application. The header contains the message identification and control information, intended to be used
Subscriber Publish-and-Subscribe (1 to n) Publisher Queue Subscriber
Subscriber
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mostly by the messaging system. A typical header would have a number of attributes. Some of the commonly occurring attributes are
Message ID/correlation ID Persistent/nonpersistent Return address Priority Segmenting/grouping information Date and time Lifetime of a message Version
The message ID and correlation ID are used to identify a specific request or reply message. As the programmer, you can move a value in one or both fields or have the messaging system create a unique ID for you. Before you put the request message in the queue, you can save the ID(s) and use it in a subsequent get operation for the reply message. The program that receives the request message copies this information into the reply message. This allows the originating program (the one that gets the reply) to instruct the messaging system to look for a specific message in the queue instead of getting the first one in the queue. Persistent messages always arrive at their destination, even when the system fails. They are hardened (that is, saved on disk). You can make a specific message persistent or all messages on a particular queue persistent. You can assign a priority to a message and thus control the order in which it is processed. The return address is important for request/reply messages. You have to tell the server program where to send the reply message. Clients and server have a one-to-many relationship and usually the server program cannot find out from the user data where the request message came from. Therefore, the client provides the reply-to queue and reply-to queue manager in the message header. Messages can be segmented or grouped. Message segmenting can be transparent to the application programmer. If permitted, the queue manager segments a large message when it does not fit in a queue. On the receiving end, the application has the option to either receive the entire message in one piece or each segment separately. This may depend on the buffer size available for the application. A second method of segmenting leaves you as the programmer in control so that you can split a message according to logical boundaries or
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