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In the last chapter we discussed distributed objects, which have overcome many of the shortcomings of RPC, described in 4. In particular, distributed objects allowed for code reuse by separating out the code for marshalling and networking into a separate software component. This separation also allowed us to move away from point-to-point integration patterns. In addition, distributed objects introduced the concept of language independence, which is important for large enterprise integration projects. Distributed objects also blurred the distinction between the client (or service consumer) and the server (or the service provider). Thus, a more peer-to-peer type of relationship can be established between applications. Finally, distributed objects allowed us to develop a rudimentary concept of a registry. Although distributed objects provided a big step forward on many fronts in the battle for enterprise applications integration, they failed to address two very import shortcomings of RPC:
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Both RPC and distributed objects employ synchronous interaction between the applications being integrated. This means that the client application is blocked from doing further work until the server application completes its work and returns control to the client application. This leads to strong coupling between applications and a lack of scalability in the integration solution. In other words, if a large number of applications need to be integrated, neither RPC nor distributed objects is the proper solution. RPC- and ORB-based communication is not reliable and there is no guarantee that the messages and return values will be delivered to the intended targets. Thus, the client application may experience a hang-up in its operation under certain circumstances (such as a break
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in the network connection or when the two applications are not up and running at the same time). In this chapter, we discuss asynchronous messaging, which overcomes these two major problems with RPC and distributed objects. In addition, asynchronous messaging has other advantages, which are elaborated upon later in this chapter. In asynchronous messaging, the client or client object sends a message to the target application but does not wait for the response to continue its work, thus leading to a certain amount of decoupling between the applications involved. Therefore, asynchronous messaging may be employed as the basis for integration if high transaction volumes are expected. We start this chapter with an overview of a messaging system. Then we discuss the various components of the messaging system in detail. To demonstrate how these components work together, some sample code is provided. Finally, we conclude by discussing certain disadvantages of the messaging approach to application integration. Overview As mentioned, in asynchronous messaging the client (or service consumer) sends a message to the server but does not wait for the response. This allows the client application to perform further work while the server is completing the request from the client. This decoupling between the client and server means that more work can be accomplished in a given timeframe. In other words, it leads to a more scalable solution. In messaging, the applications do not communicate with each other directly and do not have a dedicated communication link established between them. Instead, they communicate indirectly through queues. A queue sometimes called a channel behaves like a collection of messages that can be shared across multiple computers. Figure 6.1 shows two
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Asynchronous messaging through the use of queues/channels
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applications exchanging a message through a queue. Application A first sends a message to the queue. Application B then retrieves the message from the queue after it has been delivered by Application A. As with ORB or distributed objects, in asynchronous messaging the code for the communication and marshalling is separated out as a separate software component, which allows for code reuse (that is, multiple applications can use the same code to communicate with each other and with applications on another machine). This separate software component is often called a messaging system or messageoriented middleware (MOM). Figure 6.2 shows the basic steps involved in transmitting a message from Application A to another application (Application B) running on a separate computer. Here are the five steps involved in transmitting the message: 1. Application A creates a message and populates the message with data. 2. Application A sends the message to the queue inside the messaging system (MOM). 3. The messaging system transfers the message from the sender s computer to the receiver s computer, making it available to the receiver. 4. Application B reads the message from the queue. 5. Application B processes the message received in step 4.
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