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The ACK can now be sent, establishing the session. Note that the BRANCH field in the ACK is the same value as contained in the initial INVITE. This is so stateful proxies will be able to correlate the requests and responses for this session. The BRANCH value will always start with the same first seven digits. In this case, the first digits are z9hG4bk, indicating that this message was processed using the processes and procedures defined in RFC 3261 SIP 2.0. One last note about acceptance of a request: The CSeq header is used to ensure messages are received in proper sequence. For this reason, the UAS will always verify that the CSeq value is the expected value for a dialog. When the initial request is sent, the value is always the lowest number. Each subsequent message must be the next value greater than the initial request. If not, the message is considered out of sequence and the UAS responds with 500 SERVER INTERNAL ERROR. There can be gaps between received messages, which is allowed. This could be the case, for example, when a proxy challenges a UAC prior to forwarding the request to the UAS. The UAC sends another INVITE containing the proper credentials, and with an incremental CSeq value. This provides a simple explanation of session establishment. There are many more different possibilities for session establishment, depending on the method used; however, they all use the same basic framework described here. What we have not discussed yet is emergency session initiation (such as the case for an emergency call to 911 or 0911).

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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For routing purposes, each network must maintain a routing database identifying the proper destination address for all emergency access points (in the U.S. these are referred to as Public Safety Answering Points, or PSAPs).

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