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Figure 5.1 An Ajax application moves some of the responsibilities of the presentation tier from the server up to the browser, in a new entity that we call the client tier.
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The role of the server-side presentation tier can be much reduced and workflow control partly or completely handed over to the new client tier, written in JavaScript and hosted on the browser. This new tier in our application brings with it new possibilities, as we ve already discussed. It also brings the potential for greater complexity and confusion. Clearly, we need a way to manage this.
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5.2.3 Maintaining client-side and server-side domain models
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In an Ajax application, we still need to model the business domain on the server, close to the database and other vital centralized resources. However, to give the client code sufficient responsiveness and intelligence, we typically will want to maintain at least a partial model in the browser. This presents the interesting problem of keeping the two models in sync with one another. Adding an extra tier always adds complexity and communications overheads. Fortunately, the problem isn t entirely new, and similar issues are commonly encountered in J2EE web development, for example, in which there is a strict separation between the business tier and the presentation tier. The domain model sits on the business tier and is queried by the presentation tier, which then generates web content to send to the browser. The problem is solved in J2EE by the use of transfer objects, which are simple Java objects designed to pass data between the tiers, presenting limited views of the domain model to the presentation tier. Ajax provides us with new challenges, though. In J2EE, both tiers are written in a common language with a remote procedure mechanism provided, which is typically not the case with Ajax. We could use JavaScript on the server tier, through Mozilla s Rhino or Microsoft s JScript .NET, for example, but it is currently rather unorthodox to do so, and we d still need to communicate between the two JavaScript engines. The two basic requirements for communicating between the tiers are reading data from the server and writing data to the server. We ll look at the details of these in section 5.3 through 5.5. Before we conclude our overview of architectural issues, though, we will look at the main categories of server architecture currently in use. In particular, we ll be interested to see how they represent the domain model to the presentation tier and what restrictions this might place on an Ajaxbased design. A recent informal survey (see the Resources at the end of this chapter) listed over 60 presentation frameworks for Java alone (to be fair, Java probably suffers from this framework-itis more than any other server language). Most of these differ in the details, fortunately, and we can characterize the presentation tier
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The role of the server
(in whatever server language) as following one of several architectural patterns. Let s have a look at these now.
5.3 The big picture: common server-side designs
Server-side frameworks matter to all Ajax applications. If we choose to generate the client code from a sever-side model, it matters a great deal. If we hand-code the client code and serve it as static HTML and JavaScript pages, then the framework isn t involved in delivering the app, but the data that the application will consume still has to be dynamically generated. Also, as we noted in the previous section, the server-side framework typically contains a domain model of some sort, and the presentation tier framework stands between that model and our Ajax application. We need to be able to work with the framework in order for our application to function smoothly. Web application servers can be unkindly characterized as developers playgrounds. The problem of presenting a coherent workflow to a user through a series of web pages, while interfacing to back-end systems such as database servers, has never been adequately solved. The Web is littered with undernourished, ill-maintained frameworks and utilities, with new projects popping up on a monthly, if not weekly, basis. Fortunately, we can recognize discrete families within this chaotic mixture. Reducing this framework soup to its essentials, there are possibly four main ways to get the job done. Let s examine each in turn and see how it can be adapted to the Ajax model.
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