Ajax-Driven Web Sites: Under The Hood

Recently, a number of Web sites have begun to raise some eyebrows within the developer community. What's unique about these sites is that they behave more like a desktop application than a Web application. As you interact with them, they quickly display an endless amount of information to your browser without reloading the page. At the Google Maps site for example (http://maps.google.com/), you can click on the map, zoom in, zoom out, and move around as much as you like. Your browser continues to be fed with data from the server, yet your browser doesn't have to refresh. They're not using applets, or anything like Flash, so how are they doing it? Introducing Asynchronous JavaScript + XML, also known as Ajax. To properly describe what Ajax is, it's easiest to contrast it with what it's not. For most Web sites, interaction with a Web server is simplex communication - like talking to your buddy on a walkie-talkie. You speak while he receives, and vice versa, but never at the same time. For a Web user, when he or she fills out an online form and then clicks the submit button, the entire page is posted to the Web server and the user must wait for the server to receive the request. When the server finishes processing the request, it sends the processed content back. Only then does the user's page finally refresh (see Figure 1). Ajax is an attempt to alleviate this choppy sequence of events. When the user is at an Ajax Web site the browser can call the Web server asynchronously, behind the scenes - without posting the entire page.

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