Introduction to AJAX Technologies
A book excerpt from ASP.NET 3.5 AJAX Unleashed.
If youve purchased this book, you probably are interested in AJAX technologies. If you are not familiar with the technology or are new to AJAX, it is important that you take some time and understand where AJAX fits into the big picture of web development. This section helps bring you up to speed with AJAXs place in your web development toolbox. One problem with the design of web pages (especially ASP.NET web pages) is that to refresh the data on the page, it must postback to the server. This causes the page to flicker while the page is posting, and then the page is reloaded in the browser with the results of the post. You can view the amount of data with tools such as IEHTTPHeaders and HTTPWatch. You will quickly notice that the amount of information getting posted is quite sizeable because ASP.NET applications not only postback the controls, but also post the pages ViewState. Although the technique of a postback works, it creates a lot of traffic over the wire and inherently reduces the overall scalability of your application.
The rest of this online chapter is at InformIT.com.