The typical web application involves a refresh
cycle where a postback is sent to the server, and the response from the server is re-rendered.
In other words, the server returns a complete page of HTML to be rendered by the
CHAPTER 1 ?– INTRODUCING AJAX 7
browser, which looks kind of clunky compared to desktop apps. This is a drawback to this
type of architecture because the round-trip to and from the server is expensive in user
time and bandwidth cost, particularly for applications that require intensive updates.
What is interesting about the AJAX approach is that there is really nothing new about
it. The core technology??”the XMLHttpRequest object??”has been around since 1999 with
Internet Explorer, when it was implemented as an ActiveX plug-in. This is a standard
JavaScript object recognized by contemporary browsers, which provides the asynchronous
postback capabilities upon which AJAX applications rely. More recently, it has been
added to the Mozilla Firefox, Opera, and Safari browsers, increasing its ubiquity, and has
been covered in a World Wide Web Consortium (W3C) specification (DOM Load and
Save).
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