[FoRK] AJAX: Asynchronous JavaScript + XML

Jim Whitehead ejw at soe.ucsc.edu
Tue Mar 29 17:17:05 PST 2005


Ajax isn't a technology. It's really several technologies, each flourishing
in its own right, coming together in powerful new ways. Ajax incorporates:

    * standards-based presentation using XHTML and CSS;
    * dynamic display and interaction using the Document Object Model;
    * data interchange and manipulation using XML and XSLT;
    * asynchronous data retrieval using XMLHttpRequest;
    * and JavaScript binding everything together.

The classic web application model works like this: Most user actions in the
interface trigger an HTTP request back to a web server. The server does some
processing - retrieving data, crunching numbers, talking to various legacy
systems - and then returns an HTML page to the client. 


An Ajax application eliminates the start-stop-start-stop nature of
interaction on the Web by introducing an intermediary - an Ajax engine -
between the user and the server. It seems like adding a layer to the
application would make it less responsive, but the opposite is true.

Instead of loading a webpage, at the start of the session, the browser loads
an Ajax engine - written in JavaScript and usually tucked away in a hidden
frame. This engine is responsible for both rendering the interface the user
sees and communicating with the server on the user's behalf. The Ajax engine
allows the user's interaction with the application to happen asynchronously
- independent of communication with the server. So the user is never staring
at a blank browser window and an hourglass icon, waiting around for the
server to do something.

- Jim

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