AJAX

Asynchronous JavaScript and XML, or Ajax, is a web development technique for creating interactive web applications using a combination of:

• XHTML (or HTML) and CSS for presenting information
• The Document Object Model manipulated through JavaScript to dynamically display and interact with the information presented
• The XMLHttpRequest object to exchange data asynchronously with the web server. (XML is commonly used, although any format will work, including preformatted HTML, plain text, JSON and even EBML)

The earliest form of asynchronous remote scripting, Microsoft's Remote Scripting, was developed before XMLHttpRequest existed, and made use of a dedicated Java applet.

Like DHTML, LAMP, or SPA, Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together. In fact, derivative/composite technologies based substantially upon Ajax, such as AFLAX, are already appearing.

Ajax applications look almost as if they reside on the user's machine, rather than across the Internet on a server. The reason: pages get updated, not entirely refreshed. “Every user's action that normally would generate an HTTP request takes the form of a JavaScript call to the Ajax engine instead”, wrote Jesse James Garrett, in the essay that first defined the term. “Any response to a user action that doesn’t require a trip back to the server — such as simple data validation, editing data in memory, and even some navigation — the engine handles on its own. If the engine needs something from the server in order to respond — if it’s submitting data for processing, loading additional interface code, or retrieving new data — the engine makes those requests asynchronously, usually using XML, without stalling a user’s interaction with the application.”

Traditional web applications essentially submit forms, completed by a user, to a web server. The web server responds by sending a new web page back. Because the server must submit a new page each time, applications run more slowly and awkwardly than their native counterparts.

Ajax applications, on the other hand, can send requests to the web server to retrieve only the data that is needed, usually using SOAP or some other XML-based web services dialect. On the client, JavaScript processes the web server response. The result is a more responsive interface, since the amount of data interchanged between the web browser and web server is vastly reduced. Web server processing time is also saved, since much of it is done on the client.