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What Is AJAX?

AJAX (Asynchronous JavaScript and XML) is an umbrella term for a collection of Web development technologies used to create interactive Web applications, mostly W3C standards (the XMLHttpRequest specification is developed by WHATWG [1]):

• XHTML Ð a stricter, cleaner rendering of HTML into XML.
• CSS for marking up and adding styles.
• The Javascript Document Object Model (DOM) which allows the content, structure and style of a document to be dynamically accessed and updated.
• The XMLHttpRequest object which exchanges data asynchronously with the Web server reducing the need to continually fetch resources from the server.

Since data can be sent and retrieved without requiring the user to reload an entire Web page, small amounts of data can be transferred as and when required. Moreover, page elements can be dynamically refreshed at any level of granularity to reflect this. An AJAX application performs in a similar way to local applications residing on a userÕs machine, resulting in a user experience that may differ from traditional Web browsing.

The Origins of AJAX

 Recent examples of AJAX usage include Gmail [2], Flickr [3] and 24SevenOffice [4]. It is largely due to these and other prominent sites that AJAX has become popular only relatively recently Ð the technology has been available for some time. One precursor was dynamic HTML (DHTML), which twinned HTML with CSS and JavaScript but suffered from cross-browser compatibility issues. The major technical barrier was a common method for asynchronous data exchange; many variations are possible, such as the use of an ÒiframeÓ for data storage or JavaScript Object Notation for data transmission, but the wide availability of the XMLHttpRequest object has made it a popular solution. AJAX is not a technology, rather, the term refers to a proposed set of methods using a number of existing technologies. As yet, there is no firm AJAX standard, although the recent establishment of the Open AJAX group [5], supported by major industry figures such as IBM and Google, suggests that one will become available soon.