AJAX is not new. These techniques have been available to developers targeting Internet Explorer on the Windows platform for many years. Until recently, the technology was known as web remoting or remote scripting. Web developers have also used a combination of plug-ins, Java applets, and hidden frames to emulate this interaction model for some time. What has changed recently is that the inclusion of support for the XMLHttpRequest object has became ubiquitous in the mainstream browsers across all platforms.

Some uses for AJAX interactions are the following:

-  Real-Time Form Data Validation: Form data such as user IDs, serial numbers, postal codes, or even special coupon codes that require server-side validation can be validated in a form before the user submits a form.

-  Autocompletion: A specific portion of form data such as an email address, name, or city name may be autocompleted as the user types.

-  Master Details Operations: Based on a client event, an HTML page can fetch more detailed information on data such as a product listing that enables the client to view the individual product information without refreshing the page.

-  Sophisticated User Interface Controls: Controls such as tree controls, menus, and progress bars may be provided that do not require page refreshes.

-  Refreshing Data on the Page: HTML pages may poll data from a server for up-to-date data such as scores, stock quotes, weather, or application-specific data.

-  Server-side Notifications: An HTML page may simulate a server-side push by polling the server for event notifications that may notify the client with a message, refresh page data, or redirect the client to another page.

Ajax provides the ability to communicate with the server asynchronously, thereby freeing the user experience from the request/response cycle. With Ajax, when a user clicks a button, you can use JavaScript and DHTML to immediately update the UI, and spawn an asynchronous request to the server to perform an update or query a database. When the request returns, you can then use JavaScript and CSS to update your UI accordingly without refreshing the entire page. Most importantly, users don't even know your code is communicating with the server: the Web site feels like it's instantly responding.

While the infrastructure needed by Ajax has been available for a while, it is only recently that the true power of asynchronous requests has been leveraged. The ability to have an extremely responsive Web site is exciting as it finally allows developers and designers to create "desktop-like" usability with the standard HTML/CSS/JavaScript stack.

Source:  Sun Microsystems Inc. and Bea Systems

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