Asynchronous JavaScript and XML (AJAX) combine proven technologies including JavaScript, Extensible Markup Language (XML), dynamic HTML (DHTML), Cascading Style Sheets (CSS), and the Document Object Model (DOM) to enable the delivery of interactive Web applications. AJAX-based Web applications no longer have to reprocess and resend the entire Web page to the end user's browser every time anything changes on the page. As a result, advanced features, like seamlessly populating a Web form with the name of the city when the zip code is entered, can be delivered without having the user sit through the awkward dead time of a complete page refresh. Accelerating the adoption of AJAX is the underlying technologies that are already supported by hundreds of millions of browsers worldwide, and are ready to deliver a cool interactive surfing experience to the end users.

Is it really so simple? Not really. While AJAX adds a patina of interactivity and coolness to Web applications, its increased complexity and more "chatty" protocols can significantly impact the performance of such applications. The bottom line is that even if Web applications become slicker, end-user satisfaction of the site could actually go down because of performance issues. In today's world of instant gratification and finicky behavior, slow Web sites are just not tolerated by users regardless of the perceived coolness of the site. In fact, a Web site has to be interactive and responsive at the same time to accomplish its business objectives.

It's All About the End User
Instead of simply asking the browser to display simple HTML pages, AJAX applications place a heavier demand on an end-user's browser executing JavaScript and parsing frequent client/server XML messages. Because of the myriad browsers or browser versions being used, and knowing that each version is slightly different in the way it performs these tasks, it is very difficult for IT to ensure that the application can meet the end user's response time expectation. For example, even if the AJAX application is tested on the QA lab's Mozilla Firefox 2.0 browser and deemed to be meeting the service level, it still requires a major leap of faith on the part of developers to conclude that the application is acceptable to the majority of users running Microsoft's Internet Explorer 7 (IE7) browser.

Testing the code against all types of browsers is theoretically feasible, but it is certainly a costly ongoing effort to keep up with the rate of change in browser technologies and patch releases. Compounding the problem is the constant need to update the Web application to maintain site freshness, making the testing chore even more difficult.