AJAX which stands for Asynchronous JavaScript and XML is a way of programming for the Web that gets rid of the hourglass, i.e. downloading problem. Data, design and content are combined together into a seamless whole. When your client clicks on something on an AJAX driven application, there is very little lag time. The page simply displays what he is asking for. If you don’t believe this, visit Google Maps for a few seconds. Scroll around and watch as the map updates almost before your eyes blink. There is very little lag and you don’t have to wait for pages to refresh or reload.

The AJAX model adds more dynamic interactivity to Web applications, making them feel more like desktop applications. On the flip side, because AJAX is made up of a number of different standards implemented in slightly different ways by browsers, it is very difficult to get AJAX applications working correctly with any browser. Scripting has become a significant source of security vulnerabilities for Web applications. In January Google patched a Gmail flaw that involved Perl script. PHP has also been hit by several major security flaws.

Some of the usability issues surrounding AJAX applications are more general. For example, it can be important to let users know that their input has been registered, because the usual feedback mechanisms of the hourglass cursor and spinning browser are not applicable to XMLHttpRequest. One technique is to replace Submit butto ns with a “Now updating…” type message so that users do not repeatedly click on buttons while waiting for a response.

Another issue is that users may fail to notice that parts of the page they’re viewing have been updated. You can alleviate this problem by using a variety of visual techniques to subtly draw the user’s eye to updated areas of the page. Other issues caused by updating the page with AJAX include “breaking” the browser’s back button and the URL in the address bar not reflecting the entire state of the page, preventing bookmarking.

Implementing an AJAX User Interface in place of a regular forms-based one may considerably increase the number of requests made to the server. For instance, a regular Google Web search causes one hit on the server, occurring when the user submits the search form. However, Google Suggest, which attempts to autocomplete your search terms, sends several requests to the server as the user types.
When developing an AJAX application, be careful of how many requests you will be sending to the server and the resulting server load this will cause. You can mitigate server load by buffering requests on the client and caching server responses in the client, where applicable. You should also attempt to design your AJAX Web applications so that as much logic.

As it happens with any technology, there are ample ways to make mistakes with AJAX. Some of the problems do not have easy solutions currently but will improve as AJAX matures. As the developer community is gaining experience by developing AJAX applications, finding solutions will be easy as best practices and guidelines will be documented.

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