In each interaction with the user, an entirely new page is required. Such applications must use bandwidth, transferring constant page elements (site navigation, for example) with each page request, even though these elements were sent to the client in previous responses. Further, such applications are slower than desktop applications,because the web application is unresponsive while the results of one interaction are being processed and until the new page loads.

AJAX

(Asynchronous JavaScript and XML) refers to a group of technologies that allows for the creation of fat-client web applications, where the responsibility for a relatively large portion of the application processing is given to the client. Specifically, AJAX applications often use XHTML and CSS for semantic markup and presentation, XML to format data that will be exchanged between client and server, and JavaScript to communicate with the server and manipulate the Document Object Model (DOM).

When using AJAX, communication between the client and the server can be limited to data only, as the client has the functionality necessary to handle presentation and user interaction.

Further, such communication happens asynchronously and without a page refresh.

This allows for the client-side of the application to remain responsive even while data is being exchanged with the server. If a user performs an action that requires a request be sent to the server, the request can be sent and the application can handle further interaction while awaiting a response (even issuing more requests). This concept is illustrated in figure below.

Probably the most popular AJAX application today is Google's GMail (http://www.gmail.com), a web-based email client. In traditional web-based email clients, nearly every action by the user causes a refresh of the entire page or frame. Because GMail employs AJAX, the page never refreshes.

Rather, the interface is updated dynamically by manipulating the DOM via JavaScript based on user interaction with the application and XML communication with the GMail server. This enables, for example,a user to compose an email, click Send, and continue reading his or her email while the GMail server handles the actual processing of the sent message. While reading the rest of the email, he or she will receive a status message indicating that the message was sent successfully.