This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs for the special pre-order price, click here for more information. Aimed at everyone from enterprise developers to self-taught scripters, Real-World AJAX: Secrets of the Masters is the perfect book for anyone who wants to start developing AJAX applications.

Servers Are for Data, Not Pages
AJAX changes the role of Web pages from being merely HTML documents into "applications" that contain both HTML markup and code. And it changes the role of the "server" from merely serving HTML pages to serving data too.

In classic Web applications, Web servers serve HTML Web pages. Some of the pages are static; others are generated dynamically by server-side logic. When the application contains dynamic data, the server has to convert that data into HTML markup and send it to the browser to be displayed as HTML pages. This way the server is merely serving "screen images" to the client side while the client-side browser is merely a screen-images rendering engine.

In AJAX Web applications, servers don't have to convert data into HTML markup. They can send data directly to the client-side. The client-side code will process the data inside the browser and dynamically update the HTML display. This eliminates significant overhead on the server side, leverages the client-side processing powers, and delivers better performance and scalability, as shown in Figure 1.4.

Dynamic and Continuous User Experience
An important characteristic of AJAX is in its first letter "A" – a user experience that is "asynchronous." Asynchronous means that users continue to interact with the application while the browser is communicating with the server. No more "click, wait, and page refresh," the AJAX user experience is dynamic and continuous.

Classic Web applications deliver a "click, wait, and page refresh" user experience. Because the Web was originally designed for browsing HTML documents, a Web browser responds to user actions by discarding the current HTML page and sending an HTTP request back to the Web server. After doing some processing, the server returns a new HTML page to the browser, which then displays the new page. The cycle of "browser requests, server responds" is synchronous, meaning that it happens in real-time rather than "in the background" so the user has to wait and cannot do other tasks. Figure 1.5 illustrates the traditional HTML "click-wait-refresh" paradigm.

In AJAX-based applications, partial screen updates replace HTML's "click-wait-refresh" and asynchronous communication replaces synchronous request/response. This model decouples user interaction from server interaction, while updating only those user interface elements that have new information. This more efficient application architecture eliminates the wait so users can keep working and it makes nonlinear workflow possible. It also reduces network bandwidth consumption and server load for improved performance and scalability. Figure 1.6 illustrates the AJAX asynchronous/partial update paradigm.

This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs, click here to order.