From Jesse James Garrett, “Ajax: A New Approach to Web Applications” (February 18, 2005):
Ajax isn’t a technology. It’s really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:
- standards-based presentation using XHTML and CSS;
- dynamic display and interaction using the Document Object Model;
- data interchange and manipulation using XML and XSLT;
- asynchronous data retrieval using XMLHttpRequest;
The classic web application model works like this: Most user actions in the interface trigger an HTTP request back to a web server. The server does some processing — retrieving data, crunching numbers, talking to various legacy systems — and then returns an HTML page to the client. It’s a model adapted from the Web’s original use as a hypertext medium, but as fans of The Elements of User Experience know, what makes the Web good for hypertext doesn’t necessarily make it good for software applications.
Figure 1: The traditional model for web applications (left) compared to the Ajax model (right).
This approach makes a lot of technical sense, but it doesn’t make for a great user experience. While the server is doing its thing, what’s the user doing? That’s right, waiting. And at every step in a task, the user waits some more.
Obviously, if we were designing the Web from scratch for applications, we wouldn’t make users wait around. Once an interface is loaded, why should the user interaction come to a halt every time the application needs something from the server? In fact, why should the user see the application go to the server at all?
How Ajax is Different
An Ajax application eliminates the start-stop-start-stop nature of interaction on the Web by introducing an intermediary — an Ajax engine — between the user and the server. It seems like adding a layer to the application would make it less responsive, but the opposite is true.
Figure 2: The synchronous interaction pattern of a traditional web application (top) compared with the asynchronous pattern of an Ajax application (bottom).