Introduction to AJAX

Take a look at Google Suggest. Watch the way the suggested terms update as you type, almost instantly. Now look at Google Maps. Zoom in. Use your cursor to grab the map and scroll around a bit. Again, everything happens almost instantly, with no waiting for pages to reload.

Google Suggest and Google Maps are two examples of a new approach to web applications that we at adaptive path have been calling Ajax. The name is shorthand for Asynchronous JavaScript + XML, and it represents a fundamental shift in whats possible on the Web.

Defining Ajax

Ajax is not a technology. It is really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:

(1) standards-based presentation using XHTML and CSS.
(2) Dynamic display and interaction using the Document Object Model.
(3) Data interchange and manipulation using XML and XSLT.
(4) Asynchronous data retrieval using XMLHttpRequest.
(5) JavaScript binding everything together.

What Makes AJAX different?

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 is a model adapted from the Web original use as a hypertext medium, but as fans of The Elements of User Experience know, what makes the Web good for hypertext does not 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 does not make for a great user experience. While the server is doing its thing, what is the user doing? Thats 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 would not 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.

Instead of loading a webpage, at the start of the session, the browser loads an Ajax engine written in JavaScript and usually tucked away in a hidden frame. This engine is responsible for both rendering the interface the user sees and communicating with the server on the users behalf. The Ajax engine allows the users interaction with the application to happen asynchronously independent of communication with the server. So the user is never staring at a blank browser window and an hourglass icon, waiting around for the server to do something.



Figure 2: The synchronous interaction pattern of a traditional web application (top) compared with the asynchronous pattern of an Ajax application (bottom).

Every user action that normally would generate an HTTP request takes the form of a JavaScript call to the Ajax engine instead. Any response to a user action that does not require a trip back to the server such as simple data validation, editing data in memory, and even some navigation the engine handles on its own. If the engine needs something from the server in order to respond if its submitting data for processing, loading additional interface code, or retrieving new data the engine makes those requests asynchronously, usually using XML, without stalling a users interaction with the application.

Note Internet Explorer uses an ActiveXObject, while other browsers uses the built-in JavaScript object called XMLHttpRequest.

Code to create an XmlHttp Object

function ajaxFunction()

{

    var xmlHttp;

    try

    {

        // Firefox, Opera 8.0+, Safari

        xmlHttp=new XMLHttpRequest();

    }

    catch (e)

    {

        // Internet Explorer

        try

        {

            xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");

        }

        catch (e)

        {

            try

            {

                xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");

            }

            catch (e)

            {

                alert("Your browser does not support AJAX!");

                return false;

            }

        }

    }

  }

The onreadystatechange property stores the function that will process the response from a server.

xmlHttp.onreadystatechange=function()

{

  // We are going to write some code here

}

The readyState property holds the status of the server's response. Each time the readyState changes, the onreadystatechange function will be executed.

Here are the possible values for the readyState property