It’s almost impossible today to be involved in web application design or development and not be aware of Ajax, a technology that includes but is not limited to Asynchronous JavaScript and XML. That’s because Ajax is currently the primary technique for driving the high responsiveness and interactivity of some of the most popular applications on the web such as Google Maps and Flickr. These applications are representative of a new generation of highly responsive, highly interactive web applications, referred to as Web 2.0 applications, that often involve users collaborating online and sharing content.

Ajax has different implications for developers working in different roles. For example, component developers creating custom components for web applications build Ajax functionality into the design. Page authors use these Ajax components, along with widgets, JavaScript technology, and other techniques, to incorporate Ajax functionality into their web applications. Ajax impacts other roles too. For example, enterprise application developers need to add logic in server-side components to handle Ajax-related requests directed to the server.

What You Need to Know About Ajax
As a page author, you don’t need to understand every detail of the Ajax methodology to incorporate it into a web page. However, you should have a general idea of what Ajax is and how it works. As this article mentioned earlier, Ajax enables the high responsiveness of many web applications. For example, a web site such as Google Maps uses Ajax to provide a highly responsive user interface (UI). You can view a map, then move your cursor across it to see adjacent areas almost immediately.

Ajax enables high responsiveness because it supports asynchronous and partial refreshes of a web page. A partial refresh means that when an interaction event fires — for example, a user moves the cursor across a Google map — a web server processes the information and returns a limited response specific to the data it receives. Significantly, the server does not send back an entire page to the client of the web application — in this case a web browser — as is the case for conventional “click, wait, and refresh” web applications. The client then updates the page based on the response.

Asynchronous means that after sending data to the server, the client can continue processing while the server does its processing in the background. This means that a user can continue interacting with the client without noticing a lag in response. For example, a user can continue to move the mouse over a Google map and see a smooth, uninterrupted change in the display because extended parts of the map have been loaded asynchronously. The client does not have to wait for a response from the server before continuing, as is the case for the traditional synchronous approach.

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