Technologies of AJAX

If you search the Internet for AJAX, you are likely to notice a large number of items popping up under the AJAX name that don't seem to fit my definition. In most cases, these libraries provide the related functionality needed to finish your AJAX application, but other times, these libraries are just someone trying to jump on the AJAX bandwagon. When looking at these libraries and techniques, I divide them into three groups:

• Those directly used in AJAX

• Those closely related to AJAX

• Those that are just part of the rest of the RIA world

Most of the technologies directly related to AJAX are those that make up the "asynchronous" and the "XML" parts of the AJAX acronym. These are the libraries and techniques that provide the communication layer and the ways to encode the data that moves over it. In many cases, this is the XMLHttpRequest object, but IFrames are also popular, and cookies or embedded ActiveX/Java are yet another possibility. The "X" in AJAX is the technique used in the data encoding, and it's an area with a huge number of possibilities. Data can be transferred as plain text, HTML, XML, or any other type of format that might be convenient for the situation.

XML is extremely popular as a data format because it's supported by so many languages and because it is easy to describe arbitrary data types with it. Many times, XML is used to facilitate Remote Procedure Call (RPC) mechanisms, but it can also be used to directly describe the data being transferred. RPC allows for the direct mapping of JavaScript types to the server's data types (PHP, Java, and so on) and vice versa. XML has been used for this purpose before, so it includes many standardized formats such as those used in SOAP, XML-RPC, and WDDX. It can also include a custom XML format created for a specific AJAX implementation.

Another popular approach is to generate JavaScript directly on the server and send it to the client where it can be used directly. This approach is possible because JavaScript can run code from a string using its eval statement. This approach is often used in conjunction with others because the server-side language can't evaluate JavaScript directly, but a specific JavaScript Notation (JSON) can allow JavaScript to be used in both directions. This notation, called JavaScript Object Notation, is often used in RPC approaches because it can describe any data type JavaScript can support while still being easily parseable by other languages.

As we move beyond what is needed to allow for AJAX communication, we get the large set of related technologies. These provide the extra glue and user-interface elements that are needed for a complete AJAX application. These are generally JavaScript libraries; some are stand-alone and just provide a few features; others provide an entire framework for creating dynamic Web pages. In most cases, you'll need some sort of effects library. This library provides fades, swipes, and many other visual effects that can be used to draw attention to the HTML element that you've updated using AJAX.

In addition, there are a number of libraries that provide drag-and-drop functionality, which is less closely related to AJAX but is most useful when used in conjunction with AJAX. Some of these libraries allow AJAX communication in some setups, while they are just a related add-on in other setups. This mix-and-match of libraries is great because you use only what you need to get the job done. Keep in mind, however, that the Web browser will need to download all this JavaScript to actually run.

In the wider world of technologies related to AJAX, you get the JavaScript libraries and other technologies, such as Scalable Vector Graphics (SVG), that might be in an AJAX Web application, but they really don't have a direct relation to AJAX. These technologies make up the wider world of RIA, but they are also more expansive and harder to integrate into existing applications.