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1. Rich Internet Applications

Software evolution always seems to happen in this typical fashion: Once a technology is grounded firmly (meaning the connections between the parts work and the architecture is fundamentally sound), upgrading the end user's experience becomes a much higher priority. Application technology is in this stage, and the general term for this kind of application is a Rich Internet Application (RIA). AJAX is one means of producing Rich Internet Applications. (Microsoft Silverlight is another popular means of creating RIAs.)

The primary reason for the existence of AJAX is to improve the standard HTTP GET/POST idiom with which Web users are so familiar. That is, the standard Web protocol in which entire forms and pages are sent between the client and the server is getting a whole new addition.

Although standard HTTP is functional and well understood by Web developers, it does have certain drawbacks—the primary one is that the user is forced to wait for relatively long periods while pages refresh. This has been a common problem in all event-driven interfaces. (The Windows operating system is one of the best examples.) AJAX introduces technology that shields end users from having to wait for a whole page to post.

Think back to the way HTTP typically works. When you make a request (using GET or POST, for example), the Web browser sends the request to the server, but you can do nothing until the request finishes. That is, you make the request and wait—watching the little progress indicator in the browser. When the request returns to the browser, you can begin using the application again. The application is basically useless until the request returns. In some cases, the browser's window even goes completely blank. Web browsers have to wait for Web sites to finish an HTTP request in much the same way that Windows-based programs have to wait for message handlers to complete their processing. (Actually, if the client browser uses a multithreaded user interface such as Windows Internet Explorer, users can usually cancel the request—but that's all they can really do.) You can easily demonstrate this problem by introducing a call to System.Threading.Thread.Sleep inside the Page_Load method of an ASPX page. By putting the thread to sleep, you force the end user to wait for the request to finish.

The AJAX solution to this problem is to introduce some way to handle the request asynchronously. What if there were a way to introduce asynchronous background processing into a Web site so that the browser would appear much more responsive to the user? What if (for certain applications) making an HTTP request didn't stall the entire browser for the duration of the request, but instead seemed to run the request in the background, leaving the foreground unhindered and changing only the necessary portion of the rendered page? The site would present a much more continuous and smooth look and feel to the user. As another example, what if ASP.NET included some controls that injected script into the rendered pages that modified the HTML Document Object Model, providing more interaction from the client's point of view? Well, that's exactly what ASP.NET AJAX support is designed to do.

2. What Is AJAX?

AJAX formalizes a style of programming meant to improve the UI responsiveness and visual appeal of Web sites. Many AJAX capabilities have been available for a while now. AJAX consolidates several good ideas and uses them to define a style of programming and extends the standard HTTP mechanism that is the backbone of the Internet. Like most Web application development environments, ASP.NET takes advantage of HTTP capabilities in a very standard way. The browser usually initiates contact with the server using an HTTP GET request, followed by any number of POSTs. The high-level application flow is predicated upon sending a whole request and then waiting for an entire reply from the server. Although the ASP.NET server-side control architecture greatly improves back-end programming, users still get their information a whole page at a time. It operates almost like the mainframe/terminal model popular during the 1970s and early 1980s. However, this time the terminal is one of many modern sophisticated browsers and the mainframe is replaced by a Web server (or Web farm).

The standard HTTP round-trip has been a useful application strategy, and the Web grew up using it. While the Web was developing in the late 1990s, browsers had widely varying degrees of functionality. For example, browsers ranged all the way from the rudimentary America Online Browser (which had very limited capabilities) to cell phones and personal digital assistants (PDAs), to more sophisticated browsers such as Internet Explorer and Netscape Navigator, which were rich in capability. For instance, Internet Explorer supports higher level features such as JavaScript and Dynamic HTML. This made striking a balance between usability of your site and the reach of your site very difficult prior to the advent of ASP.NET.

However, the majority of modern computing platforms can run a decent browser that can process client-side scripting. These days, most computing environments run a modern operating system, such as the Windows Vista or Windows 7 operating systems, or even Macintosh OS X. These environments run browsers fully capable of supporting XML and JavaScript. With so many Web client platforms supporting this functionality, it makes sense to take advantage of the capabilities.

In addition to extending standard HTTP, AJAX is also a very clever way to use the Web service idiom. Web services are traditionally geared toward enterprise-to-enterprise business communications. However, Web services are also useful on a smaller scale for handling Web requests out of band. ("Out of band" simply means making HTTP requests using other methods instead of the standard page posting mechanism.) AJAX uses Web services behind the scenes to make the client UI more responsive than it is for traditional HTTP GETs and POSTs.


One of the primary changes AJAX brings to Web programming is that it depends on the browser taking an even more active role in the process. Instead of the browser simply rendering streams of HTML and executing small custom-written script blocks, AJAX includes some new client-script libraries to facilitate the asynchronous calls back to the server. AJAX also includes some basic server-side components to support these new asynchronous calls coming from the client. There's even a community-supported AJAX Control Toolkit available for the ASP.NET AJAX implementation. Figure 1 shows the organization of ASP.NET AJAX support.

Figure 1. The conceptual organization of ASP.NET AJAX support layers.

3.1. Reasons to Use AJAX

If traditional ASP.NET development is so entrenched and well established, why would you want to introduce AJAX? At first glance, AJAX seems to introduce some new complexities into the ASP.NET programming picture. In fact, it seems to reintroduce some programming idioms that ASP.NET was designed to deprecate (such as overuse of client-side script). However, AJAX promises to produce a richer experience for the user. Because ASP.NET support for AJAX is nearly seamless, the added complexities are well mitigated. When building a Web site, there are a few reasons you might choose to enable your ASP.NET site for AJAX:

  • AJAX improves the overall efficiency of your site by performing, when appropriate, parts of a Web page's processing in the browser. Instead of waiting for the entire HTTP protocol to get a response from the browser, you can push certain parts of the page processing to the client to help the client to react much more quickly. Of course, this type of functionality has always been available—as long as you're willing to write the code to make it happen. ASP.NET AJAX support includes a number of scripts so that you can get a lot of browser-based efficiency by simply using a few server-side controls.

  • ASP.NET AJAX introduces to a Web site UI elements usually found in desktop applications, such as rectangle rounding, callouts, progress indicators, and pop-up windows that work for a wide range of browsers (more browser-side scripting—but most of it has been written for you).

  • AJAX introduces partial-page updates. By refreshing only the parts of the Web page that have been updated, the user's wait time is reduced significantly. This brings Web-based applications much closer to desktop applications with regard to perceived UI performance.

  • AJAX is supported by most popular browsers—not just Internet Explorer. It works for Mozilla Firefox and Apple Safari, too. Although it still requires some effort to strike a balance between UI richness and the ability to reach a wider audience, the fact that AJAX depends on features available in most modern browsers makes this balance much easier to achieve.

  • AJAX introduces a huge number of new capabilities. Whereas the standard ASP.NET control and page-rendering model provides great flexibility and extensibility for programming Web sites, AJAX brings in a new concept—the extender control. Extender controls attach to existing server-side controls (such as the TextBox, ListBox, and DropDownList) at run time and add new client-side appearances and behaviors to the controls. Sometimes extender controls can even call a predefined Web service to get data to populate list boxes and such (for example, the AutoComplete extender).

  • AJAX improves on ASP.NET Forms Authentication and profiles and personalization services. ASP.NET support for authentication and personalization provides a great boon to Web developers—and AJAX just sweetens the offerings.

These days, when you browse different Web sites, you run into many examples of AJAX-style programming. Here are some examples:

3.2. Real-World AJAX

Throughout the 1990s and into the mid-2000s, Web applications were nearly a throwback to 1970s mainframe and minicomputer architectures. However, instead of a single large computer serving dumb terminals, Web applications consist of a Web server (or a Web farm) connected to smart browsers capable of fairly sophisticated rendering capabilities. Until recently, Web applications took their input from HTTP forms and presented output in HTML pages. The real trick in understanding standard Web applications is to see the disconnected and stateless nature of HTTP. Classic Web applications can show only a snapshot of the state of the application.

Microsoft supports standard AJAX idioms and patterns in the ASP.NET framework. However, AJAX is more a style of Web programming involving out-of-band HTTP requests than any specific technology.

You've no doubt seen sites engaging the new interface features and stylings available through AJAX programming. Examples include,, and Very often while browsing these sites, you'll see modern features such as automatic page updates that do not require you to generate a postback explicitly. Modal-type dialog boxes that require your attention appear until you dismiss them. These are all features available through AJAX-style programming patterns and the ASP.NET extensions (for example, a rich set of AJAX server-side controls and extensions) for supporting AJAX.

If you're a long-time Microsoft environment Web developer, you might be asking yourself whether AJAX is something really worthwhile or whether you might be able to get much of the same type of functionality using a tried and true technology such as DHTML.

3.3. AJAX in Perspective

Any seasoned Web developer targeting Internet Explorer as the browser is undoubtedly familiar with Dynamic HTML (DHTML). DHTML is a technology that runs at the browser for enabling Windows desktop-style UI elements in the Web client environment. DHTML was a good start, and AJAX brings the promise of more desktop-like capabilities to Web applications.

AJAX makes available wider capabilities than DHTML does. With DHTML, primarily you can change the style declarations of an HTML element through JavaScript. However, that's about as far as it goes. DHTML is very useful for implementing such UI features as having a menu open when the mouse pointer rests on it. AJAX expands on this idea of client-based UI using JavaScript as well as out-of-band calls to the server. Because AJAX is based on out-of-band server requests (rather than relying only on a lot of client script code), AJAX has the potential for much more growth in terms of future capabilities than does DHTML.
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