If you’ve been involved in Web development for any amount of
time, the latest trend of developing applications with remote scripting
capabilities via AJAX
(Asynchronous JavaScript + XML) will not be a surprise. After all, it has been
available for many years with the drawback being browser inconsistencies.
Microsoft always promoted ActiveX as the way to go, but this approach has
changed with AJAX’s rise in popularity. Let’s take a
closer look at the AJAX revolution, including Microsoft’s involvement and

What is AJAX?

Defining AJAX is not as simple as pointing to a W3C page
because it is a collection of technologies. It combines the following:

  • Standards-based
    presentation layer utilizing XHTML
    and CSS
  • The DOM (Document Object Model) is used for
    dynamic display and user interaction
  • Asynchronous
    data retrieval via XMLHttpRequest
  • XML and XSLT allow data interchange and
  • JavaScript
    is used to tie everything together

The big difference between the AJAX approach and traditional Web development
is remote scripting. Remote scripting allows client-side JavaScript to request
data from a server without refreshing the Web page. This is accomplished via JavaScript
and the XMLHttpRequest object. Remote scripting moves
some of the processing to the client (browser), which greatly reduces the number
of calls to the Web server.

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Microsoft first implemented the XMLHttpRequest
object in Internet Explorer 5 for Windows as an ActiveX object.
Outlook Web Access was first developed with this ActiveX component. Engineers
on the Mozilla project implemented a compatible
native version for Mozilla 1.0 (and Netscape 7) with Apple
adding support in their Safari 1.2 browser. Similar functionality is covered in
a proposed W3C standard. In
the meantime, the XMLHttpRequest object has become a
de facto standard.

A middle man

A traditional Web application triggers a call to the HTTP
server for a user action or request. In turn, the server performs some tasks
and returns an HTML page to the requesting client. It is a disconnected user
experience, where the user is left waiting much of the time while the server
requests finish.

AJAX applications eliminate the start-stop-start-stop nature
of Web interactions by introducing an intermediary between the user and the
server. The client browser loads the AJAX engine upon the start of a session.
The AJAX engine is written in JavaScript and is usually tucked away in a hidden
frame. It is responsible for both rendering the user interface and
communicating with the server. The AJAX engine allows the user’s interaction
with the application to happen asynchronously independent of communication with
the Web server.

AJAX in action

The AJAX approach is appealing to major companies like Google and Amazon. Google
has used the AJAX approach extensively in the development of its Gmail, Google Suggest, and Google Maps Web applications. (Actually, Google has made
a huge investment in AJAX
in all of its major recently developed or enhanced products.) Likewise, Amazon
followed a similar course with its A9 search
engine. There are many more examples available every day.

Microsoft’s AJAX

Of course, Microsoft is at work to develop a better AJAX. Atlas
is the codename for their upcoming AJAX
support. It goes well beyond the original concept, including integrated
debugging with Visual Studio. In addition, new ASP.NET server controls will be
available to easily bind client-side controls to server-side code. The Atlas
Client Script Framework will make it easy to interact with pages and such.
However, it will not be available in Visual Studio 2005.

Microsoft recently announced that the Atlas Client Script
Framework will include the following components (as detailed on the Atlas
project site):

  • Extensible
    core framework adding features to JavaScript such as lifetime management,
    inheritance, multicast event handlers, and interfaces.
  • Base
    class library for common features such as rich string manipulation,
    timers, and running tasks.
  • User
    interface framework for attaching dynamic behaviors to HTML.
  • A
    network stack simplifying server connectivity and Web service access.
  • A
    set of controls for rich user interface development–such as auto-complete
    textboxes, animation, and drag-and-drop.
  • A
    browser compatibility layer addressing browser scripting behavior

These announcements are preliminary, so they can easily
change before the actual product is released. If you
can’t wait for Microsoft, the freely available Ajax.NET
library for the Microsoft .NET Framework is available now.

Drawbacks to AJAX

The AJAX approach requires users to have JavaScript enabled
in their browser. While this should not be a major problem, it is a consideration.
Likewise, these applications have to be tested rigorously to deal with the
quirks of different browsers and platforms; however, this is already the case
with any browser-based application not contained within an Intranet where the
target browser can be controlled.

One complaint about the AJAX approach centers on it
disabling the proper function of the browser’s Back button since the dynamic
update of a page is not recognized by the browser as going to a different page.
There are many workarounds for this problem with an IFRAME’s
often utilized approach (Google Maps uses this

One last criticism that I find amusing is people claiming
AJAX is just a new term introduced as a marketing vehicle for older
technologies. While this may be true, at least the included technologies are
mature and tested.

New twist on an old approach

approach to Web development is not new, but the broad support by all major
browsers does make it more accessible and applicable to the Web development
community at large. The technologies utilized in AJAX are mature and stable. It allows you to
develop rich applications that require less server roundtrips, so the user
spends less time waiting.