Server framework, there is a collection of server controls and services that extend the ASP.NET 2.0 framework. These are commonly known as server extensions.

They include support for localization, globalization, debugging, tracing, web services, and application services and are essentially meant for reducing the roundtrips to the web server and providing incremental updates. The ASP.NET Ajax server extensions framework includes the following:

• ASP.NET Ajax server controls
• Application services bridge
• Web services bridge
  

The ASP.NET Ajax Server Controls

You can group the ASP.NET Ajax server controls into two distinct groups: Script Management and Partial Update.

The Script Management Group

• ScriptManager
• ScriptManagerProxy

The ScriptManager is the most important control for an Ajax-enabled web page. It facilitates partial page rendering of a web page using the UpdatePanel, provides access to web services, and enables registering the scripts. The other control in this group — the ScriptManagerProxy— as the name suggests, acts as a “proxy” to the ScriptManager control instance.

The Partial Update Group

• UpdatePanel
• UpdateProgress
• Timer

The UpdatePanel acts as a container of controls that you can use for specifying the portion of your web page where you need partial page rendering. This is the control that works together with the ScriptManager control to provide you with an asynchronous callback feature—that is, an asynchronous mode of operation that requires less network and server load and provides you with an awesome user experience. The UpdateProgress controls work together with the UpdatePanel control and are displayed automatically whenever an asynchronous update operation is started.

The Application Services Bridge

The application services bridge is an API that enables the different application services like authentication and profile management to be invoked from the client-side script.

The Web Services Bridge

The web services bridge is an API that provides access to external web services from the client-side script. You have JavaScript proxies available to access these web services.

Both of these bridges are responsible for asynchronous communication support.

Before .NET Ajax architecture, let’s discuss how both an Ajax and a non-Ajax web application work. In a non-Ajax web application environment, the Http request that is sent by the client browser is directly processed at the web server at the server side. The web server processes this incoming request and returns the response back to the client browser in the form of HTML and CSS. This happens synchronously—that is, the browser has to wait until the request is complete.

On the contrary, in an Ajax web application environment, the Ajax engine sits in between the client browser and the web server. It is the Ajax layer that is responsible for initiating requests both synchronously and asynchronously to the web server. The web server processes these incoming requests and returns the response to the Ajax engine rather than the client browser directly. The Ajax engine returns the HTML and CSS data to the client browser. This model facilitates faster responsiveness by minimizing postback delays and also by reducing the server overheads and network loads. Figures 2.2 and 2.3 illustrate how these two models work.

There are two perspectives to the ASP.NET Ajax architecture: the server and the client. In other words, you get two APIs with this framework: the server-side API and the client-side API. We will learn both of them as we progress through this section.

We refer to the client API as the client framework and the server API as the server framework. It should also be noted that the client framework is not tightly coupled to the server framework—that is, the client framework can work independent of the server framework. Thus, you can use the client framework to design and develop applications even in PHP or other web-based languages as well!