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AJAX, shorthand for Asynchronous JavaScript and XML, is a web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user makes a change. This is meant to increase the web page's interactivity, speed, and usability.

AJAX represents a set of commonly used techniques, like HTML/XHTML, CSS, Document Object Model(DOM), XML/XSLT, Javascript and the XMLHttpRequest object, to create RIA's (Rich Internet Applications).

The use of AJAX has led to an increase in interactive or dynamic interfaces on web pages and better quality of Web Services due to the asynchronous mode. Data is usually retrieved using the XMLHttpRequest object. Despite the name, the use of JavaScript and XML is not actually required, nor do the requests need to be asynchronous.

ASP.NET AJAX is the free Microsoft AJAX framework for building highly interactive and responsive web applications that work across all popular browsers. The ASP.NET AJAX framework includes Server-Side ASP.NET AJAX, Client-Side ASP.NET AJAX, the AJAX Control Toolkit, and the jQuery library. ASP.NET AJAX enables developers to choose their preferred method of AJAX development, whether it is server-side programming, client-side programming, or a combination of both.
AJAX is based on the following web standards:
  • XMLHttpRequest Object
  • JavaScript
  • DOM
  • XML
  • CSS
Some of the benefits of using AJAX in web-based application include the following:
  • Improved user experience, performance and efficiency by negating page postbacks
  • Reduced server hits and network laod
  • Faster page rendering and improved response time
  • Introducing partial page updates to refresh only parts of the web page
  • Support for both synchronous and asynchronous communication
  • Providing a framework with a collection of integrated server and client-side components to ease development of web applications that can leverage the power of AJAX
ASP.Net AJAX has come up with several server controls. These are the following:
  • ScriptManager
  • ScriptManagerProxy
  • UpdatePanel
  • UpdateProgress
  • Timer
The ScriptManager manages all ASP.Net AJAX resources on a page and renders the links for the ASP.Net AJAX client libraries, which lets you use AJAX functionality like PageMethods, UpdatePanels etc. It creates the PageRequestManager and Application objects, which are prominent in raising events during the client life cycle of an ASP.Net AJAX Web page. It also helps you create proxies to call web services asynchronously.
No. You can use only one ScriptManager on a page.
A page can contain only one ScriptManager control. If you have a Master-Content page scenario in your application and the MasterPage contains a ScriptManager control, then you can use the ScriptManagerProxy control to add scripts to content pages.

Also, if you come across a scenario where only a few pages in your application need to register to a script or a web service, then its best to remove them from the ScriptManager control and add them to individual pages, by using the ScriptManagerProxy control. That is because if you added the scripts using the ScriptManager on the Master Page, then these items will be downloaded on each page that derives from the MasterPage, even if they are not needed, which would lead to a waste of resources.

The UpdatePanel enables you to add AJAX functionality to existing ASP.Net applications. It can be used to update content in a page by using Partial-page rendering. By using Partial-page rendering, you can refresh only a selected part of the page instead of refreshing the whole page with a postback.
Yes, you can do that. You would want to nest update panels to basically have more control over the Page Refresh.
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