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The UpdatePanel control is probably the most important control in the ASP.NET AJAX package. It will AJAXify controls contained within it, allowing partial rendering of the area. 

Timer controls allow you to do postbacks at certain intervals. If used together with UpdatePanels, which is the most common approach, it allows for timed partial updates of your page, but it can be used for posting back the entire page as well. In this chapter we will focus on using timers with UpdatePanels, so if you have not already read the chapter on UpdatePanels, please do so now. 
Here is a small example of using the Timer control. It simply updates a timestamp every 5 seconds.

ASP.NET AJAX is a free framework to implement Ajax in web applications, for quickly creating efficient and interactive Web applications that work across all popular browsers. The Ajax Framework is powered with -

  • Reusable Ajax Controls
  • Support for all modern browsers
  • Access remote services and data from the browser without tons of complicated script. 
Versions of Ajax release 
  • ASP.NET Ajax Framework 1.0 (earlier release to this was called the Atlas) 
  • ASP.NET Ajax Framework 1.0 was available as a separate download for ASP.NET 2.0

XAJAX uses XML as a transport for data between the webpageand server, and you do not write your own JavaScript data handlers tomanipulate the data received from the server. Instead you use a .net class andbuilt in JavaScript methods, a combination that works very similar to theHTML_AJAX_Action class and haSerializer combo. XAJAX is designed for simplicityand ease of use. HTML_AJAX allows for multiple transmission types for your AJAXdata - such as urlencoding, JSON, serialization, plain text, with othersplanned, and has a system you can use to write your own serializers to meetyour specific needs. HTML_AJAX has a class to help generate JavaScript and anaction system similar to XAJAX "action pump" that allows you to avoidwriting JavaScript data handlers if you desire. But it also has the ability towrite your own data handling routines, automatically register classes andmethods using a server "proxy" script, do different types ofcallbacks including grabbing remote urls, choose between sync and asyncrequests, has iframe xmlhttprequest emulation fallback capabilities for userswith old browsers or disabled activeX, and is in active development with more featuresplanned (see the Road Map for details) HTML_AJAX has additional features suchas client pooling and priority queues for more advanced users, and even a JavaScriptutility class. Although you can use HTML_AJAX the same way you use XAJAX, theadditional features make it more robust, extensible and flexible. And it is apear package, you can use the pear installer to both install and keep it up todate. If you are asking which is better with most .net scripts it is a matterof taste and need. 

For concurrent requests, declare separate XmlHttpRequest objects for each request. For example, for request to get data from an SQL table1, use something like this... 

There are not that many tools out there that will support both client-side and server-side debugging. Below is some information on the client-side debuggers on some of the commonly used browsers-
Firefox/Mozilla/Netscape - Have a built in debugger Venkman which can be helpful but there is a Firefox add on known as FireBug which provides all the information and AJAX developer would ever need including the ability to inspect the browser DOM, console access to the JavaScript runtime in the browser, and the ability to see the HTTP requests and responses (including those made by an XMLHttpRequest). I tend to develop my applications initially on Firefox using Firebug then venture out to the other browsers. 
Safari - Has a debugger which needs to be enabled. See the Safari FAQ for details. 
Internet Explorer - There is MSDN Documentation on debugging JavaScript. A developer toolbar for Internet Explorer may also be helpful. While debuggers help a common technique knowing as "Alert Debugging" may be used. In this case you place "alert()" function calls inline much like you would a System.out.println. While a little primitive it works for most basic cases. Some frameworks such as Dojo provide APIs for tracking debug statements.

No it is not necessary. AJAX gives interaction designers more flexibility. However, the more power we have, the more caution we must use in exercising it. We must be careful to use AJAX to enhance the user experience of our applications, not degrade it. 

There are some situations when we do not have to use huge and massive ajax libraries to over-burden our website.In these situations, micro or light weight libraries play vital roles to fulfill our needs.There are several factors that are kept in mind while implementing the ajax libraries in our applications.These factors are described here-
Browser gets slower
This is the fact that most (not all) ajax based applications make the browsers damn slow (especially massive libraries). Due to this frustration, the user keep ending the request due to the delusion that the request to the web server is not acknowledge. That thing brings web pages hanged.
Search Engine crawling problem
Search engine crawlers get frustrated by waiting for the whole response to come, hence our SEO value gets down.
Unnecessary animation addition
Have you ever seen that if you request a site, usually an animated circle revolving takes place to indicate the user that your inquiry is processing and please do not re click to restart the same thing.

Reading Xml with JQuery is an easier approach if we have interest on the usage of client-side script as this script does not post back the whole page to the server and hence, speed the network performance.

It depends. Clearly the X in AJAX stands for XML, but several AJAX proponents are quick to point out that nothing in AJAX, per se, precludes using other types of payload, such as, JavaScript, HTML, or plain text. 
XML - Web Services and AJAX seem made for one another. You can use client-side API for downloading and parsing the XML content from RESTful Web Services. (However be mindful with some SOAP based Web Services architectures the payloads can get quite large and complex, and therefore may be inappropriate with AJAX techniques.) 
Plain Text - In this case server-generated text may be injected into a document or evaluated by client-side logic. 
JavaScript - This is an extension to the plain text case with the exception that a server-side component passes a fragment of JavaScript including JavaScript object declarations. Using the JavaScript eval () function you can then create the objects on the client. JavaScript Object Notation (JSON), which is a JavaScript object based data exchange specification, relies on this technique. 
HTML - Injecting server-generated HTML fragments directly into a document is generally a very effective AJAX technique. However, it can be complicated keeping the server-side component in sync with what is displayed on the client. Mashup is a popular term for creating a completely new web application by combining the content from disparate Web Services and other online API. 

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