Thursday, November 19, 2009

Using the Microsoft Ajax Library 3.5 with the CDN

Recently Microsoft has announced its Microsoft Ajax content delivery network (CDN) which can significantly improve the performance of any ASP.NET AJAX web application. When it was first announced the CDN was almost useless for the current web applications based on ASP.NET 3.5 since it did not host the Microsoft Ajax library 3.5 and did not support content delivery via SSL. However in a very short period of time Microsoft was able to fix those problems (good job!) and now web applications built on ASP.NET 3.5 can benefit from using Microsoft Ajax CDN.

So how do you make MicrosoftAjax.js file being referenced from the CDN as opposed to the embedded file? Quite easy actually with the help of the ScriptManager control. Add the following declaration to a page (or Master page) where there is a ScriptManager control:

What that declaration means is that the script with a name MicrosoftAjax.js which is always automatically referenced by a ScriptManager from the System.Web.Extensions dll now should be referenced from that location: http://ajax.microsoft.com/ajax/3.5/MicrosoftAjax.js.

This is how it is referenced by default:
<script src="/ScriptResource.axd?d=eYUqBJhfSVL41hIDYkBL0tfaps9hoQId_48PydfbcyWH41vNvL68sk-l7P9FLAPz7b4vtI8WkZ-ezAF0b_ZkyG52wt9oUtaQ5ezFfGBr7LY1&t=ffffffffef976216" type="text/javascript"></script>

and after we include the new ScriptReference declaration:
<script src="http://ajax.microsoft.com/ajax/3.5/MicrosoftAjax.js" type="text/javascript"></script>

ScriptManager is even smart enough to automatically reference the debug version MicrosoftAjax.debug.js from the CDN when debugging is enabled in the web.config file:
<compilation debug="true">
<script src="http://ajax.microsoft.com/ajax/3.5/MicrosoftAjax.debug.js" type="text/javascript"></script>

So this is clear: we have our MicrosoftAjax.js referenced from the CDN and that improves our web application performance and saves us and the visitors some bandwidth since an internet browser will reuse the same cached copy of the MicrosoftAjax.js from the CDN for different web applications that reference it.

Using the CDN conditionally

What if we only wanted to reference MicrosoftAjax.js from the CDN when the web application is deployed to a production environment and use an embedded version in a development environment? That would make sense in order for the developers to work without having to be connected to the Internet. Once again it can be done but this time we'll need to write some code. We are going to add the MicrosoftAjax.js script reference dynamically depending on the debug value in the web.config file; we only add the script reference when debug="false":

Using the CDN via SSL

Another major case is when our web application has pages that are served via SSL. In this case we want to automatically select the correct CDN URL for the MicrosoftAjax.js. In order to do that we just modify the previous code:
So that's it for now. Enjoy using the Microsoft Ajax CDN.

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