Azure CDN – Cache busting CSS image references and minification

In my previous post I discussed my AzureCdn.Me nuget package which can be used to add a cache busting query string to your CSS file references.  In this post we look at cache busting image references contained in your CSS files, and minifying the result.

Cache Busters

Inexplicably the Azure CDN doesn’t ship with a big red reset button to allow you to clear the cache for the CDN.  Meaning if you upload a new image or file that’s currently cached it may take hours or even days before the CDN refreshes the cache.

As I outlined in my previous post the easiest way round this is to add a query string to the resource reference, which differs on each deploy.  Meaning the CDN will see the resource as changed and pull down a fresh copy.

All well and good, but as often as not your CSS file may actually contain image reference within, ie

    background-image: url("./images/lolcat1.jpg");
    background-size : 100% 100%;

Now if you modify that image it won’t be updated on the CDN, which is bad news.

Introducing AzureCdn.Me.Nant

Any regular reader of my blog will know I’m a big fan of Nant.  So I thought I’d write a build task to append a cache-busting query string onto the end of the image ref.  I spent a bit of time investigating whether anyone else had addressed this problem, to my surprise I couldn’t find any.

In the course of that investigation I took a look at YUI Compressor.  This open source tool minifies your CSS and Javascript.  I downloaded the code, and realised I could enhance it to add a cache buster prior to doing the minification.  Anyone interested can check out my fork on Github here.


I packaged up my changes as a nuget package AzureCdn.Me.Nant.  You can install it into your build project.

Step 1 – You need to ensure all image references within your css are quoted, eg:

  • good – url(“image.png”)
  • good – url(‘image.png’)
  • bad – url(image.png) – no quotes

If image refs aren’t quotes the YUI Compressor code won’t pick it up

Step 2 – Add a task similar to this into your build file, change the params to suit:

<loadtasks assembly=".\lib\Yahoo.Yui.Compressor.Build.Nant.dll" verbose="true" />
<target name="CompressCss">
    <echo message="Compressing files"/>
        <include name="..\AzureCdnMe.Sample.Web\cdn\content\azurecdnme.css" />
        <include name="..\AzureCdnMe.Sample.Web\cdn\content\bootstrap.css" />
        <include name="..\AzureCdnMe.Sample.Web\cdn\content\bootstrap-responsive.css" />

The new property I’ve added is cdnQueryString.   When it runs it will both minify and cache bust your CSS image references, by appending a the supplied version number.

Referencing your code

Once you’ve minified the CSS you need to ensure your solution uses the new minified version.  If you install my AzureCdn.Me package you can find a helper method that will allow you to determine if you are in debug mode or release, eg:

if (Html.IsInDebugMode())
    <link href="@Url.AzureCdnContent("~/Content/bootstrap.css")" rel="stylesheet"  />
    <link href="@Url.AzureCdnContent("~/Content/bootstrap-responsive.css")" rel="stylesheet"  />
    <link href="@Url.AzureCdnContent("~/Content/azurecdnme.css")" rel="stylesheet"  />
    <link href="@Url.AzureCdnContent("~/Content/minified.css")" rel="stylesheet" type="text/css" />

Show me the code

You can find a working sample of the above here, where it’s hopefully obvious what is going on.


If anyone is interested it would be easy to enhance the YUI Compressor code to add an MSBuild task to add the cdnQueryString parameter.  Also the cdnQueryString param will work in the same way if you want to also minify any javascript.

Azure CDN – with AzureCdn.Me and MVC3

I’ve been doing a fair bit of work with the Azure CDN recently.  I’ve put together this blog post outlining how to get started and to give an overview of some tooling I’ve written to help you get up and running.

Azure CDN Quickstart

Currently CDNs need to be created in the original Silverlight portal and need to be attached to an existing Hosted Service or Storage Account.  We’ll attach ours to a storage account.  When creating you should click Enable CDN  and Query String (this option will invalidate the CDN cache on any resources if you vary the query string in the resource address, more on this later).  You should now have a CDN in the cloud, now to populate it.

Populating the CDN with static content

Assuming you haven’t altered the MVC standard layout, your static content is probably in the Content folder.  However, where-ever your static content is residing you’ll need to create a new folder titled CDN and move your content into it.  The Azure CDN expects to find your content in the CDN folder.  The easiest thing to do is to cut and paste your Content folder into the CDN folder.  You should now be ready to update the image references.

Introducing AzureCdn.Me

To make the process of referencing images on the Azure CDN a bit more straight-forward I created the AzureCdn.Me nuget package which includes a couple of extension methods to ease the pain.  So install-package AzureCdn.Me into your web project.  AzureCdn.Me will create a CDN folder for you and add the extensions Url.AzureCdnContent and Html.IsDebugMode and a couple of parameters into your web.config.  If we open web.config we can see the new params:

<add key="AzureCDNEndpoint" value="CDN" />
<add key="AzureCDNDebug" value="true" />

The parameters have been defaulted with the values appropriate for debug.  We can now alter our _layout file to use the extensions method.  First off you’ll need to reference the package by adding a using statement at the top of the file, eg:

@using AzureCdn.Me.Code.Extensions
<!DOCTYPE html>
    <title>AzureCdn.Me Sample</title>
    <meta charset="utf-8" />
    <link href="@Url.AzureCdnContent("~/Content/bootstrap.css")" rel="stylesheet" />
    <link href="@Url.AzureCdnContent("~/Content/bootstrap-responsive.css")" rel="stylesheet" />
    <link href="@Url.AzureCdnContent("~/Content/azurecdnme.css")" rel="stylesheet" />

Note I haven’t altered the address of the static files.  Now if we run our project everything should still be fine, however if we open Firebug, we can see that the extension method has both appended the CDN folder to the front of the string, as read from web.cofig, additionally and importantly it’s also added a querystring containing a cache busting random number.  As it’s stored in a static class this number should stay the same until you redploy.  This is very useful, as if we create subsequent versions of our website, each deploy will force the CDN to fetch a fresh version of the stylesheet.


There is also an overloaded where you can pass in a value of your choice.  For example, you could pass in todays date, which would mean that the cache would refresh itself every 24hours.  Or you could pass in the version number of the executing assembly etc.  Hopefully you get the idea.

<link href="@Url.AzureCdnContent("~/Content/bootstrap.css", DateTime.Now.ToString("ddMMyy"))" rel="stylesheet"  />

Doing it Live

So once you’re happy with your site, you want to push your site live, you’re going to need to replace the debug AzureCDNEndpoint value in web.config with the actual value of your CDN endpoint, ie  The easiest way of doing that is with web config transformations.  Once deployed your Css files, images etc will be served up from the Azure CDN, and because of the querystring component, any changes will always be picked up as soon as you push the changes live.

In my next post I’ll outline how to minify your css and how you can invalidate the cache for any images that are referenced in your CSS and Javascript .