Update Web.Config in an Azure Cloud Service package

09 August 2013

Windows Azure deployments are done using a convenient .cspkg and .cscfg files. The .cscfg is an xml config file and the .cspkg is essentially a zip file that contains your application code. This means you can build once and deploy to different environments by providing a different version of the .cscfg, making continuous deployment simple. Just keep the .cspkg file around and deploy it anywhere.

Problems arise when you need to modify something in the cspkg, such as the Web.Config for your web application. A common scenario where this is necessary is configuring Windows Identity Foundation to update a trusted issuer thumbprint or federation realm. Options to fix the problem are to create a new package for each environment or creating the package on demand as part of the deploy process. Microsoft has provided a way to create packages manually but it’s complicated to set up and involves duplicating a lot of work that’s done for us already in the MSBuild tasks for the cloud project.

The fix

An alternate approach I’ve had success with is to modify the Web.Config on role start in your web project based on values stored in the .cscfg configuration file. To do this, copy the Web.Config in your project and rename it to Web.Config_pretransform or something similar. Also stop tracking the Web.Config in your source control since it will just be generated as needed (but make sure the project still has a reference to it). Next add code to your WebRole.cs to do the file modification like so:

public override bool OnStart()
{
   UpdateConfigs();
}

Fill in the UpdateConfigs method with code to open the Web.Config_pretransform using Microsoft.Web.Administration.ServerManager.

private void UpdateConfigs()
{
  using ( var server = new ServerManager() )
  {
    Site site = server.Sites[RoleEnvironment.CurrentRoleInstance.Id + "_Web"];
    string physicalPath = site.Applications["/"].VirtualDirectories["/"].PhysicalPath;
    string inputWebConfigPath = Path.Combine( physicalPath, "web.config_pretransform" );
    string outputWebConfigPath = Path.Combine( physicalPath, "web.config" );

    File.Copy( inputWebConfigPath, outputWebConfigPath, overwrite:true );

    SetWIFWebConfigSettings( outputWebConfigPath );
  }
}

The code above grabs the physical path of the website from IIS and passes it off to the SetWIFWebConfigSettings method. This method can then parse and update the Web.Config using your favorite XML parser. Finally, the code below shows how to update the realm and requireHttps attributes using values from the .cscfg:

private static void SetWIFWebConfigSettings( string webConfigPath )
{
  var doc = XDocument.Load( webConfigPath );
  var wifConfig = doc.Descendants( "microsoft.identityModel" ).Single();

  var wsFederation = wifConfig.Descendants( "federatedAuthentication" ).Single()
    .Descendants( "wsFederation" ).Single();
  wsFederation.SetAttributeValue( "realm", CloudConfigurationManager.GetSetting( "WifWsFederationRealm" ) );
  wsFederation.SetAttributeValue( "requireHttps", "true" );

  doc.Save( webConfigPath );
}
  • It’s important to note that Azure will not actually put our instance on the load balancer until after the RoleStart method has finished which allows us to do these modifications.

One last thing we need to do is make it work locally as well. An easy fix is to copy the Web.Config.pretransform to the Web.Config location prior to building the project if it doesn’t exist.

<PropertyGroup>
   <PreBuildEvent>if not exist "$(ProjectDir)\Web.config" (copy /Y "$(ProjectDir)\Web.config_pretransform" "$(ProjectDir)\Web.config") else (echo web.config already exists in $(ProjectDir), skipping)</PreBuildEvent>
</PropertyGroup>

That’s all we need to do to modify the Web.Config on role start in an Azure Cloud Service. It lets us keep all environment specific settings in the .cscfg which means we can deploy one package to any environment.

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