Issues deploying dscExtension to Azure VMSS

I've been having some issues deploying a dscExtension to an Azure virtual machine scale set (VMSS) using a deployment template.

Here's how I've added it to my template:

{
  "name": "dscExtension",
  "properties": {
    "publisher": "Microsoft.Powershell",
    "type": "DSC",
    "typeHandlerVersion": "2.9",
    "autoUpgradeMinorVersion": true,
    "settings": {
      "ModulesUrl": "[concat(parameters('_artifactsLocation'), '/', 'MyDscPackage.zip', parameters('_artifactsLocationSasToken'))]",
      "ConfigurationFunction": "CmvmProcessor.ps1\\CmvmProcessor",
      "Properties": [
        {
          "Name": "ServiceCredentials",
          "Value": {
            "UserName": "parameters('administratorLogin')",
            "Password": "parameters('administratorLoginPassword')"
          },
          "TypeName": "System.Management.Automation.PSCredential"
        }
      ]
    }
  }
}

The VMSS itself is successfully deploying, but when I browse the InstanceView of the individual VMs, the dscExtension shows the failed status with an error message.

The problems I'm having are as follows:

The ARM deployment does not try to update the dscExtension upon redeploy. I am used to MSDeploy web app extensions where the artifacts are updated and the code is redeployed on each new deployment. I do not know how to force it to update the dscExtension with new binaries. In fact it only seems to give an error on the first deploy of the VMSS, then it won't even try again.

The error I'm getting is for old code that doesn't exist anymore.

I had a bug previously in a custom DSC Powershell script where I tried to use the -replace operator which is supposed to create a $Matches variable but it was saying $Matches didn't exist.

In any case, I've since refactored the code and deleted the entire resource group then redeployed. The dscExtension is still giving the same error. I've verified the blob storage account where my DSC .zip is located no longer has the code which is capable of producing this error message. Azure must be caching the dscExtension somewhere. I can't get it to use my new blob .zip that I upload before each deployment.

Any insight into the DSC Extension and how to force it to update on deploy?

It sounds like you may be running into multiple things here, so trying the simple one first. In order to get a VM extension to run on a subsequent deployment you have to "seed" it. (and you're right this is different than the rest of AzureRM) Take a look at this template:

https://github.com/bmoore-msft/AzureRM-Samples/blob/master/VMDSCInstallFile/azuredeploy.json

There is a property on the DSC extension called:

              "forceUpdateTag" : "changeThisToEnsureScriptRuns-maxlength=50",

The property value must be different if you ever want the extension to run again. So for example, if you wanted it to run every time you'd seed it with a random number or a guid. You could also use version numbers if you wanted to version it somehow. The point is, if the value in the template is the same as the one you're passing in, the extension won't run again.

That sample uses a VM, but the VMSS syntax should be the same. That property also applies to other extensions (e.g. custom script).

The part that seems odd is that you said you deleted the entire RG and couldn't get it to accept the new package... That sounds bad (i.e. like a bug). If the above doesn't fix it, we may need to dig deeper into the template and script. LMK...