Optimising IaaS deployments in Azure Resource Templates

Unlike most of my recent posts this one won’t have code in it. Instead I want to talk about concepts and how you should look long and hard at your templates to optimise deployment.

In my previous articles I’ve talked about how nested deployments can help apply sensible structure to your deployments. I’ve also talked about things I’ve learned around what will successfully deploy and what will give errors. Nested deployments are still key, but the continuous cycle of improvements in Azure means I can change my information somewhat around what works well and what is likely to fail. Importantly, that change allows us to drastically improve our deployment time if we have lots of virtual machines.

I’d previously found that unless I nested the extensions for a VM within the JSON of the virtual machine itself, I got lots of random deployment errors. I am happy to now report that situation has improved. The result of that improvement is that we can now separate out the extensions deployed to a virtual machines from the machine itself. That separates the configuration of the VM, which for complex environments almost certainly has a prescribed sequence, from the deployment of  the VM, which almost certainly doesn’t.

To give you a tacit example, in the latest work at Black Marble we are deploying a multi-server environment (DC, ADFS, WAP, SQL, BizTalk, Service Bus and two IIS servers) where we deploy the VMs and configure them. With my original approach, hard-fought to achieve a reliable deploy, each VM was pushed and fully configured in the necessary sequence, domain controller first.

With our new approach we can deploy all eight VMs in that environment simultaneously. We have moved our DSC and Custom Script extensions into separate resource templates and that has allowed some clever sequencing to drastically shorten the time to deploy the environment (currently around fifty minutes!).

We did this by carefully looking at what each step was doing and really focusing on the dependencies:

  • The domain controller VM created a new virtual machine. The DSC extension then installed domain services and certificate services and created the domain. The custom script then created some certificated.
  • The ADFS VM created a new virtual machine. The DSC extension then joined that server to the domain. The custom script then copied the certificate from the DC and configured ADFS.
  • The WAP VM created a new virtual machine. The DSC extension then joined that server to the domain. The custom script then copied the certificate from the DC and configured the proxy for the configured ADFS service.

Hopefully you can see what we saw: Each machine had three phases of configuration and the dependencies were different, giving us three separate sequences:

  1. The VM creations are completely independent. We could do those in parallel to save time.
  2. The DSC configuration for the DC has to be done first, to create the domain. However, the ADFS and WAP servers have DSC that are independent, so we could do those in parallel too.
  3. The custom script configurations have a definite sequence (DC – ADFS – WAP) and the DC script depends on the DC having run it’s DSC configuration first so we have our certificate services.

Once we’ve identified our work streams it’s a simple matter of declaring the dependencies in our JSON.

Top tip: It’s a good idea to list all the dependencies for each resource. Even though the Azure Resource Manager will infer the dependency chain when it parses the template, it’s much easier for humans to look at a full list in each resource to figure out what’s going on.

The end result of this tinkering? We cut our deployment time in half. The really cool bit is that adding more VMs doesn’t add much time to our deploy as it’s the creation of the virtual machines that tends to take longest.

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