how do you find a difference between an arm template and actual deployed resource?

What-If

AS you already found, the What-If is the tool designed to meet that very need, showing a list of which resources are to be:

  • added
  • changed (with property values, mmm)
  • removed
  • ignored.

.. and allows you to confirm the changes and cancel should anything be amiss.

ARM templates do support incremental deployment, keeping unscripted resources intact, and the what-if preview feature does not change that.

Regarding guarantees..

Yes, the What-if feature itself is still in preview today and little paranoia is justified. Obviously you don’t get any guarantees for preview features, but..

First, it’s optional. You still have the ARM template and don’t have to rely on What-If alone. Nor are you forced to actually deploy via What-If feature. You can check the report, but reject changes and still deploy via your usual GA-level ARM toolset. No bridges are burned (if you trust the small confirmation-asking sub-feature).

Secondly, you should test anyway. Usually you know which changes are made in ARM and those same changes should light up in What-If. And you can verify the correctness of changes made to your dev/staging envs with little risk. You will learn early if it will work in your scenarios or not.

Be informed. In addition to testing you can investigate the known shortcomings and issues and consider if any are important for your cases. See this repo: https://github.com/Azure/arm-template-whatif

Summary

What-if is immensely useful feature and seems mature enough, that I’d expect most common scenarios to work as intended. As long as you test, and have recovery plans available, then I wouldn’t worry too much.

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