When testing ARM templates, what is the difference between using --what-if and deploymentMode: 'Validate'?

Question

I'm creating a GitHub actions workflow that tests a bicep file for deploying Azure resources. While looking into template testing, I saw two different versions to use:

1.

- uses: azure/arm-deploy@v1
    with:
      scope: subscription
      template: main.bicep
      additionalArguments: "--what-if"

- uses: azure/arm-deploy@v1
    with:
      scope: subscription
      template: main.bicep
      deploymentMode: 'Validate'

Here is the link to arm-deploy docs: https://github.com/marketplace/actions/deploy-azure-resource-manager-arm-template

I tried using both of the versions but want to understand if one of these is enough. I assume one is a subset of the other, but can't seem to find the answer which one is more comprehensive. Any input would be appreciated.

Answer 1

Bicep 'validate' checks the Bicep code for compilation errors, ie validates that the code is correct in terms of syntax and dependencies. A runtime error can still be encountered, regardless of whether the validation succeeds or not.

A 'what-if' gives a complete readout of your templates changes, should it be deployed. As such it is both a validation and a printout of your changes.

For our deployments, we only use validate to stop any syntax errors.