How to run custom jobs before inheriting from an immutable YAML template

Question

We want to add the following snippet before inheriting from an immutable YAML that does the build job.

jobs:
  - job: PreBuild
    timeoutInMinutes: 120
    pool:
      name: 'poolname'
      vmImage: 'windows'
      steps:
        - checkout: self
          clean: true
          persistCredentials: true
        - task: ..........
          ......
        - task: ..........
          ......

But when we add the snippet before the extends, the YAML would prompt: Unexpected value 'extends'

extends:
  template: template.yml@TemplateRepo

So what is the best solution based on the immutable build templates to have us run several tasks before inheriting the template where template jobs always execute first? Or, are there any other approaches that we could have everything inheriting inside the template but have our custom steps run first?

Answer 1

The general approach is to negotiate with the owner of the "immutable" template to mutate it slightly in order to accept a stepList, jobList, or stageList parameter (depending on your requirements). It's common to have, for example, optional preBuildSteps and postBuildSteps parameters that accept a stepList in order to pass in steps that run before and after a build.

If it's truly immutable inasmuch as the team that owns the template will not implement such a pattern, then you won't be able to extend the template and still fulfill your requirements.