Azure Pipeline to trigger Pipeline using YAML

Question

Attempting to trigger an Azure pipeline when another pipeline has been completed using a YAML. There's documentation indicating that you can add a pipeline resource with:

resources:   # types: pipelines | builds | repositories | containers | packages
  pipelines:
  - pipeline: string  # identifier for the pipeline resource
    connection: string  # service connection for pipelines from other Azure DevOps organizations
    project: string # project for the source; optional for current project
    source: string  # source defintion of the pipeline
    version: string  # the pipeline run number to pick the artifact, defaults to Latest pipeline successful across all stages
    branch: string  # branch to pick the artiafct, optional; defaults to master branch
    tags: string # picks the artifacts on from the pipeline with given tag, optional; defaults to no tags

However, I've been unable to figure out what the "source" means. For example, I have a pipeline called myproject.myprogram:

resources:
  pipelines:
  - pipeline: myproject.myprogram
    source: XXXXXXXX

Moreover, it's unclear how you'd build based a trigger based on this.

I know that this can be done from the web-GUI, but it should be possible to do this from a YAML.

Answer 1

Microsoft documentation says that YAML is the preferred approach. So, instead of going for the build-trigger option let's understand the, little bit confusing, YAML trigger. The following tags will work from the original question and now with a bit easier documentation:

resources:
  pipelines:
  - pipeline: aUniqueNameHereForLocalReferenceCanBeAnything
    project: projectNameNOTtheGUID
    source: nameOfTheOtherPipelineNotTheDefinitionId
    trigger:
      branches:
        include:
        - master
        - AnyOtherBranch

The documentation from Microsoft is confusing and the IDs are numerous. At times they want the Project GUID at times the project name. At times they want the pipeline name and at times the pipeline definition Id. But they use the same name for the variable (project and pipeline). And on top of that they write documentation that does not make it easy to guess which one to use the best way is to trial and error.

I think to avoid the confusion in other places I'm giving example of another place in the pipeline you refer to the same variables with different values. In the DownloadArtifact task, you need to use the project GUID and the pipeline definition Id as shown below:

- task: DownloadPipelineArtifact@2
      inputs:
        source: specific (a literal constant value not the pipeline name)
        project: projectGUIDNOTtheProjectName
        pipeline: numericDefinitionIdOfPipelineNotPipelineNameOrUniqueRef
        runVersion: 'latest'

Just look at how they used the same variables in a different way, but both referring to a pipeline and in my case the same exact pipeline. That could create confusion and to avoid stumbling into the next issue I give it here for clarification.