Azure DevOps YAML Pipeline: Key Concepts, Hierarchy, and Best Practices

Azure DevOps pipeline flow

 


Azure DevOps YAML pipelines provide a powerful and flexible way to automate your CI/CD workflows. Understanding the hierarchy, variable passing, triggers, and conditions is essential for building efficient pipelines.

YAML Pipeline Hierarchy

A YAML pipeline is structured in a hierarchical manner:

1. Stages – The top-level division in a pipeline (e.g., "Build," "Test," "Deploy").

2. Jobs – A sequence of steps that run sequentially or in parallel within a stage.

3. Steps – The smallest executable unit, which can be a script or a predefined task.

1. Stages

In a YAML pipeline, the top level is stages. Each stage can contain multiple jobs, and each job consists of a series of steps.

Stages define logical boundaries in a pipeline,even if not explicitly defined, every pipeline has at least one stage.
Below are examples of stages:

• Build App
• Run Tests
• Deploy to Production

Example:

yaml

stages:  
- stage: Build App
  jobs:  
  - job: BuildJob  
    steps:  
      - script: echo "Building the app..."  

2. Jobs

A job is a set of steps that run together. Jobs can:

• Run sequentially or in parallel.
• Have dependencies (e.g., Job2 depends on Job1).
• Read more about job here

Example:

yaml

jobs:  
- job: JobA  
  steps:  
    - script: echo "Running Job A"  

- job: JobB  
  dependsOn: JobA  
  steps:  
    - script: echo "Running Job B after Job A" 

3. Steps (Tasks)

Steps are the smallest executable units in a pipeline. They can be:

• Scripts (Bash, PowerShell, etc.)
• Predefined tasks (e.g., npm install, dotnet build)

Example:

yaml

steps:  
- script: echo "Restoring dependencies..."  
- task: DotNetCoreCLI@2  
  inputs:  
    command: 'build'  

Passing Variables Across Tasks, Jobs, and Stages

1. Across Tasks in the Same Job

Use task.setvariable to pass values between tasks.

Example:

yaml

steps:  
- powershell: |  
    Write-Host "##vso[task.setvariable variable=MYVAR]abc"  
- script: echo $(MYVAR)  # Output: abc  

2. Across Different Jobs

Use isOutput=true and reference the variable via dependencies.

Example:

yaml

jobs:  
- job: Job1  
  steps:  
    - bash: |  
        echo "##vso[task.setvariable variable=MYVAR;isOutput=true]abc"  
      name: SetVarStep  

- job: Job2  
  dependsOn: Job1  
  variables:  
    MYVAR: $[ dependencies.Job1.outputs['SetVarStep.MYVAR'] ]  
  steps:  
    - script: echo $(MYVAR)  # Output: abc  

3. Across Different Stages

Use stageDependencies to pass variables between stages.

Example:

yaml

stages:  
- stage: StageA  
  jobs:  
  - job: JobA  
    steps:  
      - bash: |  
          echo "##vso[task.setvariable variable=MYVAR;isOutput=true]abc"  
        name: SetVarStep  

- stage: StageB  
  dependsOn: StageA  
  variables:  
    MYVAR: $[ stageDependencies.StageA.JobA.outputs['SetVarStep.MYVAR'] ]  
  jobs:  
  - job: JobB  
    steps:  
      - script: echo $(MYVAR)  # Output: abc  

---

Triggers in YAML Pipelines

Triggers automate pipeline execution based on events like code commits or pull requests.

1. CI Trigger (Continuous Integration)

Runs when changes are pushed to specified branches/paths.

Example:

yaml

trigger:  
  branches:  
    include:  
      - main  
  paths:  
    include:  
      - src/**  
    exclude:  
      - src/docs/**  

2. PR Trigger (Pull Request Validation)

Runs when a PR is created or updated.

Example:

yaml

pr:  
  branches:  
    include:  
      - main  
  paths:  
    include:  
      - src/**  

🔹 Best Practices for Triggers:

• Use batch: true to optimize CI runs.
• Path filters are case-sensitive.
• Wildcards (*) are not supported in path filters.

---

Conditions in Pipelines

Control when jobs, steps, or stages run using conditions.

Common Conditions:

Condition	Description
succeeded()	Runs only if previous steps succeeded (default).
always()	Runs regardless of previous failures.
failed()	Runs only if previous steps failed.
eq(variables['var'], 'value')	Runs if a variable matches a value.

Example:

yaml

steps:  
- script: echo "This runs only on main branch"  
  condition: eq(variables['Build.SourceBranch'], 'refs/heads/main')  

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