Milestone step for concurrency in Jenkins - Commands & Configuration

Introduction

When multiple builds run in parallel, sometimes you want to keep only the latest one and stop older ones. The milestone step in Jenkins helps control this by marking points in the pipeline where older builds can be discarded to avoid wasted work.

When you have frequent code changes and want to avoid running outdated builds that started earlier.

When multiple developers push code quickly and you want to keep the pipeline focused on the newest changes.

When running long pipelines and you want to save resources by canceling older builds that are no longer relevant.

When you want to ensure only one build passes a certain point before others proceed.

When you want to prevent race conditions by controlling concurrency in your pipeline.

Config File - Jenkinsfile

Jenkinsfile

pipeline {
  agent any
  stages {
    stage('Build') {
      steps {
        echo 'Building...'
        milestone 1
      }
    }
    stage('Test') {
      steps {
        echo 'Testing...'
        milestone 2
      }
    }
    stage('Deploy') {
      steps {
        echo 'Deploying...'
      }
    }
  }
}

This Jenkinsfile defines a simple pipeline with three stages: Build, Test, and Deploy.

The milestone step is used in the Build and Test stages to mark points where Jenkins will cancel older builds that have not reached these milestones yet.

This helps keep only the latest build running past these points, saving resources and avoiding conflicts.

Commands

This command updates the Jenkins pipeline job with the new Jenkinsfile containing milestone steps to control concurrency.

Terminal

jenkins-jobs --conf jenkins.ini update Jenkinsfile

Expected Output

Job updated successfully: pipeline-job

Starts a new build of the pipeline job to test the milestone concurrency control.

Terminal

jenkins build pipeline-job

Expected Output

Started build #1 for pipeline-job

Starts a second build quickly after the first to demonstrate that the milestone step will cancel the older build when it reaches the milestone.

Terminal

jenkins build pipeline-job

Expected Output

Started build #2 for pipeline-job

Build #1 canceled due to milestone step

Shows the console output of the second build to verify it proceeded while the first was canceled.

Terminal

jenkins console pipeline-job #2

Expected Output

[Pipeline] echo

Building...

[Pipeline] milestone

[Pipeline] echo

Testing...

[Pipeline] milestone

[Pipeline] echo

Deploying...

[Pipeline] End of Pipeline

Key Concept

If you remember nothing else, remember: the milestone step cancels older builds that have not reached the same milestone, keeping only the latest build running past that point.

Common Mistakes

Not placing milestone steps at key points in the pipeline.

Without milestones, Jenkins cannot cancel older builds, so concurrency control fails.

Add milestone steps at stages where you want to limit concurrency and cancel outdated builds.

Using milestone steps without understanding their order.

Milestones must be numbered in increasing order to work correctly; otherwise, builds may not cancel as expected.

Number milestone steps sequentially in the order they appear in the pipeline.

Summary

The milestone step marks points in a Jenkins pipeline to control concurrency.

Older builds that have not reached a milestone are canceled when a newer build passes it.

Place milestone steps in stages where you want to keep only the latest build running.

Practice

1. What is the main purpose of the milestone step in a Jenkins pipeline?

easy

A. To allow only the latest pipeline run to continue past a certain point

B. To trigger a new pipeline run automatically

C. To send notifications when a build fails

D. To archive build artifacts for later use

Solution

1. Step 1: Understand the role of the milestone step

   The milestone step is designed to control concurrency by ensuring only the newest pipeline run proceeds beyond the milestone point.

2. Step 2: Compare with other pipeline steps

   Other steps like triggering builds or sending notifications do not control concurrency like milestone does.

3. Final Answer:

   To allow only the latest pipeline run to continue past a certain point -> Option A

4. Quick Check:

   Milestone controls concurrency = B [OK]

Hint: Milestone lets only the newest run pass a checkpoint [OK]

Common Mistakes:

• Confusing milestone with notification steps
• Thinking milestone triggers builds
• Assuming milestone archives artifacts

2. Which of the following is the correct syntax to add a milestone step with label "BuildCheckpoint" in a Jenkins declarative pipeline?

easy

A. milestone label: 'BuildCheckpoint'

B. milestone('BuildCheckpoint')

C. milestone label = 'BuildCheckpoint'

D. milestone: 'BuildCheckpoint'

Solution

1. Step 1: Recall the correct milestone syntax

   The milestone step uses named parameters with the syntax: milestone label: 'LabelName'.

2. Step 2: Identify syntax errors in other options

   Options B, C, and D have incorrect syntax such as missing named parameter format or wrong punctuation.

3. Final Answer:

   milestone label: 'BuildCheckpoint' -> Option A

4. Quick Check:

   Correct milestone syntax uses label: 'name' [OK]

Hint: Use named parameter syntax: milestone label: 'Name' [OK]

Common Mistakes:

• Using parentheses instead of named parameters
• Using equal sign instead of colon
• Omitting the label keyword

3. Given this Jenkins pipeline snippet:

stage('Test') {
  steps {
    milestone label: 'TestStart'
    echo 'Running tests'
  }
}

// Two pipeline runs start quickly one after another

What happens to the first pipeline run when the second run reaches the same milestone?

medium

A. The first run continues normally alongside the second

B. The second run waits until the first run finishes

C. The first run is aborted and only the second run continues past the milestone

D. Both runs are aborted at the milestone

Solution

1. Step 1: Understand milestone concurrency behavior

   When multiple runs reach the same milestone label, Jenkins allows only the latest run to proceed and aborts older runs at that point.

2. Step 2: Apply this to the scenario

   The second run reaching 'TestStart' milestone causes the first run to abort at that milestone, so only the second run continues.

3. Final Answer:

   The first run is aborted and only the second run continues past the milestone -> Option C

4. Quick Check:

   Milestone aborts older runs = D [OK]

Hint: Milestone aborts older runs, keeps newest only [OK]

Common Mistakes:

• Thinking both runs continue
• Assuming second run waits
• Believing both runs abort

4. You added a milestone step in your Jenkins pipeline but it does not seem to abort older runs as expected. Which of the following is the most likely cause?

medium

A. The pipeline is missing the agent block

B. The milestone step is placed after all stages have completed

C. The milestone label is not unique and is reused in multiple places

D. The Jenkins server is offline

Solution

1. Step 1: Check milestone placement

   The milestone step is most likely placed after all stages have completed. This makes it ineffective because older runs finish before a newer run can reach the milestone to trigger aborts.

2. Step 2: Evaluate other options

   Label reuse affects log clarity but not concurrency control (ordinals matter). Missing agent prevents runs entirely. Server offline stops everything.

3. Final Answer:

   The milestone step is placed after all stages have completed -> Option B

4. Quick Check:

   Milestone placement before completion critical = A [OK]

Hint: Place milestone before critical/protected sections [OK]

Common Mistakes:

• Placing milestone after all stages
• Blaming unrelated pipeline syntax
• Assuming server offline causes milestone issues

5. You want to ensure that only the latest run of your Jenkins pipeline proceeds past the "Deploy" stage, aborting any previous runs stuck at that point. How should you use the milestone step to achieve this?

hard

A. Use milestone label: 'Deploy' only in the first stage

B. Place milestone label: 'Deploy' at the end of the Deploy stage

C. Place milestone label: 'Deploy' in every stage before Deploy

D. Place milestone label: 'Deploy' at the start of the Deploy stage

Solution

1. Step 1: Identify where to place milestone for concurrency control

   To abort older runs before deploying, place the milestone step at the start of the Deploy stage so only the newest run proceeds.

2. Step 2: Understand why other placements fail

   Placing milestone at the end or in other stages won't stop older runs before deployment. Using it only in the first stage doesn't control concurrency at Deploy.

3. Final Answer:

   Place milestone label: 'Deploy' at the start of the Deploy stage -> Option D

4. Quick Check:

   Milestone at stage start controls concurrency there [OK]

Hint: Put milestone at stage start to block old runs [OK]

Common Mistakes:

• Placing milestone too late in the stage
• Using milestone in unrelated stages
• Not placing milestone at all