Pull request builds in Jenkins - Time & Space Complexity

Time Complexity: Pull request builds

O(n)

Understanding Time Complexity

We want to understand how the time to run pull request builds changes as the number of pull requests grows.

How does the system handle more pull requests and how does that affect build time?

Scenario Under Consideration

Analyze the time complexity of the following Jenkins pipeline snippet for pull request builds.

pipeline {
  agent any
  stages {
    stage('Build PRs') {
      steps {
        script {
          def prs = getOpenPullRequests()
          prs.each { pr ->
            build job: 'Build-Job', parameters: [string(name: 'PR_ID', value: pr.id)]
          }
        }
      }
    }
  }
}

This code gets all open pull requests and triggers a build job for each one.

Identify Repeating Operations

Look for loops or repeated actions in the code.

• Primary operation: Looping over each pull request to trigger a build.
• How many times: Once for each open pull request.

How Execution Grows With Input

The number of builds triggered grows as the number of pull requests grows.

Input Size (n)	Approx. Operations
10	10 build triggers
100	100 build triggers
1000	1000 build triggers

Pattern observation: The work grows directly with the number of pull requests.

Final Time Complexity

Time Complexity: O(n)

This means the time to trigger builds grows linearly as the number of pull requests increases.

Common Mistake

[X] Wrong: "Triggering builds for pull requests happens all at once and takes the same time no matter how many PRs there are."

[OK] Correct: Each pull request triggers a separate build, so more PRs mean more builds and more time overall.

Interview Connect

Understanding how build time scales with pull requests shows you can reason about system load and efficiency, a useful skill in real projects.

Self-Check

What if we changed the code to trigger builds only for pull requests updated in the last hour? How would the time complexity change?

Practice

1. What is the main purpose of pull request builds in Jenkins?

easy

A. To automatically test code changes before merging

B. To deploy code directly to production

C. To delete old branches after merging

D. To manually review code without testing

Solution

1. Step 1: Understand pull request builds concept

   Pull request builds run tests automatically on code changes before merging to ensure quality.

2. Step 2: Compare options with purpose

   Only To automatically test code changes before merging describes automatic testing before merging, which is the main goal.

3. Final Answer:

   To automatically test code changes before merging -> Option A

4. Quick Check:

   Pull request builds = automatic pre-merge tests [OK]

Hint: Pull request builds test code before merging [OK]

Common Mistakes:

• Confusing deployment with testing
• Thinking pull request builds delete branches
• Assuming manual review replaces automated tests

2. Which Jenkins pipeline syntax correctly triggers a build on a GitHub pull request?

easy

A. triggers { githubPush() }

B. triggers { cron('H/5 * * * *') }

C. triggers { githubPullRequest() }

D. triggers { pollSCM('H/15 * * * *') }

Solution

1. Step 1: Identify trigger for pull request builds

   The correct trigger for GitHub pull request builds in Jenkins is githubPullRequest().

2. Step 2: Eliminate other triggers

   githubPush() triggers on push events, cron and pollSCM are time-based triggers, not pull request triggers.

3. Final Answer:

   triggers { githubPullRequest() } -> Option C

4. Quick Check:

   Pull request trigger = githubPullRequest() [OK]

Hint: Use githubPullRequest() for PR triggers [OK]

Common Mistakes:

• Using githubPush() instead of githubPullRequest()
• Confusing time-based triggers with PR triggers
• Omitting triggers block entirely

3. Given this Jenkinsfile snippet, what happens when a pull request is opened?

pipeline {
  agent any
  triggers {
    githubPullRequest()
  }
  stages {
    stage('Build') {
      steps {
        echo 'Building...'
      }
    }
  }
}

medium

A. The pipeline waits for manual approval before running

B. The pipeline runs automatically and prints 'Building...'

C. The pipeline runs only on push events, not pull requests

D. The pipeline fails due to syntax error

Solution

1. Step 1: Analyze trigger block

   The trigger githubPullRequest() starts the pipeline automatically on pull request events.

2. Step 2: Check pipeline steps

   The pipeline has a 'Build' stage that echoes 'Building...'. No manual approval or syntax errors are present.

3. Final Answer:

   The pipeline runs automatically and prints 'Building...' -> Option B

4. Quick Check:

   githubPullRequest() triggers automatic build [OK]

Hint: githubPullRequest() triggers auto build on PR [OK]

Common Mistakes:

• Assuming manual approval is needed
• Confusing push and pull request triggers
• Thinking syntax error exists in snippet

4. A Jenkins pipeline with githubPullRequest() trigger does not start when a pull request is created. What is the most likely cause?

medium

A. The GitHub webhook for pull requests is not configured

B. The Jenkinsfile has a syntax error in the stages block

C. The pipeline uses cron trigger instead

D. The pull request was merged before the build started

Solution

1. Step 1: Understand trigger requirements

   The githubPullRequest() trigger requires GitHub webhooks to notify Jenkins of PR events.

2. Step 2: Identify common failure reason

   If the webhook is missing or misconfigured, Jenkins won't start the build on PR creation.

3. Final Answer:

   The GitHub webhook for pull requests is not configured -> Option A

4. Quick Check:

   Missing webhook = no PR build trigger [OK]

Hint: Check GitHub webhook setup if PR builds don't start [OK]

Common Mistakes:

• Blaming syntax errors without checking webhooks
• Ignoring webhook configuration
• Assuming cron triggers handle PR events

5. You want Jenkins to build pull requests only from specific branches (e.g., 'feature/*') and skip others. Which configuration snippet achieves this?

hard

A. triggers { githubPullRequest { branchFilter { exclude 'feature/*' } } }

B. triggers { pollSCM { ignoreBranches 'feature/*' } }

C. triggers { githubPush { branch 'feature/*' } }

D. triggers { githubPullRequest { branchFilter { include 'feature/*' } } }

Solution

1. Step 1: Identify correct trigger and filter syntax

   The githubPullRequest trigger supports branchFilter with include to specify branches to build.

2. Step 2: Compare options for filtering

   triggers { githubPullRequest { branchFilter { include 'feature/*' } } } includes only 'feature/*' branches, which matches the requirement. triggers { githubPullRequest { branchFilter { exclude 'feature/*' } } } excludes them, which is opposite. Options C and D use wrong triggers or syntax.

3. Final Answer:

   triggers { githubPullRequest { branchFilter { include 'feature/*' } } } -> Option D

4. Quick Check:

   Use branchFilter include to limit PR builds [OK]

Hint: Use branchFilter include to build specific PR branches [OK]

Common Mistakes:

• Using exclude instead of include
• Using githubPush instead of githubPullRequest
• Trying to filter branches with pollSCM