Build triggers (poll SCM, webhook, timer) in Jenkins - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a build trigger in Jenkins?

A build trigger is a way to start a Jenkins job automatically based on certain events or schedules, like code changes or time intervals.

beginner

How does 'Poll SCM' trigger work in Jenkins?

Poll SCM checks the source code repository at set intervals to see if there are changes. If changes are found, it starts a build.

intermediate

What is a webhook trigger in Jenkins?

A webhook trigger starts a Jenkins build immediately when the source code repository sends a notification about a change.

beginner

Explain the timer trigger in Jenkins.

A timer trigger starts a Jenkins build at scheduled times using cron syntax, regardless of code changes.

intermediate

Which build trigger is more efficient for immediate builds after code changes: Poll SCM or Webhook?

Webhook is more efficient because it starts the build immediately after a change, while Poll SCM waits for the next scheduled check.

What does the 'Poll SCM' trigger do in Jenkins?

A Starts a build immediately when code is pushed

B Checks the source code repository at intervals for changes

C Runs a build at a fixed time every day

D Manually triggers a build

'Poll SCM' checks the repository regularly and triggers a build if changes are detected.

Which trigger uses cron syntax to schedule builds?

A Timer

B Webhook

C Poll SCM

D Manual

The Timer trigger uses cron syntax to schedule builds at specific times.

What is the main advantage of using a webhook trigger?

A It triggers builds immediately after code changes

B It polls the repository every minute

C It schedules builds at night

D It reduces build frequency

Webhook triggers start builds right after code changes, making builds faster and more efficient.

If you want Jenkins to build every hour regardless of code changes, which trigger should you use?

A Poll SCM

B Webhook

C Manual

D Timer

Timer trigger schedules builds at fixed times, like every hour, independent of code changes.

Which trigger requires the source code repository to support sending notifications?

A Timer

B Poll SCM

C Webhook

D Manual

Webhook triggers depend on the repository sending notifications to Jenkins.

Describe the differences between Poll SCM, Webhook, and Timer triggers in Jenkins.

Think about how and when each trigger starts a build.

Explain why you might choose a webhook trigger over polling SCM in a Jenkins pipeline.

Consider build speed and system load.

Practice

1. Which Jenkins build trigger checks the source code repository regularly for changes before starting a build?

easy

A. poll SCM

B. webhook

C. timer (cron)

D. manual trigger

Solution

1. Step 1: Understand the purpose of poll SCM

   poll SCM trigger makes Jenkins check the source code repository at set intervals for any changes.

2. Step 2: Compare with other triggers

   Webhook triggers build instantly on code push, timer triggers build at scheduled times, manual trigger requires user action.

3. Final Answer:

   poll SCM -> Option A

4. Quick Check:

   poll SCM = checks repo regularly [OK]

Hint: poll SCM means Jenkins checks repo regularly [OK]

Common Mistakes:

• Confusing webhook with poll SCM
• Thinking timer triggers check repo changes
• Assuming manual trigger is automatic

2. Which of the following is the correct syntax to schedule a Jenkins build every day at midnight using cron?

easy

A. * * * * 0

B. 0 0 * * *

C. 0 12 * * *

D. 0 0 0 * *

Solution

1. Step 1: Understand cron syntax

   Cron format is 'minute hour day month weekday'. To run at midnight daily, minute=0 and hour=0.

2. Step 2: Match the correct expression

   0 0 * * * "0 0 * * *" means every day at 00:00 hours (midnight).

3. Final Answer:

   0 0 * * * -> Option B

4. Quick Check:

   Midnight daily cron = 0 0 * * * [OK]

Hint: Midnight daily cron is always '0 0 * * *' [OK]

Common Mistakes:

• Mixing hour and minute fields
• Using wrong day or weekday values
• Confusing 12 PM with midnight

3. Given this Jenkins pipeline snippet, what will trigger the build?

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

medium

A. Build runs only if SCM changes detected every 15 minutes

B. Build runs every 15 minutes regardless of changes

C. Build runs once daily at 15 minutes past midnight

D. Build runs immediately after a webhook event

Solution

1. Step 1: Understand pollSCM with 'H/15 * * * *'

   This means Jenkins checks the SCM every 15 minutes for changes.

2. Step 2: Determine when build triggers

   Build triggers only if changes are detected during these checks, not every 15 minutes automatically.

3. Final Answer:

   Build runs only if SCM changes detected every 15 minutes -> Option A

4. Quick Check:

   pollSCM triggers on changes every 15 mins [OK]

Hint: pollSCM triggers only on changes, not just time [OK]

Common Mistakes:

• Assuming build runs every 15 minutes regardless
• Confusing pollSCM with timer trigger
• Thinking webhook triggers are involved here

4. A Jenkins job with this trigger configuration does not start builds as expected:

triggers {
  pollSCM('H/5 * * * *')
}

What is the most likely cause?

medium

A. Timer trigger conflicts with pollSCM

B. The cron syntax 'H/5 * * * *' is invalid

C. Webhook is required for pollSCM to work

D. SCM polling is disabled in Jenkins global settings

Solution

1. Step 1: Check pollSCM prerequisites

   pollSCM requires SCM polling enabled globally in Jenkins settings.

2. Step 2: Validate other options

   Syntax 'H/5 * * * *' is valid; webhook is separate trigger; no conflict with timer here.

3. Final Answer:

   SCM polling is disabled in Jenkins global settings -> Option D

4. Quick Check:

   pollSCM needs global SCM polling enabled [OK]

Hint: pollSCM needs global SCM polling enabled [OK]

Common Mistakes:

• Assuming cron syntax is wrong
• Thinking webhook is mandatory for pollSCM
• Believing timer conflicts cause failure

5. You want Jenkins to build your project automatically every weekday at 6 AM and also immediately when code is pushed to GitHub. Which trigger setup is best?

hard

A. Use only pollSCM with cron '0 6 * * 1-5'

B. Use webhook only without timer

C. Use timer trigger '0 6 * * 1-5' and configure GitHub webhook

D. Use pollSCM every minute

Solution

1. Step 1: Understand requirements

   Build at 6 AM weekdays and immediately on GitHub push means two triggers needed: timer and webhook.

2. Step 2: Evaluate options

   Use timer trigger '0 6 * * 1-5' and configure GitHub webhook combines timer for scheduled build and webhook for instant build on push, fulfilling both needs.

3. Final Answer:

   Use timer trigger '0 6 * * 1-5' and configure GitHub webhook -> Option C

4. Quick Check:

   Timer + webhook = scheduled + instant builds [OK]

Hint: Combine timer and webhook for schedule plus instant build [OK]

Common Mistakes:

• Using only pollSCM misses instant build
• Using webhook alone misses scheduled build
• Polling every minute is inefficient