Poll SCM configuration in Jenkins - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Understanding Poll SCM Cron Syntax

What is the effect of the following Poll SCM cron schedule in Jenkins?

H/15 * * * *

A Polls the SCM every 15 minutes at a random offset to reduce load spikes.

B Polls the SCM exactly at minute 15 of every hour.

C Polls the SCM once daily at 15 minutes past midnight.

D Polls the SCM every 15 seconds continuously.

💡 Hint

The 'H' character in Jenkins cron means a hashed value to spread load.

The 'H/15' means Jenkins will pick a random minute offset and then poll every 15 minutes from that offset, preventing all jobs from polling at the same time.

❓ Configuration

intermediate

Correct Poll SCM Configuration for Hourly Polling

Which Poll SCM cron expression correctly configures Jenkins to poll the SCM once every hour at the 0th minute?

A 0 0 * * *

B H * * * *

C */60 * * * *

D 0 * * * *

💡 Hint

Look for the expression that triggers at minute zero of every hour.

The expression '0 * * * *' means at minute 0 of every hour, poll SCM once.

❓ Troubleshoot

advanced

Poll SCM Not Triggering Builds

A Jenkins job is configured with Poll SCM using the schedule 'H/10 * * * *' but no builds are triggered even though the repository has changes. What is the most likely cause?

A The Jenkins workspace is corrupted and prevents polling from detecting changes.

B The Poll SCM schedule syntax is invalid and causes Jenkins to skip polling.

C The Jenkins user does not have permission to access the SCM repository.

D The SCM plugin is disabled in Jenkins.

💡 Hint

Check permissions and access rights first when polling does not detect changes.

If Jenkins cannot access the SCM repository due to permission issues, polling will not detect changes and no builds will trigger.

✅ Best Practice

advanced

Efficient Poll SCM Scheduling

Which Poll SCM schedule is best to reduce load on the SCM server when multiple Jenkins jobs poll frequently?

A Use 'H/5 * * * *' to poll every 5 minutes with a hashed offset.

B Use '*/5 * * * *' to poll every 5 minutes exactly on the 0,5,10... minutes.

C Use '0 * * * *' to poll once every hour at minute 0.

D Use 'H H * * *' to poll once daily at a hashed hour and minute.

💡 Hint

Hashed offsets help distribute polling times evenly.

Using 'H/5' spreads polling every 5 minutes but staggered per job, reducing simultaneous load spikes on SCM.

🧠 Conceptual

expert

Poll SCM Behavior with Multiple Branches

In a Jenkins Multibranch Pipeline, how does Poll SCM behave when configured with a schedule like 'H/15 * * * *'?

A Polling is disabled for multibranch pipelines; builds trigger only on webhooks.

B Each branch is polled independently at its own hashed offset every 15 minutes.

C Only the default branch is polled every 15 minutes; other branches are ignored.

D All branches are polled simultaneously every 15 minutes causing load spikes.

💡 Hint

Multibranch pipelines treat branches as separate jobs with their own schedules.

Each branch job gets its own hashed polling schedule, so polling is spread out evenly across branches.

Practice

1.

What is the main purpose of the Poll SCM option in Jenkins?

easy

A. To send notifications when a build fails

B. To manually start a build whenever you want

C. To deploy the application automatically after every build

D. To regularly check the source code repository for changes and trigger builds if changes are found

Solution

1. Step 1: Understand Poll SCM function

   Poll SCM is designed to check the source code repository at regular intervals for any new changes.

2. Step 2: Identify the trigger action

   When changes are detected, Jenkins triggers a build automatically.

3. Final Answer:

   To regularly check the source code repository for changes and trigger builds if changes are found -> Option D

4. Quick Check:

   Poll SCM triggers builds on repo changes [OK]

Hint: Poll SCM triggers builds only when repo changes appear [OK]

Common Mistakes:

• Confusing Poll SCM with manual build triggers
• Thinking Poll SCM deploys code automatically
• Assuming Poll SCM sends notifications

2.

Which of the following is the correct way to set a Poll SCM schedule to check every 15 minutes in Jenkins?

easy

A. */15 * * * *

B. 0 */15 * * *

C. 0 15 * * *

D. 15 * * * *

Solution

1. Step 1: Understand cron syntax for every 15 minutes

   The syntax "*/15 * * * *" means every 15 minutes at minute 0, 15, 30, 45.

2. Step 2: Compare options

   0 */15 * * * "0 */15 * * *" polls at minute 0 every 15 hours (e.g., 00:00, 15:00); A is standard cron for every 15 minutes; Options C and D specify fixed times, not every 15 minutes.

3. Final Answer:

   */15 * * * * -> Option A

4. Quick Check:

   */15 means every 15 minutes [OK]

Hint: Use */15 in first field for every 15 minutes polling [OK]

Common Mistakes:

• Using fixed minute values instead of intervals
• Putting interval */15 in the hour field instead of minute field
• Setting wrong fields for minutes

3.

Given the Poll SCM schedule H/10 * * * *, what will Jenkins do?

medium

A. Poll the SCM every 10 minutes starting exactly at minute 0

B. Poll the SCM only once every hour at minute 10

C. Poll the SCM every 10 minutes starting at a random minute each hour

D. Poll the SCM every 10 seconds

Solution

1. Step 1: Understand Jenkins H/ syntax

   The 'H' means Jenkins picks a random minute to spread load; 'H/10' means every 10 minutes starting from that random minute.

2. Step 2: Interpret the schedule

   Jenkins polls SCM every 10 minutes but start time varies per job to avoid all jobs polling simultaneously.

3. Final Answer:

   Poll the SCM every 10 minutes starting at a random minute each hour -> Option C

4. Quick Check:

   H/10 means every 10 minutes from random start [OK]

Hint: H/10 means every 10 minutes from random start minute [OK]

Common Mistakes:

• Thinking H/10 starts exactly at minute 0
• Confusing seconds with minutes
• Assuming polling happens only once per hour

4.

You set Poll SCM with schedule H/5 * * * * but Jenkins never triggers builds even when code changes. What is the most likely cause?

medium

A. The Poll SCM schedule syntax is invalid

B. The repository URL is incorrect or unreachable

C. Polling triggers builds even without changes

D. Jenkins does not support Poll SCM for Git repositories

Solution

1. Step 1: Check Poll SCM schedule validity

   Schedule 'H/5 * * * *' is valid and means poll every 5 minutes.

2. Step 2: Identify why builds don't trigger

   If no builds trigger despite changes, likely Jenkins cannot access the repo to detect changes.

3. Final Answer:

   The repository URL is incorrect or unreachable -> Option B

4. Quick Check:

   Unreachable repo stops polling builds [OK]

Hint: Check repo URL if Poll SCM triggers no builds [OK]

Common Mistakes:

• Assuming Poll SCM triggers builds without changes
• Blaming schedule syntax when it's correct
• Thinking Jenkins can't poll Git repos

5.

You want Jenkins to poll your Git repository only during office hours (9 AM to 5 PM) every 30 minutes. Which Poll SCM schedule should you use?

hard

A. H/30 9-16 * * *

B. H/30 9-17 * * *

C. H/30 9-18 * * *

D. H/30 8-17 * * *

Solution

1. Step 1: Understand hour range in cron

   Hours are 0-23; '9-16' covers 9:00 to 16:59 (9 AM to 4:59 PM), matching office hours 9 AM to 5 PM excluding the 5 PM hour.

2. Step 2: Match office hours 9 AM to 5 PM

   9-16 includes up to 4:59 PM; 9-17 includes the full 5 PM hour (up to 5:59 PM).

3. Step 3: Confirm polling every 30 minutes

   H/30 means every 30 minutes starting at a hashed minute.

4. Final Answer:

   H/30 9-16 * * * -> Option A

5. Quick Check:

   9-16 hours covers 9 AM to 4:59 PM [OK]

Hint: Use hour range 9-16 for 9 AM to 5 PM polling excluding 5 PM hour [OK]

Common Mistakes:

• Using 9-17 which includes the 5 PM hour
• Confusing hour ranges in cron syntax
• Forgetting H/30 means every 30 minutes