Poll SCM configuration in Jenkins - Step-by-Step Execution

Process Flow - Poll SCM configuration

Start Jenkins Job

↓

Check Poll SCM Schedule

↓

Is current time matching schedule?

No→Wait until next check

Yes↓

Poll SCM for changes

↓

Are there changes?

No→Do not build, wait for next poll

Yes↓

Trigger Build

↓

End Job Cycle

Jenkins checks the SCM at scheduled times; if changes are found, it triggers a build, otherwise waits for the next poll.

Execution Sample

Jenkins

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

// Jenkins polls SCM every 15 minutes

This Jenkins pipeline snippet configures polling SCM every 15 minutes to detect changes and trigger builds.

Process Table

Step	Time	Poll SCM Triggered?	SCM Changes Detected?	Action Taken
1	00:00	Yes	No	No build triggered, wait for next poll
2	00:15	Yes	Yes	Build triggered
3	00:30	Yes	No	No build triggered, wait for next poll
4	00:45	Yes	Yes	Build triggered
5	01:00	Yes	No	No build triggered, wait for next poll
6	01:15	Yes	No	No build triggered, wait for next poll

💡 Polling continues indefinitely at scheduled intervals; builds only triggered when SCM changes are detected.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6
Time	00:00	00:15	00:30	00:45	01:00	01:15	01:30
Poll SCM Triggered	Yes	Yes	Yes	Yes	Yes	Yes	Yes
SCM Changes Detected	No	Yes	No	Yes	No	No	Yes
Build Triggered	No	Yes	No	Yes	No	No	Yes

Key Moments - 3 Insights

Why doesn't Jenkins trigger a build every time it polls SCM?

Jenkins triggers a build only if SCM changes are detected during the poll, as shown in execution_table rows 1 and 3 where polling occurs but no changes are found, so no build is triggered.

What does the schedule 'H/15 * * * *' mean in Poll SCM?

It means Jenkins polls SCM every 15 minutes at a hashed time to distribute load, as reflected in the execution_table times (00:00, 00:15, 00:30, etc.).

Does Jenkins build immediately after a commit to SCM?

No, Jenkins builds only at the next scheduled poll time after detecting changes, not instantly on commit, as seen in the delay between SCM changes and build triggers in the table.

Visual Quiz - 3 Questions

Look at the execution table, at which time is a build triggered after SCM changes are detected?

A 00:15

B 00:30

C 01:00

D 01:15

💡 Hint

Check the 'Build Triggered' column for 'Yes' and match with 'SCM Changes Detected' at the same step.

At which step does Jenkins poll SCM but no changes are detected?

A Step 2

B Step 3

C Step 4

D Step 6

💡 Hint

Look at 'SCM Changes Detected?' column for 'No' while 'Poll SCM Triggered?' is 'Yes'.

If the polling schedule changed to every 30 minutes, how would the execution table change?

A SCM changes would be detected more frequently

B Builds would trigger twice as often

C Polling steps would occur every 30 minutes instead of 15

D Builds would trigger immediately on commit

💡 Hint

Polling frequency controls how often Jenkins checks SCM, not how often changes occur or builds trigger.

Concept Snapshot

Poll SCM in Jenkins:
- Configure with cron-like syntax (e.g., 'H/15 * * * *')
- Jenkins checks SCM at scheduled times
- If changes detected, triggers build
- If no changes, waits for next poll
- Polling spreads load using 'H' (hash) for timing

Full Transcript

Poll SCM configuration in Jenkins means setting a schedule for Jenkins to check the source code repository for changes. Jenkins uses a cron-like syntax to decide when to poll. At each scheduled time, Jenkins checks if there are any new commits or changes in the SCM. If changes are found, Jenkins triggers a build to update the project. If no changes are found, Jenkins waits until the next scheduled poll. This process repeats indefinitely, ensuring builds only happen when needed, saving resources and time.