Build triggers (poll SCM, webhook, timer) in Jenkins - Step-by-Step Execution

Process Flow - Build triggers (poll SCM, webhook, timer)

Start Jenkins Job Setup

↓

Choose Build Trigger Type

↓

Poll SCM

↓

Trigger Condition Met?

Yes↓

Start Build Execution

↓

Build Done

↓

End

This flow shows how Jenkins waits for a trigger (poll SCM, webhook, or timer) to start a build, then runs the build and finishes.

Execution Sample

Jenkins

pipeline {
  triggers {
    pollSCM('H/5 * * * *')
    // or
    // githubPush()
    // or
    // cron('H 4/* 0 0 1-5')
  }
  stages {
    stage('Build') { steps { echo 'Building...' } }
  }
}

This Jenkins pipeline uses triggers: polling SCM every 5 minutes, or webhook, or timer cron to start the build.

Process Table

Step	Trigger Type	Trigger Condition	Condition Result	Action Taken
1	poll SCM	SCM changed since last build?	No	Wait, no build started
2	poll SCM	SCM changed since last build?	Yes	Start build
3	Build	Build running	N/A	Build executes steps
4	Webhook	Webhook received?	No	Wait, no build started
5	Webhook	Webhook received?	Yes	Start build
6	Timer	Cron time reached?	No	Wait, no build started
7	Timer	Cron time reached?	Yes	Start build
8	Build	Build running	N/A	Build executes steps
9	End	Build finished	N/A	Job ends, waits for next trigger

💡 Execution stops after build finishes and waits for next trigger event.

Status Tracker

Variable	Start	After Step 2	After Step 5	After Step 7	Final
SCM_Changed	false	true	true	true	true
Webhook_Received	false	false	true	true	true
Cron_Time	false	false	false	true	true
Build_Running	false	true	true	true	false

Key Moments - 3 Insights

Why does Jenkins not start a build immediately after polling SCM if no changes are detected?

Because the condition 'SCM changed since last build?' is false (see execution_table row 1), Jenkins waits and does not trigger the build.

How does Jenkins know when to start a build from a webhook?

Jenkins listens for an external webhook event; when it receives one (execution_table row 5), it triggers the build immediately.

What happens if multiple triggers are configured at the same time?

Jenkins evaluates each trigger independently; if any trigger condition is true (rows 2,5,7), it starts the build.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Jenkins start a build due to SCM polling?

A Step 2

B Step 1

C Step 4

D Step 6

💡 Hint

Check the 'Action Taken' column for 'Start build' related to 'poll SCM' trigger.

According to the variable tracker, what is the value of Build_Running after step 7?

A false

B true

C undefined

D null

💡 Hint

Look at the 'Build_Running' row under 'After Step 7' column.

If the webhook trigger never receives a signal, which steps in the execution table show Jenkins waiting?

A Steps 6 and 7

B Steps 1 and 2

C Steps 4 and 5

D Steps 8 and 9

💡 Hint

Look for 'Webhook' trigger rows with 'Wait, no build started' action.

Concept Snapshot

Jenkins Build Triggers:
- poll SCM: checks source code changes on schedule
- webhook: listens for external push events
- timer: runs builds on cron schedule
Trigger conditions start builds automatically
Multiple triggers can coexist
Build runs only when trigger condition is true

Full Transcript

This visual execution shows how Jenkins build triggers work. Jenkins can start builds by polling the source code repository for changes, by receiving webhooks from external systems, or by running on a timer schedule. Each trigger type checks its condition: if SCM has changed, if a webhook event is received, or if the scheduled time is reached. When any condition is true, Jenkins starts the build. The execution table traces these checks step-by-step, showing when builds start or wait. The variable tracker shows how key variables like SCM_Changed, Webhook_Received, Cron_Time, and Build_Running change during execution. Key moments clarify common confusions, such as why builds don't start without changes or events. The quiz tests understanding by asking about specific steps and variable states. This helps beginners see exactly how Jenkins triggers control build execution.