Labels for agent selection in Jenkins - Step-by-Step Execution

Process Flow - Labels for agent selection

Start Jenkins Job

↓

Check Job's Agent Label

↓

Find Agent with Matching Label

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Assign Job to Agent

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Run Job on Agent

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Job Complete

Jenkins uses labels to find an agent that matches the job's requested label, then runs the job on that agent.

Execution Sample

Jenkins

pipeline {
  agent { label 'linux' }
  stages {
    stage('Build') {
      steps { echo 'Building on Linux agent' }
    }
  }
}

This Jenkins pipeline runs the build stage on an agent labeled 'linux'.

Process Table

Step	Action	Label Checked	Agent Found	Job Assigned	Output
1	Start job	linux	No	No	Waiting for agent
2	Search agents	linux	Agent1 (linux)	No	Found agent with label 'linux'
3	Assign job	linux	Agent1 (linux)	Yes	Job assigned to Agent1
4	Run job	linux	Agent1 (linux)	Yes	Output: Building on Linux agent
5	Job complete	linux	Agent1 (linux)	Yes	Job finished successfully

💡 Job completes after running on agent with matching label 'linux'

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Agent Selected	None	Agent1 (linux)	Agent1 (linux)	Agent1 (linux)	Agent1 (linux)
Job Status	Waiting	Waiting	Assigned	Running	Completed

Key Moments - 3 Insights

Why does the job wait at the start before running?

The job waits because Jenkins is searching for an available agent with the matching label, as shown in step 1 and 2 of the execution table.

What happens if no agent has the requested label?

The job will stay in the waiting state and not run, because Jenkins cannot assign it to any agent without the matching label, as seen in step 1 where no agent is found.

Can a job run on an agent without the requested label?

No, Jenkins strictly assigns the job only to agents that have the requested label, ensuring the job runs in the correct environment, as shown in step 3.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the job assigned to an agent?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Check the 'Job Assigned' column in the execution table.

According to the variable tracker, what is the job status after step 4?

A Waiting

B Running

C Assigned

D Completed

💡 Hint

Look at the 'Job Status' row after step 4 in the variable tracker.

If no agent has the label 'linux', what would happen to the job status?

A It would stay waiting indefinitely

B It would run on any available agent

C It would fail immediately

D It would run on the master node

💡 Hint

Refer to the key moment about what happens if no agent has the requested label.

Concept Snapshot

Jenkins uses labels to select agents for jobs.
Specify label in pipeline with agent { label 'name' }.
Jenkins finds an agent with that label.
Job runs only on matching labeled agent.
If no agent matches, job waits.
Labels ensure correct environment for jobs.

Full Transcript

This visual execution shows how Jenkins uses labels to select agents for running jobs. The job starts and Jenkins checks the requested label 'linux'. It searches for an agent with that label and finds Agent1. The job is assigned to Agent1 and runs there, producing the output 'Building on Linux agent'. The job completes successfully. Variables like 'Agent Selected' and 'Job Status' change step-by-step, showing the job moving from waiting to running to completed. Key moments explain why the job waits if no agent matches and why it cannot run on agents without the label. The quiz tests understanding of when the job is assigned, job status changes, and what happens if no matching agent exists.