Agent types (permanent, cloud) in Jenkins - Commands & Configuration

Introduction

Jenkins uses agents to run tasks. Permanent agents are always ready on your own machines. Cloud agents start when needed and stop after use, saving resources.

When you want a dedicated machine always ready to run Jenkins jobs without delay.

When you want to save costs by using cloud machines only during job execution.

When you need to run jobs on different operating systems or environments dynamically.

When you want to scale Jenkins capacity automatically based on workload.

When you want to isolate jobs to avoid interference by running them on separate cloud agents.

Config File - jenkins-agent-config.xml

jenkins-agent-config.xml

<slave>
  <name>permanent-agent-1</name>
  <description>Permanent agent on local server</description>
  <remoteFS>/home/jenkins</remoteFS>
  <numExecutors>2</numExecutors>
  <mode>NORMAL</mode>
  <retentionStrategy class="hudson.slaves.RetentionStrategy$Always"/>
  <launcher class="hudson.slaves.JNLPLauncher"/>
  <label>linux permanent</label>
</slave>

This XML configures a permanent Jenkins agent.

name: The agent's name.

remoteFS: The directory Jenkins uses on the agent machine.

numExecutors: How many jobs can run at once.

retentionStrategy: 'Always' means the agent stays connected all the time.

launcher: JNLPLauncher means the agent connects to Jenkins.

label: Used to assign jobs to this agent.

Commands

Starts the permanent agent by connecting it to the Jenkins master using the JNLP protocol and secret key.

Terminal

java -jar agent.jar -jnlpUrl http://jenkins.example.com/computer/permanent-agent-1/slave-agent.jnlp -secret 1234567890abcdef

Expected Output

[Jenkins agent] Connected

Agent permanent-agent-1 is online

→

-jnlpUrl - URL to connect the agent to Jenkins master

→

-secret - Secret key for secure connection

Installs the cloud agent plugin to enable dynamic cloud agents in Jenkins.

Terminal

jenkins-plugin-cli --plugins cloud-agent

Expected Output

Installing plugin: cloud-agent

Plugin cloud-agent installed successfully

Creates a cloud agent template node in Jenkins from an XML configuration file.

Terminal

jenkins-cli create-node cloud-agent-template < cloud-agent-template.xml

Expected Output

Node cloud-agent-template created

Lists all configured Jenkins nodes including permanent and cloud agents.

Terminal

jenkins-cli list-nodes

Expected Output

master

permanent-agent-1

cloud-agent-template

Key Concept

Permanent agents are always ready machines; cloud agents start and stop on demand to save resources.

Common Mistakes

Trying to use a permanent agent without starting its agent process.

The agent won't connect to Jenkins, so jobs assigned to it will not run.

Run the agent.jar command with correct URL and secret to connect the agent.

Not installing or configuring the cloud agent plugin before creating cloud agents.

Jenkins cannot create or manage cloud agents without the plugin.

Install the cloud agent plugin first using jenkins-plugin-cli.

Assigning jobs to cloud agents without a proper cloud agent template configured.

Jenkins cannot provision cloud agents dynamically, so jobs will fail to run.

Create and configure a cloud agent template node before assigning jobs.

Summary

Permanent agents run continuously on dedicated machines and connect to Jenkins using JNLP.

Cloud agents are created dynamically using templates and plugins to save resources.

Commands include starting agents, installing plugins, creating nodes, and listing nodes.

Practice

1. What is a key characteristic of a permanent agent in Jenkins?

easy

A. It runs only on cloud platforms like AWS or Azure.

B. It is always connected and ready to run jobs.

C. It starts only when a job is triggered and stops afterward.

D. It automatically scales based on job demand.

Solution

1. Step 1: Understand permanent agent behavior

   Permanent agents are set up to be always online and ready to accept jobs without delay.

2. Step 2: Compare with cloud agents

   Cloud agents start and stop dynamically, unlike permanent agents which stay connected.

3. Final Answer:

   It is always connected and ready to run jobs. -> Option B

4. Quick Check:

   Permanent agent = always connected [OK]

Hint: Permanent agents stay online; cloud agents start/stop [OK]

Common Mistakes:

• Confusing permanent agents with cloud agents
• Thinking permanent agents scale automatically
• Assuming permanent agents only run on cloud

2. Which of the following is the correct way to define a cloud agent in Jenkins pipeline syntax?

easy

A. agent { permanent { name 'agent1' } }

B. agent { cloud { cloudName 'aws' } }

C. agent { docker { image 'cloud-agent' } }

D. agent { label 'cloud' }

Solution

1. Step 1: Identify cloud agent syntax

   Jenkins pipeline uses agent { label 'cloud' } to target cloud agents provisioned with matching labels.

2. Step 2: Eliminate other options

   agent { cloud { cloudName 'aws' } } is invalid syntax; C is for Docker agents, not cloud agents; D is incorrect syntax.

3. Final Answer:

   agent { label 'cloud' } -> Option D

4. Quick Check:

   Cloud agent syntax uses label { 'cloud' } [OK]

Hint: Cloud agents use agent { label 'cloud' } [OK]

Common Mistakes:

• Using invalid 'cloud { cloudName }' syntax for cloud agents
• Confusing Docker agent syntax with cloud agent
• Trying to define permanent agents inside agent block

3. Given this Jenkinsfile snippet:

pipeline {
  agent { label 'cloud' }
  stages {
    stage('Build') {
      steps {
        echo 'Running on cloud agent'
      }
    }
  }
}

What will happen when this pipeline runs?

medium

A. It fails because 'cloud' label is invalid for agents.

B. It runs on a permanent agent labeled 'cloud'.

C. It runs on a cloud agent that starts and stops automatically.

D. It runs on the master node regardless of label.

Solution

1. Step 1: Understand label usage

   The pipeline requests an agent with label 'cloud', which typically refers to a cloud agent.

2. Step 2: Behavior of cloud agents

   Cloud agents start when needed and stop after the job finishes, matching the label usage.

3. Final Answer:

   It runs on a cloud agent that starts and stops automatically. -> Option C

4. Quick Check:

   Label 'cloud' triggers cloud agent start/stop [OK]

Hint: Label 'cloud' usually means dynamic cloud agent [OK]

Common Mistakes:

• Assuming label 'cloud' means permanent agent
• Thinking label usage causes failure
• Assuming master node runs all jobs

4. You configured a cloud agent in Jenkins, but jobs never run on it and stay queued. What is the most likely cause?

medium

A. The cloud agent label is missing or mismatched in the job configuration.

B. The permanent agent is offline, so cloud agents cannot start.

C. The Jenkins master node is overloaded and blocking all agents.

D. The cloud agent plugin is not installed, causing syntax errors.

Solution

1. Step 1: Check label matching

   Jobs must specify the correct label matching the cloud agent to run on it.

2. Step 2: Understand job-agent assignment

   If labels don't match, Jenkins queues jobs waiting for suitable agents.

3. Final Answer:

   The cloud agent label is missing or mismatched in the job configuration. -> Option A

4. Quick Check:

   Label mismatch causes jobs to queue [OK]

Hint: Always match job label with agent label [OK]

Common Mistakes:

• Assuming permanent agent status affects cloud agents
• Blaming Jenkins master overload without evidence
• Confusing plugin absence with job queuing

5. You want to optimize Jenkins to save resources by using cloud agents for most jobs but keep a permanent agent for critical builds. How should you configure labels and agents?

hard

A. Assign label 'critical' to the permanent agent and 'cloud' to cloud agents; assign jobs accordingly.

B. Use only permanent agents with label 'all' and avoid cloud agents to reduce complexity.

C. Assign all jobs to label 'cloud' and disable the permanent agent to save resources.

D. Configure cloud agents without labels and assign all jobs to the permanent agent.

Solution

1. Step 1: Separate job types by labels

   Use distinct labels like 'critical' for permanent agent and 'cloud' for cloud agents to control job assignment.

2. Step 2: Assign jobs to matching labels

   Critical builds use the permanent agent with 'critical' label; other jobs use cloud agents with 'cloud' label.

3. Final Answer:

   Assign label 'critical' to the permanent agent and 'cloud' to cloud agents; assign jobs accordingly. -> Option A

4. Quick Check:

   Use labels to separate permanent and cloud jobs [OK]

Hint: Label agents and jobs to control resource use [OK]

Common Mistakes:

• Using only one label for all agents
• Disabling permanent agent when critical builds exist
• Not assigning labels to cloud agents