Answer
Jenkins Master-Agent Architecture
Architecture Overview
CODE
┌─────────────────────────────────────────────────────────┐
│ Jenkins Master │
│ │
│ - Web UI & REST API - Job scheduling │
│ - Plugin management - Build queue │
│ - User authentication - Monitors agents │
│ - Configuration storage - Distributes work │
│ │
│ Communication: SSH / JNLP (Java Web Start) │
│ ┌────────────┬────────────┬────────────┐ │
│ ↓ ↓ ↓ ↓ │
│ Agent-Linux Agent-Win Agent-Docker Agent-Mac │
│ Selenium IE tests Build image iOS tests │
│ Tests Tests │
└─────────────────────────────────────────────────────────┘
Jenkins Master Responsibilities
- ✓Hosts the Jenkins web interface
- ✓Stores all job configuration
- ✓Manages plugins and credentials
- ✓Schedules jobs and assigns them to available agents
- ✓Should NOT run builds — delegates to agents
Jenkins Agent Responsibilities
- ✓Executes the actual pipeline steps
- ✓Has the required tools (JDK, Maven, Node.js, Chrome, etc.)
- ✓Reports results back to master
- ✓Can be on different OS (Linux, Windows, macOS)
Defining Agents in Jenkinsfile
GROOVY
// Run on any available agent
pipeline {
agent any
...
}
// Run on a specific label (e.g., agent with "selenium" label)
pipeline {
agent { label 'selenium-agent' }
...
}
// Run in a Docker container (agent-less infrastructure)
pipeline {
agent {
docker {
image 'maven:3.9-openjdk-17'
args '-v /root/.m2:/root/.m2' // cache Maven deps
}
}
...
}
// Run on no agent at top level — define per stage
pipeline {
agent none
stages {
stage('Build on Linux') {
agent { label 'linux' }
steps { sh 'mvn clean package' }
}
stage('Test on Windows') {
agent { label 'windows' }
steps { bat 'mvn test' }
}
}
}
Adding an Agent to Jenkins
SSH Agent Setup
- ✓Jenkins → Manage Jenkins → Manage Nodes → New Node
- ✓Name:
selenium-agent-01 - ✓Remote root directory:
/home/jenkins - ✓Labels:
selenium chrome linux - ✓Launch method:
Launch agents via SSH - ✓Host:
192.168.1.50 - ✓Credentials: SSH key pair
JNLP Agent (Agent connects to master)
Bash
# On agent machine:
java -jar agent.jar \
-jnlpUrl http://jenkins-master:8080/computer/agent-01/slave-agent.jnlp \
-secret <secret-key> \
-workDir /home/jenkins
Parallel Stages Across Agents
GROOVY
pipeline {
agent none
stages {
stage('Parallel Tests') {
parallel {
stage('Chrome Tests') {
agent { label 'agent-chrome' }
steps { sh 'mvn test -Dbrowser=chrome' }
}
stage('Firefox Tests') {
agent { label 'agent-firefox' }
steps { sh 'mvn test -Dbrowser=firefox' }
}
stage('API Tests') {
agent { label 'agent-api' }
steps { sh 'mvn test -Dsuite=api' }
}
}
}
}
}