🧠 Jenkins Master-Agent Architecture Explained: Setup, Usage & CI Pipeline Integration (Ubuntu)

Jenkins is a powerful open-source automation server used to implement CI/CD (Continuous Integration and Continuous Delivery) pipelines. One of its key features is the Master-Agent architecture, which helps distribute workloads across machines to improve scalability and performance.

This guide walks you through:

✔️ Jenkins Master and Agent concepts
✔️ Installation on Ubuntu (CLI-based)
✔️ Master-Agent connection setup
✔️ A simple CI pipeline using agents

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🧠 1. Jenkins Master: Meaning & Definition

The Master Node is the core component of a Jenkins setup. It's responsible for orchestrating and managing the entire Jenkins environment.

🔑 Key Responsibilities:

• Stores pipeline configurations

• Distributes workloads to Agents

• Manages plugins & security

• Handles:

  • Job scheduling

  • Triggering builds

  • Managing build results

  • Communication with Agent nodes

🧾 Definition:

The Jenkins Master is the central server that manages jobs, schedules builds, and delegates tasks to agents for execution.

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🧩 2. What is an Agent?

A Jenkins Agent (formerly known as "Slave" or "Worker Node") is any machine set up to execute build tasks sent by the master.

🤖 Also referred to as:

• Agent = Worker = Slave = Node

✅ Use Cases:

• Master: Manages pipelines and coordinates execution.

• Agent: Executes tasks — such as compiling code, running tests, or deploying to production.

Agents can run in Docker containers, virtual machines, or physical remote servers — making them highly flexible for distributed builds.

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🛠️ 3. Jenkins Master and Agent Installation ( on Ubuntu )

Let’s set up a basic Jenkins Master-Agent system using the command line.

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🔧 Master Installation ( on Ubuntu ):

Run the following commands on the server you want to use as the Master Node:

sudo apt update
sudo apt install openjdk-11-jdk -y
wget -q -O - https://pkg.jenkins.io/debian-stable/jenkins.io.key | sudo apt-key add -
sudo sh -c 'echo deb https://pkg.jenkins.io/debian-stable binary/ > /etc/apt/sources.list.d/jenkins.list'
sudo apt update
sudo apt install jenkins -y
sudo systemctl start jenkins

Now, Jenkins will be available on http://your_server_ip:8080.

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🧩 Agent Setup

On the Agent Machine, install Java, Docker, and Git:

bashCopyEditsudo apt install -y openjdk-17-jdk git docker.io
sudo usermod -aG docker $USER  # Optional: If you want to run container-based builds

Then go to the Jenkins UI:

Manage Jenkins → Manage Nodes → New Node

Choose Permanent Agent, and configure:

• Remote root directory: /home/jenkins

• Launch method: Launch agent via SSH

• Add SSH credentials to connect the master to the agent

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🔗 4. Agent Integration & Verification

After setting up the agent:

Go to Manage Jenkins → Nodes

• ✅ Verify the agent shows as Connected

• 🏷 Assign a label like dev-server

  

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✨ Pipeline Using Agent

Here’s a basic Jenkins pipeline that runs on a labeled agent:

groovyCopyEditpipeline {
  agent { label 'dev-server' }
  stages {
    stage('Clone') {
      steps {
        git 'https://github.com/user/app.git'
      }
    }
    stage('Build') {
      steps {
        sh 'sudo apt-get install docker.io'
      }
    }
  }
}

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📦 5. Push Docker Image to DockerHub (with PAT)

Once Jenkins is building on the agent, let’s push a Docker image to Docker Hub.

📌 Prerequisites:

• Docker installed on the agent

• Docker Hub PAT (Personal Access Token)

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🔐 Step 1: Store DockerHub Credentials

1. Go to Jenkins → Credentials → System → Global Credentials

2. Add a Username + Password, where the password is your Docker Hub PAT

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🧾 Step 2: Jenkinsfile to Build & Push

pipeline {
  agent { label 'dev-server' }
  stages {
    stage('Build & Push') {
      steps {
        script {
          withCredentials([usernamePassword(credentialsId: 'docker-creds', usernameVariable: 'DOCKER_USER', passwordVariable: 'DOCKER_PASS')]) {
            sh """
              docker build -t $DOCKER_USER/node-app:latest .
              echo $DOCKER_PASS | docker login -u $DOCKER_USER --password-stdin
              docker push $DOCKER_USER/node-app:latest
            """
          }
        }
      }
    }
  }
}

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✅ Verify:
Check your Docker Hub account to confirm the image is successfully pushed!

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🎯 Final Thoughts

Setting up a Jenkins Master-Agent system is essential for scaling your CI/CD workflows across environments. By integrating Docker, GitHub, and secure credential handling, your pipelines become more modular, powerful, and production-ready.