Introduction to Jenkins

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What is Jenkins?

Jenkins is an open-source automation server written in Java. It automates the repetitive tasks of the software development lifecycle:

• Build: Compiling the code
• Test: Running automated tests
• Analysis: Code quality (SonarQube, linting)
• Deployment: Releasing to production

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CI/CD with Jenkins

Continuous Integration (CI)

Continuous Deployment (CD)

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Architecture

Main components

Controller (Master)

The Jenkins controller:

• Manages the web interface
• Schedules builds
• Stores the configuration
• Distributes tasks to the agents

Agents (Slaves)

Agents run the builds:

• Can be on different platforms
• Connect to the controller via JNLP or SSH
• Can be static or dynamic (cloud)

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Key concepts

Job / Project

A job is an automated task. Job types:

Type	Description	Usage
Freestyle	Configuration via UI	Simple jobs
Pipeline	Groovy code (Jenkinsfile)	Complex CI/CD
Multibranch	One pipeline per branch	Git projects
Folder	Job organization	Structure

Build

A build is an execution of a job. Each build has:

• A unique number (#1, #2, ...)
• A status (Success, Failure, Unstable, Aborted)
• Logs
• Artifacts (optional)

Workspace

The workspace is the working directory where the code is cloned and the build runs.

/var/jenkins_home/workspace/
├── my-project/
│   ├── src/
│   ├── pom.xml
│   └── target/
└── another-project/

Artifact

An artifact is a file produced by a build:

• Java JAR/WAR
• Compiled binaries
• Docker images
• Test reports

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Jenkins vs other CI/CD tools

Aspect	Jenkins	GitHub Actions	GitLab CI	CircleCI
Hosting	Self-hosted	Cloud	Cloud/Self	Cloud
Configuration	Jenkinsfile/UI	YAML	YAML	YAML
Plugins	1800+	Actions	Built-in	Orbs
Cost	Free	Freemium	Freemium	Paid
Learning curve	Medium	Easy	Easy	Easy
Flexibility	⭐⭐⭐⭐⭐	⭐⭐⭐	⭐⭐⭐⭐	⭐⭐⭐

When to choose Jenkins?

• On-premise infrastructure required
• Need for extreme customization
• Specific (legacy) integrations
• Full control over the environment

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Typical workflow

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Terminology

Term	Description
Controller	Main Jenkins server
Agent	Build execution node
Job	Automated task
Build	Execution of a job
Pipeline	CI/CD workflow as code
Stage	Step of a pipeline
Step	Action within a stage
Workspace	Working directory
Artifact	Produced file
Plugin	Jenkins extension

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Benefits of Jenkins

1. Open Source and free

# Pas de licence à payer
# Communauté active
# Documentation abondante

2. Extensibility

More than 1800 plugins for:

• SCM integrations (Git, SVN, Mercurial)
• Build tools (Maven, Gradle, npm)
• Cloud (AWS, Azure, GCP)
• Containers (Docker, Kubernetes)
• Notifications (Slack, Email, Teams)
• Quality (SonarQube, Checkmarx)

3. Pipeline as Code

// Jenkinsfile versionné avec le code
pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                sh 'mvn clean package'
            }
        }
        stage('Test') {
            steps {
                sh 'mvn test'
            }
        }
        stage('Deploy') {
            steps {
                sh 'kubectl apply -f k8s/'
            }
        }
    }
}

4. Scalability

• Distributed master/agents architecture
• Dynamic agents (Kubernetes, Docker, EC2)
• Build parallelization

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Limitations of Jenkins

Limitation	Mitigation
Dated interface	Blue Ocean plugin
Complex configuration	Jenkins Configuration as Code
Server maintenance	Jenkins on Kubernetes
Sometimes unstable plugins	LTS versions
Groovy learning curve	Shared Libraries

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Summary

• Jenkins is an open-source CI/CD automation server
• Master/agent architecture for scalability
• Pipeline as Code with Jenkinsfile
• Rich plugin ecosystem
• Ideal for on-premise environments

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