How does Jenkins Pipeline work?

Question

Accepted Answer

**Jenkins Pipeline** — a way to describe a CI/CD pipeline as code (Jenkinsfile).

**Simple analogy:** A Pipeline is like a recipe: step-by-step instructions with checks at each stage.

**Declarative Pipeline (recommended):**
```groovy
pipeline {
    agent any

environment {
        APP_NAME = "myapp"
        DOCKER_REGISTRY = "ghcr.io"
        REGISTRY_CREDENTIALS = credentials('ghcr-token')
    }

stages {
        stage('Checkout') {
            steps {
                checkout scm
            }
        }

stage('Test') {
            steps {
                sh 'npm ci'
                sh 'npm test'
            }
        }

stage('Build') {
            steps {
                sh "docker build -t ${DOCKER_REGISTRY}/${APP_NAME} ."
            }
        }

stage('Deploy to Staging') {
            when {
                branch 'develop'
            }
            steps {
                sh 'kubectl apply -f k8s/staging/'
            }
        }

stage('Deploy to Production') {
            when {
                branch 'main'
            }
            input {
                message "Deploy to production?"
                ok "Yes, deploy!"
            }
            steps {
                sh 'kubectl apply -f k8s/production/'
            }
        }
    }

post {
        failure {
            slackSend channel: '#alerts', message: "Build FAILED: ${env.JOB_NAME}"
        }
        success {
            slackSend channel: '#alerts', message: "Build SUCCESS: ${env.JOB_NAME}"
        }
    }
}
```

**Scripted Pipeline (more flexible, but more complex):**
```groovy
node {
    stage('Checkout') {
        checkout scm
    }
    stage('Build') {
        sh 'npm ci && npm run build'
    }
}
```

**Key concepts:**
- **Agent**: where to run (any, docker, label).
- **Stages**: pipeline stages.
- **Steps**: commands within a stage.
- **Post**: actions after pipeline (success/failure).
- **Parallel**: parallel stages.

DevOps: How does Jenkins Pipeline work?