From Legacy Scripts to a Unified CI/CD Platform: A Practical Migration Playbook

Migrate from Jenkins, CircleCI, or custom scripts to modern GitLab CI or GitHub Actions without breaking deployments.

Why Migrate CI/CD Platforms?

Many engineering organizations find themselves trapped with legacy CI/CD systems that have become maintenance nightmares. Whether it's Jenkins instances requiring constant plugin updates, custom shell scripts scattered across repositories, or CircleCI configurations that no one dares to touch, the pain is real and growing.

Modern CI/CD platforms like GitLab CI and GitHub Actions offer compelling advantages: native Git integration, declarative pipeline definitions, built-in security scanning, and significantly lower operational overhead. But migration isn't trivial—especially when you're managing 50+ pipelines supporting production deployments.

This playbook documents the successful migration of 200+ Jenkins pipelines to GitLab CI over 12 weeks, with zero production incidents and minimal disruption to development teams. We'll share the strategy, tooling, and lessons learned.

Current State Assessment

Before migrating a single pipeline, conduct a thorough assessment of your current CI/CD landscape. Understanding what you have is critical to planning what you need.

Inventory Your Pipelines

Create a comprehensive inventory including:

• Pipeline Count & Complexity

  How many pipelines? Simple builds or complex multi-stage deployments?

• Trigger Mechanisms

  Git webhooks, scheduled jobs, manual triggers, API calls?

• External Dependencies

  Docker registries, artifact repositories, secret managers, notification systems

• Deployment Targets

  Kubernetes, EC2, Lambda, on-prem servers

• Custom Scripts & Tools

  Groovy scripts, custom plugins, proprietary tooling

Target Platform Selection

Choosing between GitLab CI, GitHub Actions, and CircleCI depends on your specific needs. Here's how they compare:

Migration Strategy: Gradual vs Big Bang

You have two approaches: migrate everything at once (big bang) or gradually migrate pipeline by pipeline. For most organizations, gradual migration is the safer choice.

Phase 1: Proof of Concept (3-5 Pipelines)

Start with low-risk pipelines to validate your migration approach, tooling, and team readiness. Choose pipelines that represent different patterns you'll encounter.

Select Your POC Pipelines

Choose pipelines that cover different scenarios:

• Simple Build-Test Pipeline

  Basic CI workflow to validate core functionality

• Docker Build & Push

  Test container registry integration

• Deploy to Staging

  Validate deployment automation and secrets management

Example: Jenkins to GitLab CI Migration

pipeline {
  agent any
  stages {
    stage('Build') {
      steps {
        sh 'npm install'
        sh 'npm run build'
      }
    }
    stage('Test') {
      steps {
        sh 'npm test'
      }
    }
    stage('Deploy') {
      when { branch 'main' }
      steps {
        sh './deploy.sh production'
      }
    }
  }
}

stages:
  - build
  - test
  - deploy

variables:
  NODE_VERSION: "18"

build:
  stage: build
  image: node:${NODE_VERSION}
  script:
    - npm ci
    - npm run build
  artifacts:
    paths:
      - dist/
    expire_in: 1 hour
  cache:
    paths:
      - node_modules/

test:
  stage: test
  image: node:${NODE_VERSION}
  script:
    - npm ci
    - npm test
  coverage: '/All files[^|]*\|[^|]*\s+([\d\.]+)/'

deploy:
  stage: deploy
  image: alpine:latest
  script:
    - apk add --no-cache bash
    - ./deploy.sh production
  only:
    - main
  environment:
    name: production
    url: https://app.example.com
  when: manual

Phase 2: Parallel Running

Run both old and new pipelines simultaneously to build confidence before fully committing. This gives you an escape hatch if issues arise.

Implementation Strategy

• Dual Webhook Configuration

  Configure Git webhooks to trigger both Jenkins and GitLab CI

• Compare Outputs

  Validate that both systems produce identical artifacts

• Monitor Metrics

  Track build times, success rates, and reliability

• Team Training

  Developers get hands-on experience with new system

Phase 3: Full Cutover

With confidence built from parallel running, switch fully to the new platform and decommission the legacy system.

Cutover Checklist

• Verify All Pipelines Migrated

  100% of production pipelines must be operational

• Update Documentation

  Developer guides, runbooks, incident response procedures

• Disable Old Webhooks

  Prevent accidental triggers of legacy system

• Archive Legacy Configuration

  Keep backups for 90 days before deletion

• Decommission Infrastructure

  Shut down old Jenkins servers, reduce costs

Lessons Learned & Common Pitfalls

After completing 200+ pipeline migrations, here are the key lessons we learned: