Lecture 143: Main tools for CI/CD (Jenkins, GitLab CI, CircleCI)

CI/CD processes don't appear magically out of thin air (as we sometimes wish). There are powerful tools that take care of all the "dirty" work: automating builds, testing, and deployment. Today we'll see how the most popular CI/CD tools work: Jenkins, GitLab CI, and CircleCI. Besides exploring what they can do, we'll figure out how to pick the right tool for your projects.

Jenkins: the automation dinosaur that's still in the game

Jenkins is an open-source automation tool you can customize for basically any need. If Jenkins were a superhero, its motto would be "I was here before everyone else." Jenkins showed up way back in 2005 under the name Hudson and over the years has earned both love and hate from developers. It's powerful and flexible, but requires patience and attention.

Here's what it can do:

• Job creation: separate jobs that perform build, test, deploy steps.
• Pipelines: ability to compose complex sequences of tasks. Jenkins supports declarative and scripted pipelines.
• Dependency management: integration with tools like Maven, Gradle, Docker.
• Flexibility: over 1500 plugins (yes, we're surprised too) to connect Jenkins to anything — SCM (e.g., Git), clouds (AWS, Azure), monitoring systems (Prometheus).

Jenkins pipeline example

pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                sh 'mvn clean package'
            }
        }
        stage('Testing') {
            steps {
                sh 'mvn test'
            }
        }
        stage('Deploy') {
            steps {
                sh 'echo "Deploying to Production!"'
            }
        }
    }
}

Jenkins is great for projects that need massive customization or for companies that have been using it for ages. But be ready to spend time configuring it and learning a bunch of plugins.

GitLab CI: automation straight from your repo

If you use GitLab, you've probably heard about its built-in CI/CD. GitLab CI lets you automate everything from writing code to deploying it, without having to install a separate tool.

GitLab CI highlights

• Built-in: GitLab CI is part of GitLab itself. You don't need to set up a separate server like with Jenkins.
• YAML-based: pipelines are created with YAML files (.gitlab-ci.yml) stored in the repository.
• Auto runners: GitLab provides its own runners (virtual machines) to run pipelines. You can also use self-hosted runners.
• Container support: Docker support out of the box.

Example .gitlab-ci.yml

stages:
  - build
  - test
  - deploy

build:
  stage: build
  script:
    - mvn clean package

test:
  stage: test
  script:
    - mvn test

deploy:
  stage: deploy
  script:
    - echo "Deploying application..."

GitLab CI is perfect for projects already hosted on GitLab. If you have a small team or want to get CI/CD up quickly without extra hassle, it's your pick.

CircleCI: the cloud master of CI/CD

CircleCI is a cloud service for automation that bets on simplicity and integration with Docker and GitHub. It's less about "flexibility like Jenkins" and more about "lightweight and easy."

CircleCI highlights

• Cloud-based: no servers to maintain, everything runs in the cloud.
• Docker support: CircleCI works great with containers; you can isolate environments in just a couple of lines.
• YAML configuration: pipelines are configured in YAML, like in GitLab CI.
• GitHub integration: CircleCI loves GitHub and syncs with it easily.

Example .circleci/config.yml

version: 2.1
jobs:
  build:
    docker:
      - image: circleci/openjdk:11-jdk
    steps:
      - checkout
      - run:
          name: Build with Maven
          command: mvn clean package
  test:
    docker:
      - image: circleci/openjdk:11-jdk
    steps:
      - checkout
      - run:
          name: Run Tests
          command: mvn test
workflows:
  version: 2
  build-and-test:
    jobs:
      - build
      - test

If you want to start CI/CD quickly and use GitHub, CircleCI can be a great choice — especially for container-driven projects.

Comparison: Jenkins, GitLab CI, and CircleCI

How to choose a CI/CD tool?

The choice of tool depends on your needs:

1. If you're in a large company and need a super-flexible tool, pick Jenkins.
2. If you're already using GitLab and want to roll out CI/CD quickly — GitLab CI is your choice.
3. If your project is cloud-based and you value easy setup — try CircleCI.

By the way, you can totally mix tools. For example, use Jenkins for complex pipelines and GitLab CI for quick branch-level checks.