Understanding GitLab CI/CD Pipelines

GitLab CI/CD Pipelines Explained

GitLab CI/CD (Continuous Integration/Continuous Deployment) is a powerful system for automating the process of building, testing, and deploying applications. The pipeline is defined in a .gitlab-ci.yml file, which resides in the root of the repository.

How GitLab CI/CD Works

1. Stages: Pipelines have different stages, such as build, test, and deploy.
2. Jobs: Each stage contains jobs that run specific tasks.
3. Runners: Jobs are executed by GitLab Runners, which are workers that process the jobs.
4. Triggers: Pipelines are triggered by events such as pushing code, merging branches, or manually starting a job.

Example of a GitLab CI/CD Pipeline (.gitlab-ci.yml)

Below is a simple GitLab CI/CD YAML file:

stages:
  - build
  - test
  - deploy

# Build Stage
build-job:
  stage: build
  image: node:16
  script:
    - echo "Installing dependencies..."
    - npm install
    - echo "Building the project..."
    - npm run build
  artifacts:
    paths:
      - dist/

# Test Stage
test-job:
  stage: test
  image: node:16
  script:
    - echo "Running tests..."
    - npm test
  dependencies:
    - build-job

# Deploy Stage
deploy-job:
  stage: deploy
  image: alpine:latest
  script:
    - echo "Deploying application..."
    - echo "Application deployed successfully!"
  only:
    - main  # Runs only on the main branch

Explanation of the .gitlab-ci.yml File

1. Defining Stages

stages:
  - build
  - test
  - deploy

• Pipelines run through these stages in sequential order.
• Each stage can have multiple jobs that run in parallel.

2. Build Stage (build-job)

build-job:
  stage: build
  image: node:16
  script:
    - echo "Installing dependencies..."
    - npm install
    - echo "Building the project..."
    - npm run build
  artifacts:
    paths:
      - dist/

• Uses Node.js 16 as the environment.
• Installs dependencies and builds the project.
• Stores build artifacts (dist/ directory) so the next stages can use them.

3. Test Stage (test-job)

test-job:
  stage: test
  image: node:16
  script:
    - echo "Running tests..."
    - npm test
  dependencies:
    - build-job

• Uses Node.js 16 for running tests.
• Ensures that tests are only executed after the build-job completes.
• Inherits build artifacts from the build-job.

4. Deploy Stage (deploy-job)

deploy-job:
  stage: deploy
  image: alpine:latest
  script:
    - echo "Deploying application..."
    - echo "Application deployed successfully!"
  only:
    - main

• Uses Alpine Linux as the environment.
• Runs a deployment script (this would usually include SSH, FTP, or cloud deployment commands).
• Runs only when changes are pushed to the main branch.

Additional Features

1. Using Environment Variables

You can define secrets such as API keys in GitLab Settings → CI/CD → Variables and reference them in .gitlab-ci.yml:

script:
  - echo "Deploying with API Key: $API_KEY"

2. Caching Dependencies

To speed up the pipeline by reusing dependencies:

cache:
  paths:
    - node_modules/

3. Running Jobs on Specific Branches

To limit jobs to specific branches:

only:
  - main
  - develop

4. Running Jobs Manually

To trigger a job manually from the GitLab UI:

manual-job:
  stage: deploy
  script:
    - echo "This job must be triggered manually."
  when: manual

Conclusion

GitLab CI/CD pipelines allow for automated testing, building, and deployment of applications. Using .gitlab-ci.yml, you can define workflows, use caching, control environments, and secure your deployment process.