Infrastructure & Operations › CI/CD & Deployment

Continuous Integration

Automatically building and testing every change.

Also known as: CI, continuous integration, CI build, build and test, automated builds

Continuous integration (CI) means every change pushed to the shared repository is built and tested automatically, within minutes. The goal: find out quickly when a change breaks something, while it’s still small and fresh in your mind.

# .github/workflows/ci.yml (GitHub Actions)
name: CI
on:
  push:
  pull_request:
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: 20 }
      - run: npm ci
      - run: npm run lint
      - run: npm test

Open a pull request, and the CI service runs these steps on a clean machine and reports a green check or a red cross. Common services: GitHub Actions, GitLab CI, CircleCI, Jenkins (GitHub Actions, CI pipeline).

What CI usually runs

• Install dependencies, from the lockfile.
• Lint and format checks, and type checks.
• Unit and integration tests.
• A build, to prove it compiles and bundles.
• Sometimes security scans and coverage reports.

The habits behind it

• Integrate often. Merge small changes to the main branch daily, not giant branches after weeks.
• Keep the main branch green. If a build breaks, fixing it is top priority.
• Make it fast. Slow CI (more than around 10 minutes) trains people to ignore or batch changes. Use caching (build caching) and run tests in parallel.
• Require it: protect the main branch so changes can’t merge until CI passes.
• Run it the same way locally, through the same scripts (npm scripts, Makefile), so you can reproduce failures.

Pitfalls

• Flaky tests that pass and fail randomly. People get used to rerunning the build, and real failures hide.
• “Works locally, fails in CI”: differences in versions, environment variables or missing files.
• Secrets in CI must use the platform’s encrypted storage and never be printed (CI secrets).
• A CI pass is evidence, not proof. Tests only catch what they cover.

CI is the first half of CI/CD. The second half, delivering changes to users, is continuous delivery.