CI/CD: continuous integration guide

Continuous integration means every developer integrates their code into the main branch of the repository multiple times a day. Each integration automatically triggers a build and test execution process. If something fails, the team receives immediate feedback and fixes it before the error accumulates.

The alternative is infrequent integration: each developer works on their branch for days or weeks, and the final integration generates massive conflicts, hard-to-trace bugs and delays. CI inverts this dynamic: problems are detected when they are small and easy to fix.

Continuous Delivery extends CI by automating the deployment process. Every change that passes CI tests becomes ready to deploy to production with a single click. Continuous Deployment goes one step further: every change that passes tests is automatically deployed to production without human intervention.

The difference between Continuous Delivery and Continuous Deployment is the final manual step. In Delivery, someone authorises the deploy. In Deployment, it is fully automatic. Most teams start with Delivery and evolve to Deployment when confidence in tests and monitoring is sufficient.

A CI/CD pipeline is a sequence of stages that code traverses from commit to production. Each stage validates a different aspect: compilation, quality, security, functionality and deployment. If a stage fails, the pipeline stops and notifies the team.

A typical pipeline includes: code checkout, dependency installation, build, unit tests, integration tests, static analysis (linting, security), deploy to staging, end-to-end tests in staging, and finally deploy to production. Stages can run in parallel when they have no dependencies on each other.

GitHub Actions is the most popular CI/CD tool for both open-source and private projects on GitHub. Its native integration with the repository and the marketplace of reusable actions make it highly productive. GitLab CI/CD offers integrated pipelines within GitLab with a powerful YAML configuration and scalable runners.

Jenkins, though older, is still used in enterprise for its total flexibility and extensive plugin ecosystem. CircleCI and Buildkite offer superior performance with parallel pipelines and intelligent caching. For Kubernetes deployments, Argo CD and Flux implement GitOps: the cluster state is defined in Git.

The most important practice is keeping the pipeline fast. If tests take 30 minutes, developers will avoid integrating frequently. Run the fastest tests first (unit), parallelise integration tests and use aggressive dependency caching. A CI pipeline should complete in under 10 minutes.

Keep the main branch always in a deployable state: if CI fails, fixing the build is the top priority. Implement automatic rollback: if the production deploy generates errors, the previous version is restored automatically. And version the pipeline configuration alongside the code: the pipeline is code, treat it as such.

Start with the minimum viable setup: a pipeline that runs existing tests on every push to the main branch. If you have no tests, start with linting and type checking: at least you will catch syntax and type errors. Then add automatic deploy to a staging environment.

When the team is comfortable with the CI flow in staging, add production deployment (manual at first, automatic later). Increase test coverage gradually and add security stages (dependency scanning, SAST) when the basic pipeline is stable.