What is CI?

Continuous Integration: every commit triggers automated build + test. The goal: catch bugs WITHIN HOURS of being introduced, not weeks later in QA.

Developer pushes commit
       ↓
CI runner starts
       ↓
clone repo at that commit
       ↓
run: build, lint, test
       ↓
report status: ✓ pass or ✗ fail
       ↓
maybe deploy (CD = Continuous Delivery/Deployment)

Modern CI providers:

• GitHub Actions (most popular for open source)
• GitLab CI (built into GitLab)
• CircleCI (independent SaaS)
• Jenkins (self-hosted, oldest, most flexible)
• Buildkite (hybrid: SaaS UI + self-hosted runners)
• Drone, Concourse, Argo Workflows (cloud-native)

Standard CI pipeline stages:

1. Setup: install dependencies, restore cache
2. Lint: code style, type check (eslint, ruff, clippy)
3. Build: compile, bundle assets
4. Test: unit, integration, e2e
5. Security scan: vulnerabilities, secrets in code
6. Artifact: upload binary, container, archive
7. Deploy (if main branch): staging, then prod

Each stage runs in isolation; failure stops the pipeline.

The CI runner is the orchestrator: pulls workflow definition, schedules jobs to executors, captures logs, reports status.