GitHub Actions — workflows, jobs, steps, secrets, OIDC

GitHub Actions puts CI/CD inside the GitHub repository. It went into beta in 2018 and moved to GA in 2019.

1. About GitHub Actions

It is a CI/CD platform run by GitHub. The traits people highlight are that workflow definitions live inside the repo at .github/workflows/*.yml, and that the marketplace lets us reuse off-the-shelf actions, which lowers the cost of getting started. Pricing is free for public repos, with a free tier for private repos plus metered usage on top. Enterprise pricing is separate.

Term	Meaning
Workflow	An automation defined by one YAML file (on: + jobs:)
Event	A trigger (push · pull_request · schedule · workflow_dispatch, etc.)
Job	A bundle of steps run serially or in parallel on the same runner
Step	One line that runs a command (run:) or an action (uses:)
Runner	The machine that actually executes a job (hosted or self-hosted)
Action	A reusable unit (uses: actions/checkout@v4)

name: ci
on:
  push:
    branches: [main]
  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 test

2. Runner

• Hosted (GitHub-hosted) — ephemeral VMs supplied by GitHub. Ubuntu, Windows, macOS. A fresh VM per job.
• Self-hosted — run the runner agent on our own server or VM. Workflows match by label.
• Larger runner — beefier hosted options (paid).

Hosted's strength is the cleanliness of a uniform environment; self-hosted's strength is cost and environment control (e.g. GPU, internal network access).

3. Matrix

Run the same job with several parameter combinations in parallel.

strategy:
  matrix:
    node: [18, 20, 22]
    os: [ubuntu-latest, windows-latest]

Combinations grow cost and time fast, so narrowing them with include and exclude is the common shape.

4. Reusable workflow · Composite action

• Reusable workflow — called from another workflow with uses:. Inputs and secrets are declared.
• Composite action — wraps multiple steps into one action. Reuse at a smaller granularity.

Both show up together when standardizing CI across larger orgs.

5. Cache and artifacts

• actions/cache — store and restore dependencies and build outputs by key. A matching key hits.
• actions/upload-artifact / download-artifact — pass outputs between jobs and across workflows.
• Many setup actions (setup-node · setup-go · setup-python, etc.) include caching internally.

Cache keys typically come from a hash of the lock file (package-lock.json · pnpm-lock.yaml, etc.). A key that is too short keeps missing the cache.

6. Secrets and environments

• Repository secrets — repo-scoped.
• Environment secrets — environment-scoped (staging, production). Approvals and waits available.
• Organization secrets — org-scoped.

Logs auto-mask secrets, but leaks are reported when echoing or forwarding to external services.

7. OIDC

A GitHub Actions-issued OIDC token is verified by a cloud (e.g. AWS, GCP, Azure), which then issues short-lived credentials. We get cloud permissions without storing static keys in secrets, and that wins on both security and operations.

permissions:
  id-token: write
  contents: read

steps:
  - uses: aws-actions/configure-aws-credentials@v4
    with:
      role-to-assume: arn:aws:iam::123:role/GitHubActions
      aws-region: ap-northeast-2

GCP's Workload Identity Federation and Azure's OIDC integration follow the same grain.

8. Other CI tools

Tool	Released	Grain
GitHub Actions	2018 beta / 2019 GA	Repo-bound · marketplace
GitLab CI	2015~	GitLab-bound · self-hosting friendly
CircleCI	2011	Workflow DSL with a fast on-ramp
Jenkins	2011 (originally Hudson 2005)	Standard for self-hosting · plugin-rich
Buildkite	2014	Self-hosted agents + SaaS UI
Travis CI	2011	Once a standard in OSS, share has shifted
Drone CI	2014	Container-native
Bitbucket Pipelines	2016	Bitbucket-bound

The grain of the choice:

• Coupling with the repo host — GitHub → Actions, GitLab → GitLab CI, Bitbucket → Pipelines.
• Self-hosting first — Jenkins, Drone, self-hosted GitLab Runner.
• Marketplace and ecosystem — Actions is widely seen as the richest.
• Cost predictability — different unit pricing and concurrent-job caps.

9. PR guards and the grain of deploy

PR guards:

• Build and unit tests.
• Lint and type checks.
• Integration tests (with service containers or Testcontainers when needed).
• Code coverage report.
• Security scans (CodeQL · Dependabot · SCA).

Deploys:

• Auto-deploy to staging on merge to main.
• Production behind an environment approval gate.
• Pull cloud permissions through OIDC.
• Pair with cloud-side tooling for canary or rolling deploys.

10. Schedule and matrix cost

Use on: schedule with cron for periodic jobs. Data pipelines, doc generation, consistency checks — all common spots. Watch out for usage limits and cost on per-minute schedules.

Keep matrix combinations of OS and language versions from blowing up, and understand what fail-fast: true and continue-on-error: false actually mean.

11. Common stumbles

Secret leaks — echo $SECRET or forwarding to an external service can break masking. Block them with explicit add-mask or by minimizing permissions.

OIDC permission breadth — if the assumable role is overly permissive, the OIDC win shrinks.

Secrets on fork PRs — secrets are not exposed by default to PRs from forks (security). When needed, pull_request_target exists, but it carries security traps.

Infinite triggers — workflows that push their own changes re-trigger themselves. Block with paths or if.

Runner environment drift — the default tooling and versions on ubuntu-latest change over time. Pinning is safer.

Cache miss — too long a key almost always misses; too short uses the wrong cache.

Self-hosted isolation — letting self-hosted runners take fork PRs is dangerous. Off by default.

Concurrency — fast pushes on the same branch start parallel jobs that collide. Use a concurrency: group with cancel.

Action provenance — uses: on an untrusted action grants code execution. Pinning by SHA is recommended.

Closing thoughts

CI/CD operational load grows with the amount adopted. Starting with a single PR guard and only expanding into deploys and schedules once it has settled in is the safer path. When pulling in marketplace actions, double-check provenance and pin by SHA.

Next

• vitest-pytest-real-world

See GitHub Actions docs, Workflow syntax, Reusable workflows, OIDC integration, actions/cache, and GitLab CI.