What is Merge Queue in CI/CD?

Question

Accepted Answer

**Merge Queue** — mechanism for sequential PR merging, ensuring `main` always stays green.

**Problem:** PR passed checks, but by the time of merge `main` is already broken (conflict, flaky test).

**How it works:**
1. PR is added to the queue.
2. CI runs on `main + PR` version.
3. If green — it is merged.
4. If red — PR is removed from the queue.

**GitHub Merge Queue:**
```yaml
# .github/workflows/ci.yml
on:
  pull_request:
    branches: [main]
  merge_group:
    branches: [main]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npm ci && npm test

deploy-staging:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npm ci && npm run build
      - run: ./deploy-staging.sh
```

**GitHub Configuration:**
- Branch protection → Require merge queue.
- Minimum time to merge.
- Maximum pull requests.
- Batch size (how many PRs to merge at once).

**Advantages:** No flaky checks, parallel CI for queued PR, guaranteed stable `main`.

**Alternatives:** Bors (legacy), Mergify, custom solutions.

DevOps: What is Merge Queue in CI/CD?