Monorepo CI with path-based job filtering

In a monorepo, every CI run builds and tests all packages even when only one package changed. As the repo grows, CI time scales with the number of packages rather than the number of changes.

Use Turborepo's --filter with git diff to run only affected packages:

jobs:
  ci:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0  # needed for turbo to compute affected packages

      - uses: actions/setup-node@v4
        with:
          node-version: '20'

      - run: npm ci

      - name: Run affected tests
        run: |
          npx turbo run test build lint \
            --filter='...[origin/main]' \
            --cache-dir=.turbo

      - uses: actions/cache@v4
        with:
          path: .turbo
          key: turbo-${{ github.sha }}
          restore-keys: turbo-

For nx monorepos: npx nx affected --target=test --base=origin/main

--filter='...[origin/main]' tells Turbo to run only packages that have changed relative to main, plus their dependents. This scales CI time to the size of the change, not the size of the repo.