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Keep the Makefile! If the developer types make test or yarn build or npm run serve today, maybe they still can. Avoid changing the topmost user-facing part of the tooling where possible. Most product developers aren’t interested in build system details and don’t care about whatever change you’ve made, they don’t want it reflected in what they need to type. Retraining is expensive and burns goodwill. Also remember that many workflows take place in an editor. Bazel disrupts the paths on disk where editor extensions look for libraries. Be aware of this problem and be proactive to find and advertise workarounds for keeping editors happy.
Aspect’s rules_py is a good example: it creates a standard Python virtualenv with a site-packages folder so that tools like editors find the structure they expect and keep working.

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Watch Mode

As another example, provide a watch mode with bazel-watcher. It’s really nice to have a “live” development server where code changes are immediately visible. To make this possible, we need Bazel to run in “watch mode” where changes to sources are automatically reflected in the bazel-out tree without us having to manually run bazel after every edit. Bazel doesn’t have a watch mode built-in, but bazel-watcher adds this capability. That project releases a tool named ibazel, short for “interactive Bazel”. Our example repo has ibazel installed and on the PATH, thanks to direnv. We have a podcast episode on the technique used to manage the PATH: https://www.youtube.com/watch?v=TDyUvaXaZrc&list=PLLU28e_DRwdtpojOqWM5UeFyxad7m9gCF&index=2 It can also be installed in these alternative ways:
  • Mac: brew install ibazel
  • If you have npm installed: npm install -g @bazel/ibazel
  • Download a binary from the releases page and install it as ibazel on your $PATH.

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Exercise: A live devserver

Let’s practice using it for the frontend, since JavaScript engineers are accustomed to having watch mode. If you didn’t write a BUILD file for the frontend, you can checkout the packages branch. The target we run has to be aware of the Bazel-watcher protocol:
  • The target must be tagged to indicate its awareness: tags = ["ibazel_notify_changes"] This tag prevents the binary being restarted every time the files change.
  • The program must read from stdin to find out when a build has finished. Bazel-watcher will write lines like IBAZEL_BUILD_COMPLETED SUCCESS
js_run_devserver is one such rule that’s aware of the protocol.
Click the link to open the site in the browser, then make some edits to logger/frontend/index.ts, for example change the Get Server Logs text that appears on the button.