Recently, this question came up during the discussion. “How many source-code repositories should a startup have?”

There are two extreme answers, a single monorepo for all the code or repository for each library/microservice. Uber, for example, had 8000 git repositories with only 200 engineers! I think both extremes are wrong. Too many repositories make it hard to find code and one single repository makes it harder to do simple things like testing, bisecting (to find buggy commit), deciding repository owners.

Here’s what I have seen works best.

1. Backend code - ideally, one single repository.
2. Frontend (web) code - one single repository. This can be separately tested and even outside contractors can access this. The web code is anyways shipped to the client, so, leaking it isn’t that big of a worry.
3. Mobile code - one repository per platform (Android, iOS, etc.) if there is no code dependency. Or one single repository if a common codebase like React Native or Flutter is being used. Again, those who access the mobile code won’t need access to the backend code. And given that this code in compiled form is sent to the users, there is less worry around leaking it.
4. Open-source code - One repository per open-source package.

As opposed to a monorepo setup, this setup makes it harder to make simultaneous changes to backend and frontend, or backend and mobile apps. And that should be the right behavior anyways. Since backend services and frontend can be, or eventually will be, deployed independently of each other. Mobile apps are deployed independently anyways, so, the backend has to provide backward-incompatibility for that.