Google stores billions of lines of code in a single repository (2016) [pdf]
There's a lot of love for monorepos nowadays, but after more than a decade of writing software, I still strongly believe it is an antipattern.1. The single version dependencies are asinine. We are migrating to a monorepo at work, and someone bumped the version of an open source JS package that introduced a regression. The next deploy took our service down. Monorepos mean loss of isolation of dependencies between services, which is absolutely necessary for the stability of mission-critical business services.2. It encourages poor API contracts because it lets anyone import any code in any service arbitrarily. Shared functionality should be exposed as a standalone library with a clear, well-defined interface boundary. There are entire packaging ecosystems like npmjs and pypi for exactly this purpose.3. It encourages a ton of code churn with very low signal. I see at least one PR every week to code owned by my team that changes some trivial configuration, library call, or build directive, simply because some shared config or code changed in another part of the repo and now the entire repo needs to be migrated in lockstep for things to compile.I've read this paper, as well as watched the talk on this topic, and am absolutely stunned that these problems are not magnified by 100x at Google scale. Perhaps it's simply organizational inertia that prevents them from trying a more reasonable solution.