[heavenlyhash]

Be careful not to mistake Google's use of a monorepo with consideration of whether $DAYJOB or $FOSSPROJECT should use a monorepo.

Google has a lot of tooling and some very thoroughly-considered and reinforced policies and cultures around their use of a monorepo. Trying to use a monorepo without those tools and ingrained policies may not be very likely to lead to similar results.

[rkangel]

The default should be keeping code together, and justifying why you are splitting it up, not the other way around.

While what you said is true, the codebase of most organisations is not large enough that they run into those scale constraints for a long time. Instead, if you split up your code you immediately get organisational headaches of managing changes across multiple codebases. If you keep it together, the scale challenges can be managed later, if they occur (c.f. YAGNI).

If you are splitting, codebases should be split across organisational boundaries, not technical ones. If you look at the FOSS world, the obvious conclusion is that each library is in its own repo, and that is partly true. In practice what is happening is that each OSS project is its own organisational team, and so it lives together. If you have parts of your company on different continents working on different things (and maybe you need different access) then splitting up may make sense. Otherwise, you're probably just making your life harder for little to no gain.

[heavenlyhash]

I think I'd more or less agree with that.

For example, one of the questions I generally ask in a meeting room that's considering this topic, and has for example microservices in flight, is: "Are you willing to put in the work to make Service A support more than one version at a time in Service B?", and if not, then that's a very (very) strong indicator that the level of coupling and lack of organizational boundaries will result in pain from anything but a monorepo.

I prefer to split things up when possible. When it does fit, there's many benefits. There's also the concern that building too much culture and tooling that presumes a lack of splitting of repos can become its own form of trap which becomes increasingly hard to navigate out of, even if you later want to, as the situation self-iterates. But I agree that splitting things out needs justification, and the "default" stance should include that.

[arlk]

for reference https://ai.google/research/pubs/pub45424