[hobs]

And that would be the solution you'd absolutely abhor. Database sharding has a bunch of gotchas and things to think about because you must consider query access patterns along with your sharding (unless you want the devs to get owned or have very weird behavior.)

Building something super simple can be ok for the base use case but if you are a multi-billion dollar company you can probably afford a few dbas to actually make your platform good.

[robust-cactus]

Complexity is a great reason to implement something like this in-house. It's probably better to understand (and fully control) the sharding and transaction mechanism than to trust a third party with such a core piece of infra.

As companies get larger they move further up the stack whether it's sharding techniques, databases, custom orchestration software, their own networking hardware, etc.