[kikki]

There are _many_ databases on each server, last I checked there was around 8 servers (or: "clusters") - and we have it so the traffic is somewhat evenly distributed across each server. There are reasonable capacity limits, and when servers get full we spin up a new one and start accepting new wikis there. I am not in OPS, and they do a lot of work behind the scenes to make this all run smoothly - but from an eng perspective we rarely have issues with this at scale.

Some of this was open source before we unified all of our wiki products, which has a lot of the selection / db logic, at https://github.com/Wikia/app.

[spookthesunset]

How do you update the schema on 300,000 databases?

[kikki]

It doesn't change often, if we do we just have large automated rollout plans - but we've done mass changes enough times there are good procedures around large DB migrations.