It's federated and decentralized to a degree via the fact that content is federated across instances, and regardless of the source instance being up or down tha

It's federated and decentralized to a degree via the fact that content is federated across instances, and regardless of the source instance being up or down that content will still be visible on other instances (assuming there are followers of that content there). Mastodon depends on postgres; whether your postgres is multi-master or such is entirely up to you, the instance admin. Hosting it is much like hosting any other modern rails app. Some admins use rds, some use postgres-in-docker on a single machine, etc etc. I'm sure some have more fascinating setups there. However, that's wholly outside of what mastodon claims to do or be; it's decentralized and federated in more meaningful ways; there is no central authority listing users or allowing logins.