How we migrated the database behind every Vercel build

Every build on Vercel starts in the build warm pool, which is a set of standby containers that let builds begin without waiting for new compute. The pool runs on state that tracks which containers are ready, the tokens each one uses to authenticate, and the mapping that ties every running build back to the deployment that gets billed for it. When we built the pool, we put all of that in Redis, which was fast and made sense at the time. Over the years, though, that state turned into a liability. Tokens and container statuses can be rebuilt if they get lost, but the billing mappings can't, and a