How Big Tech Checks Username Uniqueness at Billion-User Scale
When you type a username and instantly see 'already taken', there's a multi-layer stack doing the work: a probabilistic filter, an in-memory cache, a distributed index, and a load balancer in front of all of them. This post breaks down each data structure, its trade-offs, and how Google, Meta, and Amazon combine them to answer one simple yes/no question in milliseconds across billions of users.
