Global username uniqueness at signup
Design the username/handle reservation service for a platform with 300M users where signups can hit any of several regional API endpoints. The hard requirement: a username is globally unique — two people signing up at the same instant in different regions must never both get '@alex'. Signups are bursty but modest (hundreds/sec); reads (is this handle taken / profile lookup) are heavy and can be slightly stale. How do you guarantee uniqueness without making the whole system synchronous?
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify4:00 left
Estimate4:00 planned
Design11:30 planned
Deep dive9:30 planned
Failure6:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.