System designMediumsd-g211

Subject Consistency modelsLevel Mid–Senior~35 minCommon in Databases & SQL interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.