System designHardsd-g203

Subject Distributed locksLevel Senior–Staff~40 minCommon in Concurrency · Distributed systems interviewsIndustries Technology, Software development

Question

An inventory service for a flash-sale platform must guarantee a unit of a limited-stock SKU is sold at most once across 60 app servers in two regions. The team currently uses a Redis distributed lock (single-node SETNX with TTL) around the decrement. Under load they occasionally oversell. Redesign the locking/coordination so overselling is impossible, and explain precisely why the current scheme fails.

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.