System designHardsd-g119

Subject Fraud detectionLevel Senior–Staff~50 minCommon in ML systems · Distributed systems interviewsIndustries Technology

Question

Design a real-time fraud scoring system that must return an approve/decline/review decision within 100ms p99 inline in the payment authorization path, at ~10,000 transactions/sec. Decisions use velocity features (cards-per-device-in-1h, amount-per-account-in-24h), a model score, and rules. Features must be fresh (a card used 3 times in the last minute should reflect that). Describe the architecture, the feature store, and how you keep features both fast and fresh without blocking the auth path.

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.