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Design a Flash Sale system for 10 million users hitting Buy on one product at exactly 12:00:00 — cover overselling prevention, DB protection, and idempotency.

A real Walmart interview scenario testing whether you can combine four separate fixes into one coherent architecture.

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Design a Flash Sale system for 10 million users hitting Buy on one product at exactly 12:00:00 — cover overselling prevention, DB protection, and idempotency.

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