Design global flash-sale coupon system with equal odds

Q: Design global flash-sale coupon system with equal odds

This is a System Design interview question from ByteDance for Software Engineer roles. View the full question and solution on PracHub.

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Question

Problem

Design the backend for a limited-time coupon抢购 (flash sale) system.

There are K coupons available during a sale window (e.g., 10:00:00–10:00:10 UTC).
Users are globally distributed across multiple countries/regions .
The key requirement: each eligible user should have the same probability of winning a coupon , regardless of geography/network latency.

Requirements

Functional

Users can attempt to claim during the sale window.
Each user can win at most 1 coupon (assume one account = one person).
Users should get a result: WIN (coupon assigned) or LOSE (not assigned), ideally within seconds.

Non-functional

Handle very high QPS during the window.
Prevent overselling (never assign > K coupons).
Be resilient to retries, duplicated requests, and partial failures.
Provide an auditable fairness story (how we ensure “equal odds”).

Deliverables

Explain:

APIs and data model
High-level architecture (global traffic, regional services)
The algorithm that ensures equal odds
Concurrency control, idempotency, and anti-abuse
Tradeoffs and failure handling