Design an online auction platform
Overview
This question evaluates competencies in designing scalable, low-latency real-time distributed systems, covering data modeling, consistency and concurrency control, ordering and sharding strategies, real-time push/fanout, anti-fraud and escrow/payment flows within the System Design and real-time services domain.
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Sep 6, 2025, 12:00 AM
Software Engineer
Onsite
System Design
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System Design: Real-Time English Auction Platform
Goal
Design an online auction platform that supports English-style auctions with:
- Reserve price
- Bid increments
- Proxy bidding (max auto-bid)
- Soft close (extend by X seconds when a bid arrives near the end)
- Optional Buy-It-Now
Provide:
- Functional APIs: CreateAuction, PlaceBid, GetAuction, SubscribeAuctionEvents, CloseAuction.
- Data model: Users, Items, Auctions, Bids, Watchlists, Payments.
- Real-time push mechanism for live bid updates.
- Consistency model for determining the winning bid; idempotency and concurrency control for PlaceBid.
- Anti-sniping (soft-close) and fraud prevention measures.
- Escrow/payment flow and auditability.
- A scalable architecture for 500k concurrent auctions, 5M watchers, and p99 bid-to-UI latency < 200 ms.
- Sharding/partitioning strategy, ordering of bids across partitions, failure recovery for auction finalization, backfill/replay, security, and key metrics/alerts.
Assume auctions are public, with authenticated users, and a single logical region for writes (multi-AZ) with read replicas/edges for fanout. Buy-It-Now is available until reserve is met or the first qualifying bid is placed (configurable).
Solution
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