Design an online auction platform

Q: Design an online auction platform

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Question

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).

Design an online auction platform

System Design: Real-Time English Auction Platform

Goal

Solution (Locked)

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