System Design: Real-Time Online Auction Platform

You are asked to design an online auction system that supports real-time bidding at web scale. Assume a mix of desktop and mobile clients, global users, and many concurrent auctions.

Functional Requirements

• Listing lifecycle: create, view, edit (before first valid bid), cancel (with restrictions), close.
• Bidding: real-time bids, enforced minimum increments, reserve prices, buy-it-now, and anti-sniping protections.
• Closeout: determine winner, collect payment, notify parties, and finalize auction state.
• Real-time updates: push current price, bid events, reserve met status, time-left/extension, and closeout results to watchers.

Non-Functional Requirements

• High availability and scalability across many concurrent auctions and watchers.
• Strong consistency for winner determination and money-related state.
• Low-latency bid processing and fanout (<200–300 ms p95 event-to-update as a target).
• Observability, fraud detection, and reliable payments/settlement.

Deliverables

1. APIs (HTTP and real-time) for listings, bids, and buy-it-now.
2. Data model (schemas/entities, indexes) and event model.
3. Consistency model for winning bids and concurrency control strategy.
4. Real-time update design (WebSocket/streaming), ordering, and backpressure.
5. Anti-sniping policy and auction closeout flow.
6. Fraud detection hooks and controls.
7. Payments and settlement flows with idempotency.
8. Scalability, fault tolerance, and monitoring strategy.

Assume minimal context as needed (e.g., auctions are priced in a single currency; times are UTC).