System design interview questionHardE-Commerce / Realtime
Design an Online Auction System like eBay
Design an eBay-style auction: users bid in near real time, current price updates live, and the final winner is recorded durably.
Interview scope
Client → Realtime Gateway (+ Gateway/LB for REST). App → Cache (high bid) + DB (auction/bids) + Message Queue (outbid/win). No Client → DB.
Functional requirements
- Place a bid on an active auction.
- Broadcast current high bid to watchers.
- Close auction and persist the winner.
Scale and quality goals
- Bid updates need low-latency push (Realtime Gateway).
- Current high bid is hot — Cache helps.
- Final outcome must be durable in DB; Message Queue for outbid emails.
Capacity assumption
Design for this scale
Assume a hot auction gets ~100k bids in ~5 minutes (~333 bids/s) plus many more websocket viewers.
Practice more than a diagram
Place components on the Froquiz canvas, connect the data flow, and see which decisions the scenario-specific review finds missing. The full exercise and review unlock with Premium.
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