System Design — Online Auction / eBay

Auction with concurrent bidding, real-time price updates, and close-out semantics. Pub/sub + SSE-pushed bid notifications and queue-partitioning by auction-id are the canonical answer. Requirements Functional: list item, bid, get real-time price updates, end auction, declare winner. Decisions: Bid ingestion: pub/sub partitioned by auction_id so all bids for an auction are ordered. Real-time updates: SSE (one-way push) preferred over WebSocket for unidirectional bid stream; argue trade-off. Close-out: scheduled job at end time; final-bid race-condition handling. Anti-snipe extension: end-time auto-extension when a bid lands within last N seconds. Notes Celebrity / hot-item asymmetry pushes interviewers toward pessimistic locking for the celebrity's bid path. An optimistic-locking pitch is a likely challenge prompt: justify it with contention math and expect a follow-up asking for the fallback when retries explode. Reported success criteria: name pub/sub + SSE + partition-by-auction-id explicitly. Interviewer accepts as soon as those three appear. QPS estimation is asked but doesn't need to be exact — anchor on auctions × peak_bidders × bids/sec. Preparation Have a 3-sentence SSE-vs-WebSocket-vs-long-poll comparison ready. Sketch the partition + ordering invariant on a whiteboard until automatic.