How to Design a Scalable Online Auction System: Key Architecture Decisions

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An online auction system allows sellers to list items with a starting price and time window, while buyers compete by placing incrementally higher bids. The core engineering challenge is handling concurrent bids correctly, ensuring that when two users bid simultaneously, only the highest valid bid is recorded as the winner. Each auction follows a lifecycle from creation through active bidding to winner determination and payment completion. Bid history is preserved in the database rather than overwritten, supporting auditing, dispute resolution, and fraud detection. To handle large audiences — potentially millions of users watching a single auction — current auction state can be cached in a system like Redis to reduce direct database load.

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