# Design an Online Book Seller Platform Design a service for browsing and searching books, managing inventory, placing orders, taking payments, and tracking order state. ### Constraints & Assumptions - Inventory cannot be oversold under concurrent checkout. - Payment callbacks may be delayed or duplicated. ### Clarifying Questions to Ask - Are books physical only and can an order span warehouses? - When does an inventory reservation expire? ### What a Strong Answer Covers - Catalog/search, inventory reservations, orders, payments, and state transitions. - Idempotency, consistency boundaries, recovery, and observability. ### Follow-up Questions - How would cancellations and refunds work? - How would flash-sale traffic change the design?