Design an Online Bookstore System

Design a bookstore system. The report gives only this one-word prompt, so settle the scope with the interviewer first. A common reading is an online bookstore: customers browse and search a catalog of books, see prices and availability, add books to a cart, check out and pay, and the store tracks inventory and fulfils orders. ```hint The last copy Decide when stock is set aside for a customer, and what happens when two customers try to buy the last copy of a book at the same moment. ``` ```hint Browsing is not buying Traffic to browse and search is much larger than traffic to buy. Decide which data can be served from caches or a search index, and which must be read from the source of truth. ``` ### Constraints and Clarifications - Not given: catalog size, traffic, physical or electronic books, one seller or many, regions, and which features beyond buying are in scope. Ask, or state your assumptions. ### Clarifying Questions - Physical books shipped from warehouses, e-books, or both? One store or a marketplace of sellers? - How many titles, daily visitors and orders a day, and what peaks (a major release, holiday sales)? - Which features are in scope: search, reviews and ratings, recommendations, wishlists, pre-orders? - How many warehouses, and may the store oversell and backorder? - Is payment handled by an external payment provider? ### What a Strong Answer Covers - Settled scope and rough capacity estimates - A data model for books and editions, inventory per location, carts, orders and payments - Browse and search paths: a search index kept in sync with the catalog, caching, and a CDN for images - Checkout: stock reservation, idempotent payment, and an order state machine with compensation on failure - Strong consistency where it matters (stock, orders, payments) and eventual consistency elsewhere - Scaling for peaks, failure handling and monitoring ### Follow-up Questions - A highly anticipated release opens for pre-order and traffic spikes far above normal. What breaks first, and how do you protect checkout? - How do you keep search results consistent with price and stock changes? - E-books are added. What changes in inventory and fulfilment? - The payment succeeds but the order service times out before recording it. What does the customer see, and how is it resolved?