## Design Inventory Availability and Order Placement Design the inventory portion of an e-commerce system with two core operations: 1. Query the currently available quantity for a product. 2. Place an order for one or more units without overselling under concurrent requests. Focus on data contention, transactional boundaries, and the tradeoffs between optimistic and pessimistic concurrency control. ### Constraints & Assumptions - Inventory is tracked per product SKU at one logical fulfillment location. - A placement request has an idempotency key and either reserves all requested units or none. - Concurrent orders may target the same popular SKU. - Canceled or expired orders release their reservation. - Payment processing is outside scope, but reservation state must support a later confirm or cancel operation. ### Clarifying Questions to Ask - May availability reads be slightly stale, or must they reflect every committed reservation? - Is an order for multiple SKUs atomic across all items? - How long does a reservation live before expiration? - What is the expected contention level for the hottest products? - Should a failed placement return the exact unavailable SKU and current quantity? ### Hints - Distinguish on-hand, reserved, and available inventory. - Make the capacity check and reservation write one atomic decision. - Compare retry behavior under optimistic control with lock wait behavior under pessimistic control. ### What a Strong Answer Covers - Inventory and reservation/order records with clear invariants. - Concrete query, place, confirm, cancel, and expire contracts. - Atomic no-oversell logic and idempotent retries. - Optimistic versus pessimistic behavior under low and high contention. - Multi-SKU deadlock/order concerns, expiration, observability, and failure recovery. ### Follow-up Questions - How would you prevent deadlocks when an order contains several SKUs? - When would a database row lock outperform optimistic retries? - How would you reconcile inventory after a crash between reservation and event publication? - What availability should the query show while a reservation is pending payment?