Design Inventory Availability and Order Placement

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.