Katkat
Back

Design an Inventory Service with Stock Reservations That Prevents Overselling

InstacartSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenSystem Design

Last updated

A system design discussion of an inventory service that reserves stock for orders, centered on reservations, concurrent updates and preventing overselling. It probes how stock and holds are modeled, how a reservation stays atomic under contention, how abandoned holds are released and how hot items scale.

Design an inventory service for a store that sells items in limited stock. When customers place orders, the service must reserve stock for them so that the same unit is never promised to two orders. The discussion centered on three topics: how inventory is reserved, how concurrent requests are handled, and how overselling is prevented.

Constraints and Clarifications

  • Not given: the number of items and locations, order rates, how long a reservation should hold, and where the authoritative stock count comes from. Ask, or state your assumptions.

Clarifying Questions

  • Is stock tracked per item overall, or per item at each location such as a warehouse or store?
  • When is stock reserved (when an item is added to a cart, at checkout, or at payment), and how long does a reservation hold?
  • Is overselling ever acceptable with a later substitution or refund, or must the system prevent it strictly?
  • What are the peak order rates, and are there hot items for which many buyers compete for a few units?
  • Where do stock counts come from, and how often are they corrected by deliveries or physical counts?

Hints

Hint 1

Promised is not sold: Separate the units physically on hand from the units promised to orders that have not completed, and decide what happens to a promise that is never fulfilled.

Hint 2

Two buyers, one unit: Walk through two requests that both see one unit left at the same moment, and decide exactly where in your design that race is settled.

Follow-up questions

  • Thousands of buyers try to reserve the last few hundred units of one item within seconds. How does your reserve path hold up, and what would you change?
  • The payment call times out after the reservation succeeded. How do you avoid both a leaked hold and a double charge?
  • A physical count finds fewer units than the system shows, and some of them are already reserved. What happens to the open reservations?
  • An order has several items and one of them cannot be reserved. How do you avoid holding the others longer than necessary?