Design an inventory management system
Company: Instacart
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
Design an inventory management system for an e-commerce business.
The system must support:
- Updating stock when inventory is received, adjusted, or sold.
- Reserving items when a shopper starts checkout (to prevent overselling).
- A “shopper collects items” workflow (e.g., curbside pickup / warehouse picking): an API call that marks items as collected/picked and updates inventory accordingly.
Focus areas:
- Database schema design.
- Achieving high consistency (avoid overselling, correct reservation handling under concurrency).
- Core APIs and their request/response semantics.
- Handling race conditions, retries, and idempotency.
- Reasonable scaling approach (multiple SKUs, multiple warehouses/locations).
Overview: This question evaluates competency in system design, data modeling, distributed consistency, concurrency control, API design, idempotency, and scalability for backend inventory and reservation workflows.