Instacart·Software Engineer·Onsite - System Design / Architecture
Jul 2026
System design round at Instacart for a software engineer role. The whole thing was basically one giant question about inventory management, and they wanted you to go deep on pretty much every layer of the stack. Felt like they were testing whether you could hold a lot of complexity in your head at once without losing the thread.
- Design a scalable, highly available inventory management system for an e-commerce platform that tracks stock across multiple warehouses, supports Reserve, Release, and Purchase operations, prevents overselling under concurrent load, and can handle flash-sale traffic spikes up to 50k RPS reads and 5k RPS writes.
- How would you handle consistency trade-offs in this system, specifically when to use strong consistency versus eventual consistency and why?
- Walk through your strategy for preventing race conditions when multiple concurrent requests try to reserve the same inventory item.
- How would you design the caching layer for product availability reads, and how would you handle cache invalidation?
- How would you ensure idempotency for the Reserve, Release, and Purchase APIs and provide at-least-once delivery semantics for order events?
- Describe your approach to scaling the write path, including sharding, write queues, and CQRS if applicable.
- How would you handle partial failures and retries across this system, and how do you achieve exactly-once semantics at critical boundaries?
- What observability strategy would you put in place for this system, and how would you approach disaster recovery and multi-region deployment?
“This was the whole interview.”