Databricks System Design Interview: Best-Price Book Purchasing Across Many Bookstores

The interviewer opens

Hi, and thanks for joining. Welcome to your interview for the software engineer role at Databricks. We'll design a system together, and you can sketch on the whiteboard on the right of your screen as you talk. If you need to step out or take a break, the Pause button at the top right will pause the interview.

The problem is in the Problem tab, about a service that finds the best bookstore price and places the order. Please read through the requirements and ask me anything you'd like clarified before you start drawing.

The problem

Design a service where a user submits an ISBN and a maximum price. It asks many independent bookstores for their price, picks the best offer within the limit, and places the order with that store.

Must do

• Accept a purchase request: ISBN, maximum price, and the user's identity
• Ask the participating bookstores for their current price and availability
• Pick the best qualifying offer and place the order with that store
• Let the user see the outcome: ordered (store, price), no qualifying offer, or failed

Must hold

• Slow or unavailable stores must not stall a user's purchase
• A user must never be charged for the same book twice