Design an Ad Assignment API
Company: eBay
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design an API that exposes the ad-assignment function as a production service.
The service receives a user, a list of browser positions, and a set of candidate ads. It should randomly assign eligible ads to positions while respecting each ad's per-user display limit. The service must also persist and update per-user ad display state.
Discuss the following:
1. What API endpoint or endpoints would you expose?
2. Would you use REST, RPC, or another protocol? Why?
3. Which HTTP method should be used for generating an assignment?
4. What should the URL design look like?
5. What should the request and response schemas contain?
6. Where should per-user ad display state be stored?
7. How would you update state safely when there are concurrent requests for the same user?
8. What errors should the API expose to clients?
9. How would you make the API reliable, observable, and maintainable?
Quick Answer: This question evaluates API and system-design competencies including API contract and schema design, stateful per-user data management, concurrency control, scalability and observability within production services.