Build a geo-distributed Ad Serving Pipeline
Last updated: November 7, 2025
Quick Overview
Design a geo-distributed ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
OpenAI
System Design
Machine Learning Engineer
OpenAI
November 7, 2025Machine Learning Engineer
Technical Screen
System Design
Easy
126
3
112 solved
Design a geo-distributed ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at OpenAI typically last 45-60 minutes. You are expected to drive the conversation, starting from requirements gathering through to a detailed architecture. The interviewer will evaluate your ability to handle ambiguity and make practical engineering decisions.
What the Interviewer Expects
- Clearly define functional and non-functional requirements
- Propose a reasonable high-level architecture with core components
- Choose appropriate data storage solutions with basic justification
- Discuss basic scaling strategies (horizontal scaling, caching)
- Identify potential bottlenecks and suggest simple solutions
Key Topics to Cover
Message queues and async processing
Load balancing and horizontal scaling
High-level architecture and component design
Security and authentication
Consistency models and replication
Database selection and data modeling
How to Approach This
- Start by clarifying functional and non-functional requirements with the interviewer.
- Estimate the scale: QPS, storage, bandwidth. This drives your design decisions.
- Draw a high-level architecture first, then deep dive into 1-2 critical components.
- Discuss trade-offs explicitly (e.g., consistency vs availability, SQL vs NoSQL).
- Address failure scenarios, monitoring, and how the system handles 10x traffic spikes.
Possible Follow-up Questions
- What happens if one of your database nodes goes down?
- How do you ensure data consistency across multiple services?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle a region-wide outage?
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Requirements
Functional Requirements
- Ad Request Handling: The system must handle millions of ad requests per second from various geographical locations.
- Ad Selection Algorithm: Implement an ad se...
Capacity Estimation
Back-of-Envelope Calculations
- Traffic Estimation:
- Assume 10 million daily active users, each making 10 ad requests per day.
- Total requests = 10 million users * 10 requests = 100 m...
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