Build a real-time Ad Serving Pipeline
Last updated: November 15, 2025
Quick Overview
Design a real-time ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
OpenAI
System Design
Software Engineer
OpenAI
November 15, 2025Software Engineer
System Design Round
System Design
Easy
10
7
2,385 solved
Design a real-time 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
Caching strategies (local, distributed, CDN)
Consistency models and replication
Monitoring, logging, and alerting
Partitioning and sharding strategies
Load balancing and horizontal scaling
Requirements gathering and capacity estimation
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
- How would you handle a 10x increase in traffic overnight?
- What would the deployment pipeline look like for this system?
- How would you handle schema migrations with zero downtime?
- What monitoring and alerting would you set up on day one?
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Requirements
- Functional Requirements:
- Serve real-time ads based on user queries with low latency (less than 100ms).
- Support targeting based on user demographics, interests, and behavior.
- Provide ...
Capacity Estimation
- User Estimation: Assume 10 million daily active users.
- Request Calculation: If each user generates an average of 20 ad requests per day, we would have:
- Total Requests = 10,000,000 user...
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