Design Ad Serving Infrastructure for global users
Last updated: April 8, 2026
Quick Overview
Design a fault-tolerant ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Square/Block
System Design
Software Engineer
Square/Block
April 8, 2026Software Engineer
Technical Screen
System Design
Medium
180
3
2,441 solved
Design a fault-tolerant ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Square/Block asks this during the Technical Screen to assess your architectural thinking. They want to see how you decompose a complex problem, choose appropriate technologies, and reason about failure modes. Strong candidates proactively discuss monitoring, alerting, and operational concerns.
What the Interviewer Expects
- Systematically gather requirements and estimate capacity (QPS, storage, bandwidth)
- Design a scalable architecture with clear component responsibilities
- Make well-reasoned database and caching decisions with trade-off analysis
- Address consistency vs availability trade-offs specific to the use case
- Discuss partitioning strategy, replication, and data modeling
- Cover failure handling, monitoring, and alerting strategies
Key Topics to Cover
Load balancing and horizontal scaling
Partitioning and sharding strategies
API design and rate limiting
Security and authentication
High-level architecture and component design
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 migrate from a monolithic to a microservices architecture?
- How would you handle a 10x increase in traffic overnight?
- What happens if one of your database nodes goes down?
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Requirements
- Functional Requirements:
- Serve targeted ads to global users based on user behavior and preferences.
- Support real-time bidding for advertisers to optimize ad placements.
- Provide detai...
Capacity Estimation
- User Base: Estimate 1 billion active users, with 10% making ad requests.
- Requests Per Second (QPS):
- 100 million daily active users (10% of 1 billion)
- Average of 10 ad requests pe...
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