Design a Ad Serving Service
Last updated: September 14, 2025
Quick Overview
Design a scalable ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Microsoft
System Design
Software Engineer
Microsoft
September 14, 2025Software Engineer
Onsite
System Design
Easy
33
10
4,761 solved
Design a scalable ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Microsoft 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
Requirements gathering and capacity estimation
Partitioning and sharding strategies
Consistency models and replication
Monitoring, logging, and alerting
Caching strategies (local, distributed, CDN)
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?
- How would you optimize costs as the system scales?
- How would you handle a region-wide outage?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements:
- Ad Request Handling: The system must accept millions of ad requests per second from various clients and respond with relevant ads based on targeting criteria.
- **...
Capacity Estimation
Back-of-Envelope Calculations:
- Request Volume: Assume we expect 1 billion ad requests per day. That translates to approximately 11,574 requests per second (1,000,000,000 requests / 86,400 se...
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