Design a multi-tenant Ad Serving System
Last updated: December 28, 2025
Quick Overview
Design a multi-tenant ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Discord
System Design
Machine Learning Engineer
Discord
December 28, 2025Machine Learning Engineer
System Design Round
System Design
Medium
27
9
954 solved
Design a multi-tenant ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Discord 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
- 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
Database selection and data modeling
Caching strategies (local, distributed, CDN)
Load balancing and horizontal scaling
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
- What happens if one of your database nodes goes down?
- How would you handle a 10x increase in traffic overnight?
- How do you ensure data consistency across multiple services?
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Requirements
Functional Requirements:
- Multi-Tenant Support: The system should support multiple advertisers, each with their own campaigns and targeting parameters.
- Ad Serving: Serve ads based on ...
Capacity Estimation
- User Base: Assume Discord has 150 million active users.
- Ad Requests: If each user generates an average of 10 ad requests per day, that leads to:
- Total Requests per Day = 150M users ...
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