Design a Ad Serving Service
Last updated: November 6, 2025
Quick Overview
Design a scalable ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Twilio
November 6, 20258
11
95 solved
Design a scalable ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Twilio 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
- Drive the design discussion proactively with minimal interviewer guidance
- Perform detailed capacity estimation and use it to inform design decisions
- Design for global scale with multi-region deployment and data consistency
- Deep dive into 2-3 critical components with implementation-level detail
- Address complex trade-offs: CAP theorem, eventual consistency, conflict resolution
- Discuss operational excellence: deployment strategy, chaos engineering, SLOs/SLIs
- Propose a phased rollout plan from MVP to full-scale system
Key Topics to Cover
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 monitoring and alerting would you set up on day one?
- What would the deployment pipeline look like for this system?
- How do you ensure data consistency across multiple services?
- How would you handle schema migrations with zero downtime?
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Requirements Clarification
Before diving into the architecture, clarify the scope with the interviewer. For Ad Serving Service, key functional requirements include: what are the...
Capacity Estimation
Estimate the scale to drive design decisions. Assume 100M DAU with an average of 10 actions per user per day = 1B requests/day ~ 12K QPS average, ~36K...
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