Architect a fault-tolerant Ad Serving Engine
Last updated: May 6, 2026
Quick Overview
Design a fault-tolerant ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
HRT
System Design
Software Engineer
HRT
May 6, 2026Software Engineer
Onsite
System Design
Hard
4
15
2,969 solved
Design a fault-tolerant ad serving system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at HRT 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
API design and rate limiting
Partitioning and sharding strategies
Monitoring, logging, and alerting
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 handle schema migrations with zero downtime?
- 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 ads to users based on targeting criteria (location, interests, demographics).
- Support real-time bidding (RTB) for ad placements.
- Enable advert...
Capacity Estimation
- Traffic Predictions:
Assume HRT expects up to 100 million unique users per day, with an average of 10 ad requests per user, leading to 1 billion requests per day.- Peak traffic is antic...
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