Design a Content Delivery Service
Last updated: April 15, 2026
Quick Overview
Design a distributed content delivery system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Citadel
April 15, 20261
7
3,051 solved
Design a distributed content delivery system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
ML system design at Citadel goes beyond model selection. This Technical Screen question evaluates your ability to design end-to-end ML pipelines, from data collection to model serving, while considering production constraints like latency and reliability.
What the Interviewer Expects
- Design the full ML lifecycle from data collection to model monitoring
- Address cold start, exploration/exploitation, and model freshness
- Discuss multi-objective optimization and ranking systems
- Plan for model debugging, fairness, and bias mitigation
- Design the feature store and training pipeline for scale
- Address model versioning, canary deployments, and rollback strategies
- Discuss the data flywheel and long-term system evolution
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 would you do if model performance degrades over time?
- How would you ensure fairness and reduce bias in the model?
- How would you debug a model that works well offline but poorly online?
- How would you handle a 10x increase in prediction requests?
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Requirements
Functional Requirements
- Content Delivery: The system must efficiently deliver dynamic and static content to users based on their requests.
- Request Handling: Handle millions of concur...
Capacity Estimation
Assuming Citadel has 1 million active users who generate an average of 5 requests per minute:
- Requests per minute: 1,000,000 users * 5 requests = 5,000,000 requests/minute.
- **Requests per seco...
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