Design a Email for PlanetScale
Last updated: September 21, 2025
Quick Overview
Design a low-latency email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
PlanetScale
September 21, 202542
7
4,990 solved
Design a low-latency email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
PlanetScale asks this during the System Design Round to assess your understanding of the full ML lifecycle. They want to see how you translate a business problem into an ML objective, design the feature pipeline, and plan for model monitoring and retraining.
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 is your model retraining strategy?
- How would you handle the cold start problem?
- How would you ensure fairness and reduce bias in the model?
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Requirements
Functional Requirements
- Email Sending and Receiving: Users should be able to send and receive emails reliably.
- User Authentication: Implement OAuth and multi-factor authentication fo...
Capacity Estimation
Assuming PlanetScale has roughly 1 million users with an average of 10 emails sent and received per user per day:
- Total Emails: 1,000,000 users x 10 emails = 10,000,000 emails per day.
- **Peak ...
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