Design a large-scale Email Platform
Last updated: November 22, 2025
Quick Overview
Design a scalable email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Twitter/X
November 22, 202548
4
4,779 solved
Design a scalable email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
ML system design at Twitter/X 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
- Define clear ML objectives with appropriate loss functions and metrics
- Design a comprehensive feature engineering pipeline
- Discuss model selection with trade-offs (complexity vs interpretability vs latency)
- Plan online and offline evaluation strategies including A/B testing
- Address serving infrastructure: batch vs real-time, latency requirements
- Consider data quality, labeling strategy, and feedback loops
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
- How would you ensure fairness and reduce bias in the model?
- What would you do if model performance degrades over time?
- What is your model retraining strategy?
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Requirements
Functional Requirements
- User Registration & Authentication: Users should be able to register and log in to manage their email settings.
- Email Sending/Receiving: Users can send and re...
Capacity Estimation
Assuming Twitter/X has about 500 million active users and each user sends an average of 0.2 emails per day:
- Daily Emails: 500 million users * 0.2 emails/user = 100 million emails/day.
- **Peak H...
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