Design a Email Service
Last updated: March 5, 2026
Quick Overview
Design a fault-tolerant email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Scale AI
System Design
Software Engineer
Scale AI
March 5, 2026Software Engineer
Technical Screen
System Design
Hard
7
5
3,877 solved
Design a fault-tolerant email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Scale AI asks this during the Technical Screen to assess your architectural thinking. They want to see how you decompose a complex problem, choose appropriate technologies, and reason about failure modes. Strong candidates proactively discuss monitoring, alerting, and operational concerns.
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
High-level architecture and component design
Monitoring, logging, and alerting
Caching strategies (local, distributed, CDN)
Requirements gathering and capacity estimation
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?
- How would you handle schema migrations with zero downtime?
- How would you migrate from a monolithic to a microservices architecture?
- What happens if one of your database nodes goes down?
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Requirements
- Functional Requirements:
- User registration and authentication to send/receive emails.
- Sending emails with attachments, ensuring file size limits are enforced.
- Receiving and storing e...
Capacity Estimation
- User Base: Assuming Scale AI targets 10 million users, with an expected 50% active usage.
- Requests per Second (RPS): Each active user sends and receives an average of 5 emails per day, tra...
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