Design a fault-tolerant Email System
Last updated: April 20, 2026
Quick Overview
Design a fault-tolerant email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Elastic
System Design
Software Engineer
Elastic
April 20, 2026Software Engineer
Onsite
System Design
Easy
10
4
2,922 solved
Design a fault-tolerant email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Elastic asks this during the Onsite 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
- Clearly define functional and non-functional requirements
- Propose a reasonable high-level architecture with core components
- Choose appropriate data storage solutions with basic justification
- Discuss basic scaling strategies (horizontal scaling, caching)
- Identify potential bottlenecks and suggest simple solutions
Key Topics to Cover
High-level architecture and component design
Consistency models and replication
Database selection and data modeling
Requirements gathering and capacity estimation
Message queues and async processing
Partitioning and sharding strategies
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 the deployment pipeline look like for this system?
- How would you implement rate limiting to protect the system?
- How would you optimize costs as the system scales?
- How would you handle schema migrations with zero downtime?
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Solve on CodemiaSample Answer
Requirements
- Functional Requirements:
- Users can send and receive emails with attachments.
- Users should be able to organize emails into folders and perform searches.
- Support for real-time...
Capacity Estimation
To estimate capacity:
- Assume 10 million users, with each user sending and receiving an average of 10 emails per day.
- Total emails processed daily = 10 million users * 10 emails = 100 million...
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