Architect a fault-tolerant Authentication Engine
Last updated: July 5, 2025
Quick Overview
Design a fault-tolerant authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Atlassian
System Design
Software Engineer
Atlassian
July 5, 2025Software Engineer
System Design Round
System Design
Medium
81
5
2,339 solved
Design a fault-tolerant authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Atlassian asks this during the System Design Round 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
- Systematically gather requirements and estimate capacity (QPS, storage, bandwidth)
- Design a scalable architecture with clear component responsibilities
- Make well-reasoned database and caching decisions with trade-off analysis
- Address consistency vs availability trade-offs specific to the use case
- Discuss partitioning strategy, replication, and data modeling
- Cover failure handling, monitoring, and alerting strategies
Key Topics to Cover
API design and rate limiting
High-level architecture and component design
Database selection and data modeling
Monitoring, logging, and alerting
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 optimize costs as the system scales?
- What monitoring and alerting would you set up on day one?
- How would you handle a 10x increase in traffic overnight?
- What would the deployment pipeline look like for this system?
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Requirements
Functional Requirements
- User Authentication: Support login and logout functionalities for users.
- Multi-Factor Authentication (MFA): Allow users to enable MFA for enhanced security. 3...
Capacity Estimation
Back-of-Envelope Calculations
- User Base: Assume 10 million active users, with an average of 10 logins per user per day.
- Requests per Second (QPS):
- Total logins per day = 10 mil...
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