Design Authentication Infrastructure for global users
Last updated: July 14, 2025
Quick Overview
Design a fault-tolerant authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Slack
System Design
Software Engineer
Slack
July 14, 2025Software Engineer
System Design Round
System Design
Medium
28
10
2,960 solved
Design a fault-tolerant authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Slack typically last 45-60 minutes. You are expected to drive the conversation, starting from requirements gathering through to a detailed architecture. The interviewer will evaluate your ability to handle ambiguity and make practical engineering decisions.
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
Load balancing and horizontal scaling
Caching strategies (local, distributed, CDN)
Security and authentication
High-level architecture and component design
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
- How would you implement rate limiting to protect the system?
- How would you handle schema migrations with zero downtime?
- What happens if one of your database nodes goes down?
- How would you handle a region-wide outage?
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Requirements
- Functional Requirements:
- User authentication via email/password and OAuth (Google, Microsoft, etc.).
- Multi-factor authentication (MFA) for added security.
- Session management includin...
Capacity Estimation
- User Base: Assume 10 million active users globally.
- Authentication Requests: If each user authenticates 5 times a day, that results in approximately 50 million authentication requests dail...
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