Architect a high-throughput Authentication Engine
Last updated: February 27, 2026
Quick Overview
Design a high-throughput authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Slack
System Design
Software Engineer
Slack
February 27, 2026Software Engineer
Onsite
System Design
Medium
6
10
278 solved
Design a high-throughput authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Slack 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
- 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
Database selection and data modeling
Consistency models and replication
Failure handling and fault tolerance
Message queues and async processing
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 optimize costs as the system scales?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements
- User Authentication: Support for username/password, OAuth, and SSO.
- Multi-Factor Authentication (MFA): Support MFA for enhanced security.
- Rate Limiting...
Capacity Estimation
Back-of-Envelope Calculations
- User Base: Assume 10 million active users.
- Peak QPS: Assume peak usage during work hours results in 10% of users authenticating simultaneously, leading to...
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