Design a scalable Authentication System
Last updated: May 21, 2026
Quick Overview
Design a scalable authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Plaid
May 21, 20264
12
144 solved
Design a scalable authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Plaid 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
- 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
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?
- What happens if one of your database nodes goes down?
- What would the deployment pipeline look like for this system?
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Requirements
Functional Requirements
- User Registration: Allow users to create an account securely.
- User Login: Support multi-factor authentication (MFA) for enhanced security.
- **Session Manage...
Capacity Estimation
Assuming Plaid has approximately 10 million users:
- Peak Load: 1 million concurrent requests during peak hours (10% of users).
- Requests per Second (RPS): If we consider a 1-hour peak, that ...
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