Architect a scalable Authentication Engine
Last updated: April 28, 2026
Quick Overview
Design a scalable authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Booking.com
System Design
Software Engineer
Booking.com
April 28, 2026Software Engineer
Technical Screen
System Design
Hard
8
12
388 solved
Design a scalable authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Booking.com asks this during the Technical Screen 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
- 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
Partitioning and sharding strategies
High-level architecture and component design
Message queues and async processing
API design and rate limiting
Security and authentication
Requirements gathering and capacity estimation
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 optimize costs as the system scales?
- What would the deployment pipeline look like for this system?
- How do you ensure data consistency across multiple services?
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Requirements
- Functional Requirements:
- User registration, login, and logout functionalities.
- Support for multi-factor authentication (MFA) and password reset flows.
- OAuth 2.0 support for third-par...
Capacity Estimation
- User Base: Assume 200 million registered users.
- Daily Requests: Estimate 10 million login requests daily, translating to about 115 requests per second on average. During peak hours, this c...
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