Design a Authentication Service
Last updated: August 25, 2025
Quick Overview
Design a real-time authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Notion
August 25, 2025186
15
2,447 solved
Design a real-time authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This ML system design question from Notion's Technical Screen tests your ability to think about ML systems at scale. The interviewer expects discussion of data quality, feature stores, model serving infrastructure, and A/B testing strategy.
What the Interviewer Expects
- Define clear ML objectives with appropriate loss functions and metrics
- Design a comprehensive feature engineering pipeline
- Discuss model selection with trade-offs (complexity vs interpretability vs latency)
- Plan online and offline evaluation strategies including A/B testing
- Address serving infrastructure: batch vs real-time, latency requirements
- Consider data quality, labeling strategy, and feedback loops
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
- How would you ensure fairness and reduce bias in the model?
- What is your model retraining strategy?
- How would you debug a model that works well offline but poorly online?
- How would you handle the cold start problem?
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Requirements
Functional Requirements
- User Registration: Users can create accounts via email, Google, or other OAuth providers.
- Login: Support multi-factor authentication (MFA) and single sign-on ...
Capacity Estimation
Assuming Notion has 10 million active users and each user logs in 5 times per day:
- Daily Requests = 10M users * 5 logins = 50M logins/day
- Hourly Requests = 50M / 24 hours = ~2M logins/hour
- Peak ...
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