Architect a scalable Authentication Engine
Last updated: May 9, 2026
Quick Overview
Design a scalable authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Instacart
May 9, 202620
11
61 solved
Design a scalable authentication system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Instacart 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
- 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 happens if one of your database nodes goes down?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle schema migrations with zero downtime?
- How would you handle a region-wide outage?
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Requirements
Functional Requirements:
- User Registration: Users can register with email/phone and password.
- User Login: Users can log in using their credentials, with support for multi-factor auth...
Capacity Estimation
Estimation Breakdown:
- User Base: Assume 10 million active users.
- Daily Requests: If each user logs in twice a day, that results in 20 million login requests/day.
- Peak Load: Assum...
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