Design a fault-tolerant Feature Flag System
Last updated: May 25, 2026
Quick Overview
Design a fault-tolerant feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Redfin
May 25, 202634
7
4,264 solved
Design a fault-tolerant feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Redfin 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
- Clearly define functional and non-functional requirements
- Propose a reasonable high-level architecture with core components
- Choose appropriate data storage solutions with basic justification
- Discuss basic scaling strategies (horizontal scaling, caching)
- Identify potential bottlenecks and suggest simple solutions
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 optimize costs as the system scales?
- How would you migrate from a monolithic to a microservices architecture?
- How do you ensure data consistency across multiple services?
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Requirements
- Functional Requirements:
- Ability to create, read, update, and delete feature flags.
- Rollout feature flags to specific user segments or geographies.
- Toggle feature flags in real...
Capacity Estimation
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Assuming Redfin has approximately 10 million active users and each user makes an average of 10 feature flag queries per day, we can estimate the daily requests.
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Daily Requests:
10 mill...
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