Design a Feature Flag Service
Last updated: October 29, 2025
Quick Overview
Design a event-driven feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Reddit
System Design
Software Engineer
Software Engineer
System Design Round
System Design
Medium
50
7
996 solved
Design a event-driven feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Reddit asks this during the System Design Round 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
- Systematically gather requirements and estimate capacity (QPS, storage, bandwidth)
- Design a scalable architecture with clear component responsibilities
- Make well-reasoned database and caching decisions with trade-off analysis
- Address consistency vs availability trade-offs specific to the use case
- Discuss partitioning strategy, replication, and data modeling
- Cover failure handling, monitoring, and alerting strategies
Key Topics to Cover
Message queues and async processing
Consistency models and replication
Failure handling and fault tolerance
High-level architecture and component design
Caching strategies (local, distributed, CDN)
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?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle a 10x increase in traffic overnight?
- How would you implement rate limiting to protect the system?
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Requirements
- Functional Requirements:
- Ability to create, update, and delete feature flags with associated metadata (e.g., description, targeting rules).
- Ability to roll out features to specific...
Capacity Estimation
- Estimated Traffic:
Given Reddit's user base of over 500 million monthly active users, let’s estimate peak traffic on the feature flag system.
Assuming 10% of users check feature flags ev...
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