Design a Feature Flag Service
Last updated: May 1, 2026
Quick Overview
Design a fault-tolerant feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Snapchat
System Design
Software Engineer
Snapchat
May 1, 2026Software Engineer
Technical Screen
System Design
Hard
4
12
1,849 solved
Design a fault-tolerant feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Snapchat 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
Requirements gathering and capacity estimation
Failure handling and fault tolerance
Consistency models and replication
API design and rate limiting
Message queues and async processing
High-level architecture and component design
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 do you ensure data consistency across multiple services?
- How would you optimize costs as the system scales?
- What happens if one of your database nodes goes down?
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Requirements
Functional Requirements
- Feature Flag Creation: Ability to create, update, and delete feature flags via a RESTful API.
- User Targeting: Support targeting feature flags based on user se...
Capacity Estimation
Scale Assumptions
- User Base: Assume Snapchat has 500 million monthly active users.
- Feature Flags: Assume an average of 10 feature flags per user.
- Requests: For each user, assu...
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