Design a Feature Flag Service
Last updated: October 18, 2025
Quick Overview
Design a fault-tolerant feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Zscaler
System Design
Software Engineer
Zscaler
October 18, 2025Software Engineer
Technical Screen
System Design
Hard
2
5
4,179 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 Zscaler 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
- 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
Database selection and data modeling
Message queues and async processing
Failure handling and fault tolerance
Partitioning and sharding strategies
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 handle a region-wide outage?
- How would you handle schema migrations with zero downtime?
- How would you migrate from a monolithic to a microservices architecture?
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Requirements
Functional Requirements:
- Feature Flag Creation and Management: Users can create, read, update, and delete feature flags.
- Targeting Rules: Ability to define targeting rules based on u...
Capacity Estimation
Estimation Process:
- User Base: Assume Zscaler has 1 million users. Each user checks feature flags an average of 10 times per day.
- Daily Requests: 1,000,000 users * 10 requests = 10 m...
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