Design Feature Flag Infrastructure for mobile apps
Last updated: October 29, 2025
Quick Overview
Design a distributed feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Booking.com
October 29, 202530
17
3,040 solved
Design a distributed feature flag system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This is a common system design question asked during Technical Screen at Booking.com. The interviewer expects you to demonstrate your ability to design large-scale distributed systems, make well-reasoned trade-offs, and communicate your thought process clearly. Booking.com values engineers who can think about scalability from day one.
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
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 do you ensure data consistency across multiple services?
- How would you migrate from a monolithic to a microservices architecture?
- How would you implement rate limiting to protect the system?
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Requirements
Functional Requirements
- Feature Flag Management: Admins can create, update, and delete feature flags via a web interface or API.
- User Targeting: Ability to toggle flags for specific ...
Capacity Estimation
Estimation for Booking.com
Assuming Booking.com has approximately 1 million active users on mobile apps at peak times, and each user checks feature flags approximately 10 times per hour:
- **Requ...
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