Design a System for Feature Flags/Toggles
Design a system that allows engineers to enable/disable features dynamically for specific user groups (e.g., percentage, region, beta users).
Why Interviewers Ask This
Apple interviewers ask this to evaluate your ability to design systems that balance extreme reliability with granular control. They specifically want to see if you can architect a solution that ensures zero downtime during feature rollouts while handling high-scale traffic without adding latency. The question tests your understanding of consistency models, data partitioning strategies for user targeting, and how to manage the complexity of dynamic configuration across global services.
How to Answer This Question
Key Points to Cover
- Prioritizing low-latency reads through aggressive client-side caching strategies
- Implementing consistent hashing for deterministic user targeting across sessions
- Designing a fallback mechanism to ensure app stability during service outages
- Separating write-heavy management operations from read-heavy query paths
- Ensuring atomic updates to prevent race conditions during feature toggling
Sample Answer
Common Mistakes to Avoid
- Ignoring the performance impact of synchronous flag checks on every API call
- Failing to address how to handle stale cache data when a flag is disabled urgently
- Overlooking the need for audit logs to track who changed which flag and when
- Not considering how to support complex targeting logic like time-based rollouts
- Assuming a single database can handle global scale without discussing sharding
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