Design a Feature Flag Service

Requirements

• •Allow defining feature flags with targeting rules (user ID, user segment, percentage)
• •Client SDKs evaluate flags locally using a cached rule set fetched from the service
• •Support gradual rollout: enable a flag for X% of users consistently (same user always gets same variant)
• •Support multiple environments (dev, staging, production) with independent flag states
• •Provide a management UI API for creating, updating, and archiving flags

Non-Functional Requirements

• •Flag evaluation latency < 1ms (local SDK cache)
• •Flag change propagation within 30 seconds to all SDK instances
• •Support 10,000 SDK instances polling the service
• •System handles 1 billion flag evaluations/day across all SDKs

Constraints

• •Flags are boolean or multi-variant (A/B/C)
• •Client SDKs are embedded in backend services (not browsers)
• •Flag changes must propagate to all SDK instances within 30 seconds

Evaluation Criteria