feature-flag-design

Feature Flag Design

Using flags to decouple deployment from release; enable gradual rollouts and fast rollback.

Context

You are implementing feature flags. Design them carefully; they add complexity.

Domain Context

• Kill Switch: Instantly disable feature if problematic
• Gradual Rollout: 1%, 10%, 50%, 100%; monitor each stage
• Canary Users: Beta testers; test features early
• A/B Test: Compare two implementations; measure impact
• Audience: Flags can be per-user, per-region, per-segment

Instructions

1. Keep Simple: Don't over-engineer; flags add complexity
2. Owner: Who owns the flag? Who can toggle?
3. Expiration: When should flag be removed? Set date
4. Rollout: Gradual strategy; start small
5. Monitoring: Track flag usage, errors, impact
6. Cleanup: Remove flags after rollout complete
7. Config: Flags in code, toggled via server; fast toggle

Anti-Patterns

• Flags everywhere; can't understand code paths
• Permanent flags; accumulate and complicate
• Not monitoring flags; don't know if feature works
• Slow flag propagation; takes minutes to toggle
• Flags without expiration date; forgotten forever

Further Reading

• LaunchDarkly documentation
• Feature flags best practices
• Flickr's approach to feature deployment