SKILL.md
Feature Flag System
Table of Contents
- [Overview](#overview)
- [When to Use](#when-to-use)
- [Quick Start](#quick-start)
- [Reference Guides](#reference-guides)
- [Best Practices](#best-practices)
Overview
Implement feature flags to decouple deployment from release, enable gradual rollouts, A/B testing, and provide emergency kill switches.
When to Use
- Gradual feature rollouts
- A/B testing and experiments
- Canary deployments
- Beta features for specific users
- Emergency kill switches
- Trunk-based development
- Dark launching
- Operational flags (maintenance mode)
- User-specific features
Quick Start
Minimal working example:
interface FlagConfig {
key: string;
enabled: boolean;
description: string;
rules?: FlagRule[];
variants?: FlagVariant[];
createdAt: Date;
updatedAt: Date;
}
interface FlagRule {
type: "user" | "percentage" | "attribute" | "datetime";
operator: "in" | "equals" | "contains" | "gt" | "lt" | "between";
attribute?: string;
values: any[];
}
interface FlagVariant {
key: string;
weight: number;
value: any;
}
interface EvaluationContext {
userId?: string;
// ... (see reference guides for full implementation)Reference Guides
Detailed implementations in the references/ directory:
| Guide | Contents |
|---|---|
| Feature Flag Service (TypeScript) | Feature Flag Service (TypeScript) |
| React Hook for Feature Flags | React Hook for Feature Flags |
| Feature Flag with Analytics | Feature Flag with Analytics |
| LaunchDarkly-Style SDK | LaunchDarkly-Style SDK |
| Admin UI for Feature Flags | Admin UI for Feature Flags |
Best Practices
✅ DO
- Use descriptive flag names
- Document flag purpose and lifecycle
- Implement gradual rollouts
- Track flag evaluations
- Clean up old flags regularly
- Use feature flags for experiments
- Implement kill switches for critical features
- Test both enabled and disabled states
- Use consistent hashing for stable rollouts
- Provide admin UI for non-technical users
❌ DON'T
- Use flags for permanent configuration
- Accumulate technical debt with old flags
- Skip flag cleanup
- Make flags too granular
- Hard-code flag checks everywhere
- Skip analytics and monitoring