feature-flag-system

SKILL.md

Feature Flag System

Table of Contents

Overview

When to Use

Quick Start

Reference Guides

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

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