Feature Flag System Implementation

You are a platform engineer who builds feature flag systems. Design a feature flag implementation for this application.

Application type: [WEB / API / MOBILE]
Team size: [DEVELOPERS]
Release frequency: [DAILY / WEEKLY]
Existing solution: [NONE / LAUNCHDARKLY / CUSTOM]
Use cases: [GRADUAL ROLLOUT / A-B TESTING / KILL SWITCH / OPS]

Provide:

**Feature Flag Architecture**
```
Client → Flag Service → Rules Engine → Variation
                              ↓
                         User Context
```

**Flag Definition Schema**
```json
{
  "key": "new-checkout-flow",
  "name": "New Checkout Flow",
  "description": "...",
  "type": "boolean",
  "defaultValue": false,
  "variations": [
    { "value": true, "name": "Enabled" },
    { "value": false, "name": "Disabled" }
  ],
  "rules": [
    {
      "conditions": [...],
      "variation": 0,
      "percentage": 10
    }
  ],
  "environments": ["production", "staging"]
}
```

**SDK Implementation**

Server-side:
```[language]
class FeatureFlags {
  async isEnabled(key: string, context: UserContext): Promise<boolean>
  async getVariation(key: string, context: UserContext): Promise<any>
  async getAllFlags(context: UserContext): Promise<Record<string, any>>
}
```

Client-side:
```javascript
// React hook
function useFeatureFlag(key, defaultValue) {
  // Implementation
}

// Provider component
<FeatureFlagProvider>
```

**Targeting Rules**
- User attributes (id, email, role)
- Percentage rollout
- Geographic targeting
- Device/platform targeting
- Custom attributes

**Gradual Rollout Pattern**
```
Week 1: 5% (internal + beta)
Week 2: 25%
Week 3: 50%
Week 4: 100%
```

**A/B Testing Integration**
- Experiment assignment
- Metric tracking
- Statistical significance
- Winner selection

**Operational Flags**
- Kill switches
- Maintenance mode
- Rate limiting adjustments
- Debug modes

**Admin Interface**
- Flag management UI
- Audit logging
- Change history
- Scheduled changes

**Best Practices**
- Flag naming conventions
- Flag lifecycle (create → test → release → remove)
- Technical debt management
- Documentation requirements

**Monitoring**
- Flag evaluation metrics
- Performance impact
- Error tracking per variation

You are a platform engineer who builds feature flag systems. Design a feature flag implementation for this application.

Application type: [WEB / API / MOBILE]
Team size: [DEVELOPERS]
Release frequency: [DAILY / WEEKLY]
Existing solution: [NONE / LAUNCHDARKLY / CUSTOM]
Use cases: [GRADUAL ROLLOUT / A-B TESTING / KILL SWITCH / OPS]

Provide:

**Feature Flag Architecture**
```
Client → Flag Service → Rules Engine → Variation
                              ↓
                         User Context
```

**Flag Definition Schema**
```json
{
  "key": "new-checkout-flow",
  "name": "New Checkout Flow",
  "description": "...",
  "type": "boolean",
  "defaultValue": false,
  "variations": [
    { "value": true, "name": "Enabled" },
    { "value": false, "name": "Disabled" }
  ],
  "rules": [
    {
      "conditions": [...],
      "variation": 0,
      "percentage": 10
    }
  ],
  "environments": ["production", "staging"]
}
```

**SDK Implementation**

Server-side:
```[language]
class FeatureFlags {
  async isEnabled(key: string, context: UserContext): Promise<boolean>
  async getVariation(key: string, context: UserContext): Promise<any>
  async getAllFlags(context: UserContext): Promise<Record<string, any>>
}
```

Client-side:
```javascript
// React hook
function useFeatureFlag(key, defaultValue) {
  // Implementation
}

// Provider component
<FeatureFlagProvider>
```

**Targeting Rules**
- User attributes (id, email, role)
- Percentage rollout
- Geographic targeting
- Device/platform targeting
- Custom attributes

**Gradual Rollout Pattern**
```
Week 1: 5% (internal + beta)
Week 2: 25%
Week 3: 50%
Week 4: 100%
```

**A/B Testing Integration**
- Experiment assignment
- Metric tracking
- Statistical significance
- Winner selection

**Operational Flags**
- Kill switches
- Maintenance mode
- Rate limiting adjustments
- Debug modes

**Admin Interface**
- Flag management UI
- Audit logging
- Change history
- Scheduled changes

**Best Practices**
- Flag naming conventions
- Flag lifecycle (create → test → release → remove)
- Technical debt management
- Documentation requirements

**Monitoring**
- Flag evaluation metrics
- Performance impact
- Error tracking per variation