name: feature-flag-system description: Implement feature flags (toggles) for controlled feature rollouts, A/B testing, canary deployments, and kill switches. Use when deploying new features gradually, testing in production, or managing feature lifecycles.

Feature Flag System

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

Implementation Examples

1. Feature Flag Service (TypeScript)

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;
  email?: string;
  attributes?: Record<string, any>;
  timestamp?: number;
}

class FeatureFlagService {
  private flags: Map<string, FlagConfig> = new Map();

  constructor() {
    this.loadFlags();
  }

  private loadFlags(): void {
    // Load from database or config
    this.flags.set('new-dashboard', {
      key: 'new-dashboard',
      enabled: true,
      description: 'New dashboard UI',
      rules: [
        {
          type: 'percentage',
          operator: 'lt',
          values: [25] // 25% rollout
        }
      ],
      createdAt: new Date(),
      updatedAt: new Date()
    });

    this.flags.set('premium-features', {
      key: 'premium-features',
      enabled: true,
      description: 'Premium features for paid users',
      rules: [
        {
          type: 'attribute',
          operator: 'equals',
          attribute: 'plan',
          values: ['premium', 'enterprise']
        }
      ],
      createdAt: new Date(),
      updatedAt: new Date()
    });

    this.flags.set('beta-feature', {
      key: 'beta-feature',
      enabled: true,
      description: 'Beta feature',
      rules: [
        {
          type: 'user',
          operator: 'in',
          values: ['user1', 'user2', 'user3']
        }
      ],
      createdAt: new Date(),
      updatedAt: new Date()
    });
  }

  isEnabled(flagKey: string, context: EvaluationContext = {}): boolean {
    const flag = this.flags.get(flagKey);

    if (!flag) {
      console.warn(`Flag not found: ${flagKey}`);
      return false;
    }

    if (!flag.enabled) {
      return false;
    }

    if (!flag.rules || flag.rules.length === 0) {
      return true;
    }

    return this.evaluateRules(flag.rules, context);
  }

  getVariant(flagKey: string, context: EvaluationContext = {}): any {
    const flag = this.flags.get(flagKey);

    if (!flag || !this.isEnabled(flagKey, context)) {
      return null;
    }

    if (!flag.variants || flag.variants.length === 0) {
      return true;
    }

    return this.selectVariant(flag.variants, context);
  }

  private evaluateRules(rules: FlagRule[], context: EvaluationContext): boolean {
    return rules.every(rule => this.evaluateRule(rule, context));
  }

  private evaluateRule(rule: FlagRule, context: EvaluationContext): boolean {
    switch (rule.type) {
      case 'user':
        return this.evaluateUserRule(rule, context);

      case 'percentage':
        return this.evaluatePercentageRule(rule, context);

      case 'attribute':
        return this.evaluateAttributeRule(rule, context);

      case 'datetime':
        return this.evaluateDateTimeRule(rule, context);

      default:
        return false;
    }
  }

  private evaluateUserRule(rule: FlagRule, context: EvaluationContext): boolean {
    if (!context.userId) return false;

    return rule.values.includes(context.userId);
  }

  private evaluatePercentageRule(rule: FlagRule, context: EvaluationContext): boolean {
    const hash = this.hashContext(context);
    const percentage = (hash % 100) + 1;

    return percentage <= rule.values[0];
  }

  private evaluateAttributeRule(rule: FlagRule, context: EvaluationContext): boolean {
    if (!rule.attribute || !context.attributes) return false;

    const value = context.attributes[rule.attribute];

    switch (rule.operator) {
      case 'equals':
        return rule.values.includes(value);

      case 'contains':
        return rule.values.some(v => String(value).includes(v));

      case 'gt':
        return value > rule.values[0];

      case 'lt':
        return value < rule.values[0];

      default:
        return false;
    }
  }

  private evaluateDateTimeRule(rule: FlagRule, context: EvaluationContext): boolean {
    const now = context.timestamp || Date.now();

    if (rule.operator === 'between') {
      return now >= rule.values[0] && now <= rule.values[1];
    }

    return false;
  }

  private selectVariant(variants: FlagVariant[], context: EvaluationContext): any {
    const hash = this.hashContext(context);
    const totalWeight = variants.reduce((sum, v) => sum + v.weight, 0);
    const position = hash % totalWeight;

    let cumulative = 0;
    for (const variant of variants) {
      cumulative += variant.weight;
      if (position < cumulative) {
        return variant.value;
      }
    }

    return variants[0].value;
  }

  private hashContext(context: EvaluationContext): number {
    const str = context.userId || context.email || 'anonymous';
    let hash = 0;

    for (let i = 0; i < str.length; i++) {
      const char = str.charCodeAt(i);
      hash = ((hash << 5) - hash) + char;
      hash = hash & hash;
    }

    return Math.abs(hash);
  }

  async createFlag(config: Omit<FlagConfig, 'createdAt' | 'updatedAt'>): Promise<void> {
    this.flags.set(config.key, {
      ...config,
      createdAt: new Date(),
      updatedAt: new Date()
    });
  }

  async updateFlag(key: string, updates: Partial<FlagConfig>): Promise<void> {
    const flag = this.flags.get(key);
    if (!flag) {
      throw new Error(`Flag not found: ${key}`);
    }

    this.flags.set(key, {
      ...flag,
      ...updates,
      updatedAt: new Date()
    });
  }

  async deleteFlag(key: string): Promise<void> {
    this.flags.delete(key);
  }

  getAllFlags(): FlagConfig[] {
    return Array.from(this.flags.values());
  }
}

// Usage
const featureFlags = new FeatureFlagService();

// Simple boolean check
if (featureFlags.isEnabled('new-dashboard', { userId: 'user123' })) {
  console.log('Show new dashboard');
}

// With user attributes
const hasPremiumFeatures = featureFlags.isEnabled('premium-features', {
  userId: 'user123',
  attributes: { plan: 'premium' }
});

// Get variant for A/B testing
const buttonColor = featureFlags.getVariant('button-color-test', {
  userId: 'user123'
});

2. React Hook for Feature Flags

import { createContext, useContext, ReactNode } from 'react';

interface FeatureFlagContextType {
  isEnabled: (key: string) => boolean;
  getVariant: (key: string) => any;
}

const FeatureFlagContext = createContext<FeatureFlagContextType | null>(null);

export function FeatureFlagProvider({
  children,
  userId,
  attributes
}: {
  children: ReactNode;
  userId?: string;
  attributes?: Record<string, any>;
}) {
  const flagService = new FeatureFlagService();

  const context: FeatureFlagContextType = {
    isEnabled: (key: string) => {
      return flagService.isEnabled(key, { userId, attributes });
    },
    getVariant: (key: string) => {
      return flagService.getVariant(key, { userId, attributes });
    }
  };

  return (
    <FeatureFlagContext.Provider value={context}>
      {children}
    </FeatureFlagContext.Provider>
  );
}

export function useFeatureFlag(key: string): boolean {
  const context = useContext(FeatureFlagContext);
  if (!context) {
    throw new Error('useFeatureFlag must be used within FeatureFlagProvider');
  }
  return context.isEnabled(key);
}

export function useFeatureVariant(key: string): any {
  const context = useContext(FeatureFlagContext);
  if (!context) {
    throw new Error('useFeatureVariant must be used within FeatureFlagProvider');
  }
  return context.getVariant(key);
}

// Feature component wrapper
export function Feature({
  flag,
  fallback = null,
  children
}: {
  flag: string;
  fallback?: ReactNode;
  children: ReactNode;
}) {
  const isEnabled = useFeatureFlag(flag);

  return isEnabled ? <>{children}</> : <>{fallback}</>;
}

// Usage in components
function Dashboard() {
  const hasNewDashboard = useFeatureFlag('new-dashboard');
  const theme = useFeatureVariant('theme-experiment');

  return (
    <div>
      {hasNewDashboard ? <NewDashboard /> : <OldDashboard />}

      <Feature flag="premium-features" fallback={<UpgradePrompt />}>
        <PremiumContent />
      </Feature>

      <div style={{ backgroundColor: theme?.backgroundColor }}>
        Content with experiment theme
      </div>
    </div>
  );
}

3. Feature Flag with Analytics

interface FlagEvaluationEvent {
  flagKey: string;
  userId?: string;
  result: boolean;
  variant?: any;
  timestamp: number;
  duration: number;
}

class FeatureFlagServiceWithAnalytics extends FeatureFlagService {
  private analytics: Analytics;

  constructor(analytics: Analytics) {
    super();
    this.analytics = analytics;
  }

  isEnabled(flagKey: string, context: EvaluationContext = {}): boolean {
    const startTime = Date.now();
    const result = super.isEnabled(flagKey, context);
    const duration = Date.now() - startTime;

    this.trackEvaluation({
      flagKey,
      userId: context.userId,
      result,
      timestamp: Date.now(),
      duration
    });

    return result;
  }

  getVariant(flagKey: string, context: EvaluationContext = {}): any {
    const startTime = Date.now();
    const variant = super.getVariant(flagKey, context);
    const duration = Date.now() - startTime;

    this.trackEvaluation({
      flagKey,
      userId: context.userId,
      result: variant !== null,
      variant,
      timestamp: Date.now(),
      duration
    });

    return variant;
  }

  private trackEvaluation(event: FlagEvaluationEvent): void {
    this.analytics.track('feature_flag_evaluated', {
      flag_key: event.flagKey,
      user_id: event.userId,
      result: event.result,
      variant: event.variant,
      duration_ms: event.duration
    });
  }

  async getAnalytics(flagKey: string, timeRange: { start: Date; end: Date }): Promise<{
    evaluations: number;
    uniqueUsers: number;
    enabledRate: number;
    variantDistribution: Record<string, number>;
  }> {
    return this.analytics.getFlagAnalytics(flagKey, timeRange);
  }
}

4. LaunchDarkly-Style SDK

from typing import Dict, Any, Optional
import hashlib
import json

class FeatureFlagClient:
    def __init__(self, sdk_key: str, config: Optional[Dict] = None):
        self.sdk_key = sdk_key
        self.config = config or {}
        self.flags: Dict[str, Dict] = {}
        self.initialize()

    def initialize(self):
        """Load flags from API or cache."""
        # In production, fetch from API
        self.flags = {
            'new-feature': {
                'enabled': True,
                'rollout': {
                    'percentage': 50
                }
            },
            'premium-feature': {
                'enabled': True,
                'targeting': {
                    'attribute': 'plan',
                    'values': ['premium', 'enterprise']
                }
            }
        }

    def variation(
        self,
        flag_key: str,
        user: Dict[str, Any],
        default: bool = False
    ) -> bool:
        """Evaluate flag for user."""
        flag = self.flags.get(flag_key)

        if not flag or not flag.get('enabled'):
            return default

        # Check targeting rules
        if 'targeting' in flag:
            if not self._evaluate_targeting(flag['targeting'], user):
                return False

        # Check percentage rollout
        if 'rollout' in flag:
            return self._evaluate_rollout(flag['rollout'], user, flag_key)

        return True

    def variation_detail(
        self,
        flag_key: str,
        user: Dict[str, Any],
        default: Any = None
    ) -> Dict[str, Any]:
        """Get flag variation with details."""
        value = self.variation(flag_key, user, default)

        return {
            'value': value,
            'variation_index': 0 if value else 1,
            'reason': {
                'kind': 'RULE_MATCH' if value else 'OFF'
            }
        }

    def _evaluate_targeting(self, targeting: Dict, user: Dict) -> bool:
        """Evaluate targeting rules."""
        attribute = targeting.get('attribute')
        values = targeting.get('values', [])

        user_value = user.get(attribute)
        return user_value in values

    def _evaluate_rollout(
        self,
        rollout: Dict,
        user: Dict,
        flag_key: str
    ) -> bool:
        """Evaluate percentage rollout."""
        percentage = rollout.get('percentage', 0)
        user_id = user.get('id', user.get('email', 'anonymous'))

        # Consistent hashing for stable rollout
        hash_value = self._hash_user(user_id, flag_key)
        bucket = hash_value % 100

        return bucket < percentage

    def _hash_user(self, user_id: str, flag_key: str) -> int:
        """Hash user ID for consistent bucketing."""
        combined = f"{flag_key}:{user_id}"
        hash_bytes = hashlib.sha256(combined.encode()).digest()
        return int.from_bytes(hash_bytes[:4], byteorder='big')

    def track(self, event_name: str, user: Dict, data: Optional[Dict] = None):
        """Track custom event."""
        # Send to analytics
        pass

    def identify(self, user: Dict):
        """Identify user."""
        # Update user context
        pass

    def flush(self):
        """Flush events."""
        pass

    def close(self):
        """Close client."""
        pass


# Usage
client = FeatureFlagClient(sdk_key='your-sdk-key')

user = {
    'id': 'user-123',
    'email': 'user@example.com',
    'plan': 'premium'
}

# Check if feature is enabled
if client.variation('new-feature', user):
    print("New feature enabled!")

# Get detailed information
detail = client.variation_detail('premium-feature', user)
print(f"Value: {detail['value']}, Reason: {detail['reason']}")

# Track event
client.track('feature-used', user, {'feature': 'new-feature'})

5. Admin UI for Feature Flags

interface FlagFormData {
  key: string;
  description: string;
  enabled: boolean;
  rolloutPercentage?: number;
  targetUsers?: string[];
  targetAttributes?: Record<string, any>;
}

function FeatureFlagDashboard() {
  const [flags, setFlags] = useState<FlagConfig[]>([]);
  const flagService = new FeatureFlagService();

  useEffect(() => {
    loadFlags();
  }, []);

  const loadFlags = async () => {
    const allFlags = flagService.getAllFlags();
    setFlags(allFlags);
  };

  const toggleFlag = async (key: string) => {
    const flag = flags.find(f => f.key === key);
    if (flag) {
      await flagService.updateFlag(key, { enabled: !flag.enabled });
      await loadFlags();
    }
  };

  return (
    <div className="dashboard">
      <h1>Feature Flags</h1>

      <table>
        <thead>
          <tr>
            <th>Flag</th>
            <th>Description</th>
            <th>Status</th>
            <th>Rollout</th>
            <th>Actions</th>
          </tr>
        </thead>
        <tbody>
          {flags.map(flag => (
            <tr key={flag.key}>
              <td>{flag.key}</td>
              <td>{flag.description}</td>
              <td>
                <Switch
                  checked={flag.enabled}
                  onChange={() => toggleFlag(flag.key)}
                />
              </td>
              <td>{getRolloutPercentage(flag)}%</td>
              <td>
                <button onClick={() => editFlag(flag)}>Edit</button>
                <button onClick={() => deleteFlag(flag.key)}>Delete</button>
              </td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
}

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

Resources

• LaunchDarkly
• Split.io
• Flagsmith
• Unleash