.

devops/release/feature-flags/SKILL.md

@@ -0,0 +1,472 @@
+---
+name: feature-flags
+description: Implement feature flags for progressive feature rollout using LaunchDarkly, Unleash, or custom solutions. Control feature visibility, perform A/B testing, and enable trunk-based development. Use when implementing gradual rollouts, feature toggles, or experimentation platforms.
+license: MIT
+metadata:
+  author: devops-skills
+  version: "1.0"
+---
+
+# Feature Flags
+
+Control feature releases and enable progressive rollout with feature flag systems.
+
+## When to Use This Skill
+
+Use this skill when:
+- Implementing gradual feature rollouts
+- Enabling trunk-based development
+- Running A/B tests and experiments
+- Managing feature lifecycles
+- Implementing kill switches for production
+
+## Prerequisites
+
+- Application code access
+- Feature flag service or self-hosted solution
+- Basic understanding of deployment patterns
+
+## Feature Flag Types
+
+| Type | Purpose | Example |
+|------|---------|---------|
+| Release | Control feature visibility | New checkout flow |
+| Experiment | A/B testing | Button color test |
+| Ops | Runtime configuration | Rate limiting |
+| Permission | User access control | Premium features |
+| Kill Switch | Emergency disable | Third-party integration |
+
+## LaunchDarkly
+
+### SDK Setup (Node.js)
+
+```javascript
+const LaunchDarkly = require('launchdarkly-node-server-sdk');
+
+const client = LaunchDarkly.init(process.env.LAUNCHDARKLY_SDK_KEY);
+
+await client.waitForInitialization();
+
+// Evaluate flag
+const user = {
+  key: 'user-123',
+  email: 'user@example.com',
+  custom: {
+    plan: 'premium',
+    company: 'acme'
+  }
+};
+
+const showNewFeature = await client.variation('new-checkout', user, false);
+
+if (showNewFeature) {
+  // New feature code
+} else {
+  // Existing code
+}
+```
+
+### React SDK
+
+```javascript
+import { withLDProvider, useFlags, useLDClient } from 'launchdarkly-react-client-sdk';
+
+// Provider setup
+export default withLDProvider({
+  clientSideID: 'your-client-side-id',
+  user: {
+    key: 'user-123',
+    email: 'user@example.com'
+  }
+})(App);
+
+// Using flags in component
+function FeatureComponent() {
+  const { newCheckout, experimentVariant } = useFlags();
+  const ldClient = useLDClient();
+
+  // Track events
+  const handleClick = () => {
+    ldClient.track('checkout-started');
+  };
+
+  if (newCheckout) {
+    return <NewCheckout onClick={handleClick} />;
+  }
+  return <OldCheckout onClick={handleClick} />;
+}
+```
+
+### Targeting Rules
+
+```yaml
+# LaunchDarkly targeting configuration
+flag: new-checkout
+targeting:
+  # Individual users
+  targets:
+    - variation: true
+      values: ['user-123', 'user-456']
+  
+  # Rules
+  rules:
+    # Beta users
+    - variation: true
+      clauses:
+        - attribute: email
+          op: endsWith
+          values: ['@company.com']
+    
+    # Premium plan
+    - variation: true
+      clauses:
+        - attribute: plan
+          op: in
+          values: ['premium', 'enterprise']
+    
+    # Percentage rollout
+    - variation: true
+      rollout:
+        variations:
+          - variation: true
+            weight: 20000  # 20%
+          - variation: false
+            weight: 80000  # 80%
+  
+  # Default
+  fallthrough:
+    variation: false
+```
+
+## Unleash
+
+### Server Setup
+
+```yaml
+# docker-compose.yml
+version: '3.8'
+
+services:
+  unleash:
+    image: unleashorg/unleash-server:latest
+    ports:
+      - "4242:4242"
+    environment:
+      - DATABASE_URL=postgres://postgres:password@db/unleash
+      - DATABASE_SSL=false
+    depends_on:
+      - db
+
+  db:
+    image: postgres:15
+    environment:
+      - POSTGRES_PASSWORD=password
+      - POSTGRES_DB=unleash
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+
+volumes:
+  postgres-data:
+```
+
+### SDK Setup (Node.js)
+
+```javascript
+const { initialize } = require('unleash-client');
+
+const unleash = initialize({
+  url: 'http://localhost:4242/api',
+  appName: 'my-app',
+  customHeaders: {
+    Authorization: 'your-api-token'
+  }
+});
+
+unleash.on('ready', () => {
+  // Check feature
+  const isEnabled = unleash.isEnabled('new-checkout');
+  
+  // With context
+  const context = {
+    userId: 'user-123',
+    properties: {
+      plan: 'premium'
+    }
+  };
+  
+  const isEnabledForUser = unleash.isEnabled('new-checkout', context);
+  
+  // Get variant
+  const variant = unleash.getVariant('experiment-flag', context);
+  console.log(variant.name); // 'control' or 'treatment'
+});
+```
+
+### Activation Strategies
+
+```yaml
+# Standard strategies
+strategies:
+  - name: default
+    # On/off for everyone
+    
+  - name: userWithId
+    parameters:
+      userIds: 'user-1,user-2,user-3'
+    
+  - name: gradualRolloutUserId
+    parameters:
+      percentage: 25
+      groupId: 'new-feature'
+    
+  - name: gradualRolloutRandom
+    parameters:
+      percentage: 50
+    
+  - name: flexibleRollout
+    parameters:
+      rollout: 30
+      stickiness: userId
+      groupId: 'checkout-exp'
+```
+
+## Custom Implementation
+
+### Database-Backed Flags
+
+```python
+# models.py
+from django.db import models
+
+class FeatureFlag(models.Model):
+    name = models.CharField(max_length=100, unique=True)
+    enabled = models.BooleanField(default=False)
+    rollout_percentage = models.IntegerField(default=0)
+    allowed_users = models.JSONField(default=list)
+    rules = models.JSONField(default=dict)
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+# service.py
+import hashlib
+
+class FeatureFlagService:
+    def __init__(self):
+        self._cache = {}
+    
+    def is_enabled(self, flag_name, user_id=None, context=None):
+        flag = self._get_flag(flag_name)
+        
+        if not flag or not flag.enabled:
+            return False
+        
+        # Check user allowlist
+        if user_id and user_id in flag.allowed_users:
+            return True
+        
+        # Check rules
+        if context and self._evaluate_rules(flag.rules, context):
+            return True
+        
+        # Check percentage rollout
+        if flag.rollout_percentage > 0 and user_id:
+            return self._is_in_rollout(flag_name, user_id, flag.rollout_percentage)
+        
+        return flag.rollout_percentage == 100
+    
+    def _is_in_rollout(self, flag_name, user_id, percentage):
+        hash_input = f"{flag_name}:{user_id}"
+        hash_value = int(hashlib.md5(hash_input.encode()).hexdigest(), 16)
+        return (hash_value % 100) < percentage
+    
+    def _evaluate_rules(self, rules, context):
+        for rule in rules.get('rules', []):
+            if self._evaluate_rule(rule, context):
+                return True
+        return False
+```
+
+### Redis-Backed Flags
+
+```python
+import redis
+import json
+
+class RedisFeatureFlags:
+    def __init__(self, redis_url):
+        self.redis = redis.from_url(redis_url)
+        self.prefix = 'feature_flag:'
+    
+    def set_flag(self, name, config):
+        key = f"{self.prefix}{name}"
+        self.redis.set(key, json.dumps(config))
+    
+    def is_enabled(self, name, user_id=None):
+        key = f"{self.prefix}{name}"
+        data = self.redis.get(key)
+        
+        if not data:
+            return False
+        
+        config = json.loads(data)
+        
+        if not config.get('enabled', False):
+            return False
+        
+        # User allowlist
+        if user_id in config.get('users', []):
+            return True
+        
+        # Percentage rollout
+        percentage = config.get('percentage', 0)
+        if percentage == 100:
+            return True
+        
+        if percentage > 0 and user_id:
+            return self._hash_user(name, user_id) < percentage
+        
+        return False
+    
+    def _hash_user(self, flag, user_id):
+        import hashlib
+        hash_input = f"{flag}:{user_id}"
+        return int(hashlib.sha256(hash_input.encode()).hexdigest(), 16) % 100
+```
+
+## Testing with Feature Flags
+
+### Unit Testing
+
+```javascript
+// Jest mocking
+jest.mock('launchdarkly-node-server-sdk', () => ({
+  init: jest.fn(() => ({
+    waitForInitialization: jest.fn().mockResolvedValue(undefined),
+    variation: jest.fn()
+  }))
+}));
+
+describe('Checkout', () => {
+  it('shows new checkout when flag enabled', async () => {
+    const ldClient = require('launchdarkly-node-server-sdk').init();
+    ldClient.variation.mockResolvedValue(true);
+    
+    const result = await renderCheckout(user);
+    expect(result).toContain('NewCheckout');
+  });
+  
+  it('shows old checkout when flag disabled', async () => {
+    const ldClient = require('launchdarkly-node-server-sdk').init();
+    ldClient.variation.mockResolvedValue(false);
+    
+    const result = await renderCheckout(user);
+    expect(result).toContain('OldCheckout');
+  });
+});
+```
+
+### Integration Testing
+
+```python
+# pytest fixtures
+import pytest
+
+@pytest.fixture
+def feature_flags():
+    """Provide controllable feature flags for testing."""
+    flags = {}
+    
+    class TestFlags:
+        def set(self, name, value):
+            flags[name] = value
+        
+        def is_enabled(self, name, **kwargs):
+            return flags.get(name, False)
+    
+    return TestFlags()
+
+def test_new_checkout(feature_flags):
+    feature_flags.set('new-checkout', True)
+    
+    response = client.get('/checkout')
+    assert 'new-checkout-form' in response.content
+```
+
+## Monitoring and Analytics
+
+### Flag Usage Tracking
+
+```javascript
+// Track flag evaluations
+const flagMetrics = {
+  evaluations: new Map(),
+  
+  track(flagName, variation, user) {
+    const key = `${flagName}:${variation}`;
+    const count = this.evaluations.get(key) || 0;
+    this.evaluations.set(key, count + 1);
+    
+    // Send to analytics
+    analytics.track('feature_flag_evaluated', {
+      flag: flagName,
+      variation: variation,
+      userId: user.key
+    });
+  }
+};
+```
+
+### Stale Flag Detection
+
+```python
+from datetime import datetime, timedelta
+
+def detect_stale_flags():
+    """Find flags that haven't been evaluated recently."""
+    stale_threshold = timedelta(days=30)
+    now = datetime.utcnow()
+    
+    stale_flags = []
+    for flag in FeatureFlag.objects.all():
+        if flag.last_evaluated:
+            age = now - flag.last_evaluated
+            if age > stale_threshold:
+                stale_flags.append({
+                    'name': flag.name,
+                    'last_evaluated': flag.last_evaluated,
+                    'age_days': age.days
+                })
+    
+    return stale_flags
+```
+
+## Common Issues
+
+### Issue: Inconsistent Flag Evaluation
+**Problem**: Same user sees different variations
+**Solution**: Use consistent hashing, check caching strategy
+
+### Issue: Flag Debt Accumulation
+**Problem**: Too many old flags in codebase
+**Solution**: Implement flag lifecycle, regular cleanup sprints
+
+### Issue: Performance Impact
+**Problem**: Flag evaluation slowing requests
+**Solution**: Use local caching, batch evaluations
+
+## Best Practices
+
+- Use consistent naming conventions
+- Document flag purpose and owner
+- Set expiration dates for temporary flags
+- Implement flag lifecycle management
+- Use gradual rollouts (not 0→100)
+- Monitor flag evaluation metrics
+- Clean up old flags regularly
+- Test both variations in CI
+
+## Related Skills
+
+- [blue-green-deploy](../blue-green-deploy/) - Deployment strategies
+- [git-workflow](../git-workflow/) - Trunk-based development
+- [alerting-oncall](../../observability/alerting-oncall/) - Monitoring rollouts