feature-flags.md

Feature Flag Implementation Standards

This document outlines Bayat's standards and best practices for implementing and managing feature flags across all platforms and project types.

Table of Contents

• Introduction
• Feature Flag Types
• Implementation Patterns
• Naming and Organization
• Lifecycle Management
• Testing Strategies
• Monitoring and Observability
• Security Considerations
• Platform-Specific Implementation
• Tools and Libraries
• Integration with CI/CD
• Governance and Documentation

Introduction

Feature flags (also known as feature toggles or feature switches) are a powerful technique that enables teams to modify system behavior without changing code. They facilitate:

• Continuous delivery and deployment
• A/B testing and experimentation
• Progressive rollouts to users
• Operational control (kill switches)
• Managing technical debt

This document provides comprehensive standards for implementing feature flags consistently across Bayat projects.

Feature Flag Types

Release Flags

Used to enable/disable features that are under active development.

• Purpose: Hide incomplete features in production
• Lifespan: Short-term (removed after feature is stable)
• Scope: Usually binary (on/off)
• Decision Point: Build-time or startup for simple cases, runtime for more flexibility

Operational Flags

Used to control operational aspects of the system in production.

• Purpose: Emergency shutoff, load management, maintenance mode
• Lifespan: Long-term or permanent
• Scope: Binary (on/off) or multi-variate
• Decision Point: Runtime, often dynamically updated

Experiment Flags

Used for A/B testing, multivariate testing, or experimentation.

• Purpose: Test variations of a feature with different user segments
• Lifespan: Medium-term (duration of experiment)
• Scope: Multi-variate (multiple variations)
• Decision Point: Runtime, dynamically determined per user/session

Permission Flags

Used to control feature access for specific users or user segments.

• Purpose: Premium features, early access, beta programs
• Lifespan: Long-term or permanent
• Scope: Binary (on/off) or multi-variate
• Decision Point: Runtime, determined by user attributes

Implementation Patterns

Standard Structure

Feature flags should follow a consistent structure:

Feature Flag = {
  key: String,                // Unique identifier
  name: String,               // Human-readable name
  description: String,        // Purpose and expected behavior
  type: Enum,                 // Release, Operational, Experiment, Permission
  enabled: Boolean,           // Default state
  variants: Array<Variant>,   // For multi-variate flags
  conditions: Object,         // Rules for targeting
  owner: String,              // Team/individual responsible
  createdAt: DateTime,        // Creation timestamp
  expireAt: DateTime,         // Optional planned removal date
}

Decision Models

1. Simple Boolean Toggle:

   if (featureFlags.isEnabled('new-search-algorithm')) {
     // Use new algorithm
   } else {
     // Use old algorithm
   }

2. Multi-Variant Selection:

   const variant = featureFlags.getVariant('pricing-model', user);
   switch (variant) {
     case 'control':
       return standardPricing();
     case 'experiment-a':
       return discountedPricing();
     case 'experiment-b':
       return tieredPricing();
     default:
       return standardPricing();
   }

3. Targeting Rules:

   // Pseudocode for flag configuration
   {
     key: 'premium-feature',
     conditions: {
       operator: 'ALL',
       rules: [
         { attribute: 'subscription', operator: 'EQUALS', value: 'premium' },
         { attribute: 'region', operator: 'IN', values: ['US', 'EU'] }
       ]
     }
   }

4. Gradual Rollout:

   // Pseudocode for flag configuration
   {
     key: 'new-ui',
     rollout: {
       percentage: 25,
       attribute: 'userId'  // Attribute used for consistent bucketing
     }
   }

Architectural Considerations

1. Centralized vs. Distributed:

   • Centralized: A single source of truth for all feature flags
   • Distributed: Flags defined close to their usage

2. Storage Options:

   • Configuration files (for simple, static flags)
   • Database (for dynamic, runtime-updatable flags)
   • Specialized feature flag services

3. Evaluation Location:

   • Backend evaluation (more secure, consistent)
   • Frontend evaluation (more responsive, less load on backend)
   • Hybrid approach (initial load from backend, caching on frontend)

4. Performance Considerations:

   • Cache flag configuration to avoid repeated lookups
   • Implement fallback mechanisms for service unavailability
   • Consider flag evaluation latency in critical paths

Naming and Organization

Naming Conventions

1. Key Format: Use kebab-case for flag keys:

   • new-checkout-flow
   • enhanced-search-algorithm
   • premium-user-dashboard

2. Namespace Structure:

   • [product].[feature].[subfeature] for large organizations with multiple products
   • [feature].[subfeature] for single-product teams

3. Prefixing for Purpose:

   • release: for release flags
   • ops: for operational flags
   • exp: for experiment flags
   • perm: for permission flags

Organizational Structure

1. Group by Domain/Feature:

   • Group related flags together within a domain or feature area
   • e.g., search.algorithm, search.ui, search.filters

2. Group by Team Ownership:

   • Organize flags by the team responsible for them
   • Helps with lifecycle management and accountability

3. Group by Life Expectancy:

   • Temporary vs. permanent flags
   • Helps with technical debt management

Lifecycle Management

Creation Process

1. Creation Request:

   • Specify purpose, expected behavior, target audience
   • Define success metrics and evaluation criteria
   • Document testing approach

2. Implementation:

   • Add flag to codebase with default off (for new features)
   • Update documentation
   • Create tracking ticket for removal (for temporary flags)

3. Activation:

   • Test functionality with flag on and off
   • Implement gradual rollout if needed
   • Monitor for issues after activation

Retirement Process

1. Identification:

   • Regularly review active flags
   • Flag candidates for removal: fully deployed features, completed experiments

2. Cleanup:

   • Remove conditional code
   • Remove flag configuration
   • Update documentation

3. Verification:

   • Verify functionality after flag removal
   • Update tests to reflect permanent state

Technical Debt Prevention

1. Flag Expiration Dates:

   • Set planned removal dates for temporary flags
   • Regular audits of active flags

2. Ownership Assignment:

   • Clearly assign ownership for each flag
   • Make flag cleanup part of feature completion

3. Regular Cleanup Sprints:

   • Dedicate time periodically for flag cleanup
   • Track flag debt metrics

Testing Strategies

Testing with Feature Flags

1. Test Combinations:

   • Test all flag combinations for critical paths
   • Test default on and off states for all flags

2. Parameterized Tests:

   • Run tests with different flag configurations
   • Identify critical flag combinations

3. Integration Tests:

   • Test interaction between multiple flags
   • Verify no unexpected side effects

Test Environments

1. Environment-Specific Configurations:

   • Default configs for different environments
   • Dev/staging environments with easy flag toggling

2. Testing Tools:

   • UI for toggling flags in test environments
   • API endpoints for automation

3. Override Mechanisms:

   • Query parameter overrides for testing
   • User-specific overrides for QA

Monitoring and Observability

Key Metrics

1. Flag Usage Metrics:

   • Which flags are being evaluated and how often
   • Which code paths are being executed

2. Performance Impact:

   • Latency introduced by flag evaluation
   • System performance with different flag configurations

3. Business Metrics:

   • Conversion rates, engagement, revenue impact
   • Experiment outcomes

Logging and Tracing

1. Flag Decision Logging:

   • Log all flag evaluations with context
   • Include user attributes that influenced decision

2. Distributed Tracing:

   • Include flag decisions in trace context
   • Track flag impact across services

3. Debugging Support:

   • Tools to view active flags for a session/user
   • Ability to override flags for troubleshooting

Security Considerations

Access Control

1. Role-Based Access:

   • Admin: Create/delete flags, modify production
   • Developer: Create/delete flags, modify non-production
   • Viewer: View flag configurations

2. Environment Restrictions:

   • Controlled promotion between environments
   • Approval workflows for production changes

3. Audit Trail:

   • Log all flag configuration changes
   • Record who made changes and when

Data Protection

1. Sensitive Data Handling:

   • Don't use PII in flag rules without proper controls
   • Encrypt sensitive targeting attributes

2. Privacy Compliance:

   • Consider GDPR/CCPA implications of user targeting
   • Implement appropriate consent mechanisms

Platform-Specific Implementation

Web Applications (JavaScript)

// React example with a feature flag hook
function useFeatureFlag(flagKey, defaultValue = false) {
  const [enabled, setEnabled] = useState(defaultValue);
  
  useEffect(() => {
    // Initial load
    const isEnabled = featureFlags.isEnabled(flagKey);
    setEnabled(isEnabled);
    
    // Subscribe to changes
    const unsubscribe = featureFlags.subscribe(flagKey, (newValue) => {
      setEnabled(newValue);
    });
    
    return () => unsubscribe();
  }, [flagKey]);
  
  return enabled;
}

// Usage
function SearchComponent() {
  const newAlgorithmEnabled = useFeatureFlag('new-search-algorithm');
  
  return (
    <div>
      {newAlgorithmEnabled ? <NewSearchUI /> : <LegacySearchUI />}
    </div>
  );
}

Mobile Applications

1. Native iOS (Swift):

   // Swift example
   if FeatureFlags.shared.isEnabled("new-profile-ui") {
       showNewProfileUI()
   } else {
       showLegacyProfileUI()
   }

2. Native Android (Kotlin):

   // Kotlin example
   if (featureFlags.isEnabled("new-profile-ui")) {
       showNewProfileUI()
   } else {
       showLegacyProfileUI()
   }

3. React Native:

   // React Native example
   const NewFeature = () => {
     const featureEnabled = useFeatureFlag('new-feature');
     
     if (!featureEnabled) return null;
     
     return (
       <View>
         <Text>New Feature UI</Text>
       </View>
     );
   };

Backend Services

1. Java/Spring:

   @Service
   public class PaymentService {
       private final FeatureFlagService featureFlagService;
       
       @Autowired
       public PaymentService(FeatureFlagService featureFlagService) {
           this.featureFlagService = featureFlagService;
       }
       
       public ProcessResult processPayment(Payment payment, User user) {
           if (featureFlagService.isEnabled("new-payment-processor", user)) {
               return newPaymentProcessor.process(payment);
           } else {
               return legacyPaymentProcessor.process(payment);
           }
       }
   }

2. Node.js:

   // Express middleware for feature flags
   function featureFlagMiddleware(req, res, next) {
     req.featureFlags = {};
     
     // Evaluate all relevant flags for this user
     req.featureFlags.newDashboard = featureFlags.isEnabled('new-dashboard', {
       userId: req.user?.id,
       userRole: req.user?.role,
       region: req.geo?.region
     });
     
     next();
   }
   
   // Usage in route
   app.get('/dashboard', featureFlagMiddleware, (req, res) => {
     if (req.featureFlags.newDashboard) {
       return res.render('new-dashboard');
     }
     return res.render('dashboard');
});

Tools and Libraries

Self-Hosted Solutions

1. Unleash:

   • Open-source feature flag platform
   • Supports all flag types and complex rules
   • Self-hosted with client libraries for multiple languages

2. Flagsmith:

   • Open-source feature flag management
   • Strong support for user segmentation
   • Available as self-hosted or SaaS

Cloud Services

1. LaunchDarkly:

   • Enterprise feature flag management
   • Comprehensive targeting and experimentation
   • Robust SDKs for all major platforms

2. Split:

   • Feature delivery platform
   • Strong analytics and experimentation
   • Enterprise-grade security and compliance

3. Optimizely:

   • Experimentation focused
   • Statistical analysis built in
   • Strong enterprise support

Simplified/Custom Solutions

For projects with simpler needs, consider:

1. Database-backed solution:

   • Store flags in database with caching
   • Simple admin UI for management
   • Basic targeting capabilities

2. Configuration-based solution:

   • YAML/JSON configuration files
   • Environment-specific overrides
   • CI/CD pipeline integration

Integration with CI/CD

Pipeline Integration

1. Feature Branch Development:

   • Develop behind feature flags in trunk
   • CI tests feature with flag on and off

2. Deployment Process:

   • Deploy code with flags off
   • Enable flags after deployment verification
   • Progressive rollout via flag targeting

3. Automated Testing:

   • Test matrix with different flag combinations
   • Performance tests with critical flags

Deployment Strategies

1. Dark Launches:

   • Deploy code behind flag (off)
   • Enable for internal users
   • Gradually roll out to production

2. Canary Releases:

   • Enable feature for small percentage
   • Monitor for issues
   • Gradually increase percentage

3. Ring-Based Deployment:

   • Inner ring: Internal users
   • Middle ring: Beta users
   • Outer ring: All users

Governance and Documentation

Flag Inventory

Maintain a central inventory of all feature flags:

1. Required Metadata:

   • Key, name, description
   • Type and purpose
   • Owner and creation date
   • Expected lifetime
   • Current status

2. Documentation Location:

   • Centralized wiki/knowledge base
   • Service/component documentation
   • Within flag management system

Regular Reviews

1. Flag Review Process:

   • Quarterly review of all active flags
   • Identify candidates for removal
   • Update documentation

2. Metrics Review:

   • Flag usage statistics
   • Performance impact
   • Technical debt assessment

3. Cleanup Planning:

   • Prioritize flag cleanup tasks
   • Schedule cleanup work
   • Track cleanup progress

Best Practices

1. Do:

   • Use feature flags for all significant changes
   • Set expiration dates for temporary flags
   • Test with flags in both states
   • Document all flags and their purpose

2. Don't:

   • Nest feature flags (avoid flag dependencies)
   • Use flags for configuration that rarely changes
   • Leave obsolete flags in the codebase
   • Create overly complex targeting rules