This PR implements a complete feature flags system for the Product Construction Service (PCS) that enables subscription-specific feature toggles, gradual rollouts, and A/B testing capabilities with administrative bulk operations.

🎯 Key Features

Core Infrastructure

• Predefined Feature Flags Registry: Static FeatureFlags class with rebase strategy flag and metadata
• Redis-backed Storage: Subscription-scoped feature flags with optional expiration support using existing IRedisCacheFactory
• Type-safe Access: Strongly-typed IFeatureFlagClient with compile-time validation and per-request caching
• Comprehensive Validation: All flag operations validate against the predefined registry
• Simplified Model: Streamlined FeatureFlagValue with only essential fields (SubscriptionId, FlagName, Value, CreatedAt)

REST API Endpoints

Added FeatureFlagsController in the v2020_02_20 API with full CRUD operations:

• POST /api/feature-flags - Set feature flags with optional expiry
• GET /api/feature-flags/{subscriptionId} - Get all flags for a subscription
• GET /api/feature-flags/{subscriptionId}/{flagName} - Get specific flag
• DELETE /api/feature-flags/{subscriptionId}/{flagName} - Remove flag
• GET /api/feature-flags - Admin endpoint to list all flags
• GET /api/feature-flags/available - List available flags with metadata
• GET /api/feature-flags/by-flag/{flagName} - List all subscriptions with a specific flag
• DELETE /api/feature-flags/by-flag/{flagName} - Remove flag from all subscriptions (admin)

CLI Integration

Integrated 7 new commands into the existing PCS CLI tool using the ProductConstructionServiceApi client:

# Set the rebase strategy flag with optional expiry
pcs feature-flag-set --subscription-id "guid" --flag "enable-rebase-strategy" --value "true" --expiry-days 30

# Get flags for a subscription
pcs feature-flag-get --subscription-id "guid" [--flag "specific-flag"]

# Remove a flag
pcs feature-flag-remove --subscription-id "guid" --flag "flag-name"

# List flags (admin operation)
pcs feature-flag-list [--subscription-id "guid"]

# Show available flags with descriptions
pcs feature-flag-available

# List subscriptions that have a specific flag
pcs feature-flag-list-by-flag --flag "enable-rebase-strategy"

# Remove flag from all subscriptions (admin)
pcs feature-flag-remove-from-all --flag "enable-rebase-strategy"

🏗️ Technical Implementation

Simplified Feature Flag Model

• Streamlined Structure: FeatureFlagValue(SubscriptionId, FlagName, Value, CreatedAt)
• Removed Complexity: Eliminated Expiry and UpdatedAt fields from the model
• Redis-only Expiration: Expiration handled entirely by Redis TTL, no manual validation needed
• Default Behavior: Feature flags never expire by default (Redis TTL = null)
• User Control: Optional ExpiryDays parameter in API allows custom expiration when needed

Bulk Operations

Added two new service methods for administrative operations:

• GetSubscriptionsWithFlagAsync() - Find all subscriptions with a specific flag using Redis pattern FeatureFlags_*:{flagName}
• RemoveFlagFromAllSubscriptionsAsync() - Remove a flag from all subscriptions and return count of removed flags

Redis Key Structure

Uses consistent key pattern: FeatureFlags_{subscriptionId}:{flagName} with automatic expiration cleanup handled by Redis TTL.

CLI Architecture

The CLI commands use the generated ProductConstructionServiceApi client and follow established patterns:

• All feature flag operations inherit from PcsStatusOptions for consistent authentication
• Commands use standard PCS authentication options (--isCi, --pcsUri) instead of direct Redis access
• Operations use _client.FeatureFlags.* methods to interact with the API
• Consistent Logging: All CLI operations use ILogger instead of Console.WriteLine following PCS CLI patterns

Service Integration

The IFeatureFlagClient is designed for easy integration into existing PCS components:

public async Task ProcessSubscription(Guid subscriptionId)
{
    await _featureFlagClient.InitializeAsync(subscriptionId);
    
    var useRebaseStrategy = await _featureFlagClient.GetBooleanFlagAsync(
        FeatureFlags.EnableRebaseStrategy, defaultValue: false);
    
    if (useRebaseStrategy)
    {
        // Use rebase strategy for codeflow conflicts
    }
    else
    {
        // Use standard merge strategy
    }
}

Available Feature Flag

• enable-rebase-strategy - Enables rebase strategy for codeflow conflict resolution instead of merge

🧪 Testing

Added comprehensive unit tests (17 tests) covering:

• API controller endpoints with mocked dependencies
• Service layer functionality with Redis integration
• CLI command integration and error handling
• Feature flag validation and type safety
• Bulk operations for administrative use cases

🔄 Integration with Existing Systems

• Leverages existing Redis infrastructure (IRedisCacheFactory)
• CLI follows established patterns with the ProductConstructionServiceApi client and Azure authentication
• Uses existing dependency injection and logging patterns
• Maintains API versioning consistency
• Simplified internal implementation with Redis handling all expiration logic
• Improved Error Handling: Proper exception propagation in bulk operations with detailed logging

📝 Recent Updates

• Removed Success Field: Eliminated redundant Success boolean from RemoveFlagFromAllResponse - HTTP status codes indicate success/failure
• Better Error Handling: RemoveFlagFromAllSubscriptionsAsync now properly propagates exceptions instead of silently continuing
• Consistent CLI Logging: All feature flag CLI operations now use ILogger following established PCS patterns
• Improved Documentation: Updated API documentation to be more precise about return values

This implementation provides a robust foundation for enabling rebase strategy in codeflow conflict resolution on a per-subscription basis, with administrative capabilities for bulk flag management across all subscriptions. The streamlined model reduces complexity while maintaining full functionality.

Fixes #5261.

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.