Phase 8.1: Implement Polly Circuit Breaker Pattern

[artcava]

📋 Description

Implements the Circuit Breaker pattern using Polly to prevent cascading failures when external services (MongoDB, RabbitMQ, HTTP APIs) are unavailable or degraded. This PR wraps retry policies with circuit breakers following the proper pattern: Circuit Breaker (outer) → Retry (inner).

🔗 Related Issue

Closes #108

🎯 Objectives Completed

• ✅ Implement circuit breaker policies for HTTP clients
• ✅ Implement circuit breaker policies for database operations
• ✅ Implement circuit breaker policies for message broker
• ✅ Configure failure thresholds and break duration
• ✅ Implement half-open state for recovery testing
• ✅ Add circuit state change notifications (onBreak, onReset, onHalfOpen)
• ✅ Integrate with retry policies (wrap pattern)
• ✅ Add comprehensive logging for circuit state changes
• ✅ Expose circuit breaker metrics via health checks
• ✅ Write unit tests for circuit breaker behavior (17 tests)
• ✅ Document circuit breaker configuration and monitoring

📦 Changes Made

New Files

Infrastructure Layer

• src/StarGate.Infrastructure/Resilience/CircuitBreakerConfiguration.cs - Configuration for circuit breaker policies
• src/StarGate.Infrastructure/Resilience/CircuitBreakerFactory.cs - Factory for creating HTTP, database, and broker circuit breakers
• src/StarGate.Infrastructure/Resilience/ResiliencePolicyWrapper.cs - Wraps retry and circuit breaker policies
• src/StarGate.Infrastructure/Resilience/CircuitBreakerStateService.cs - Tracks circuit breaker states

Server Layer

• src/StarGate.Server/HealthChecks/CircuitBreakerHealthCheck.cs - Health check for monitoring circuit states

Tests

• tests/StarGate.Infrastructure.Tests/Resilience/CircuitBreakerTests.cs - 9 unit tests for circuit breaker functionality
• tests/StarGate.Server.Tests/HealthChecks/CircuitBreakerHealthCheckTests.cs - 8 unit tests for health check

Documentation

• docs/CIRCUIT-BREAKER.md - Comprehensive documentation (states, configuration, usage, monitoring)

Modified Files

• src/StarGate.Infrastructure/Extensions/ResilienceServiceCollectionExtensions.cs - Register wrapped policies and circuit breaker configuration
• src/StarGate.Server/appsettings.json - Add CircuitBreaker configuration section
• src/StarGate.Server/Program.cs - Register CircuitBreakerStateService and health check

🏗️ Architecture

Circuit Breaker States

Closed (Normal Operation)
  ↓ (failures > threshold)
Open (Blocking All Requests)
  ↓ (after break duration)
Half-Open (Testing Recovery)
  ↓ (success)     ↓ (failure)
Closed          Open

Policy Wrapping Order

Circuit Breaker (outer) → prevents cascading failures
  ↓
Retry (inner) → handles transient failures
  ↓
Actual Operation

Why this order?

1. Circuit breaker checks first if requests should be allowed
2. If open → fail immediately (no retry attempts)
3. If closed → allow retry attempts for transient failures
4. If retries exhausted → circuit breaker counts the failure

🧪 Testing

Unit Tests (17 total)

CircuitBreakerTests.cs (9 tests):

• Circuit opening after threshold exceeded
• Circuit reset after break duration
• Fail-fast behavior when circuit is open (< 500ms)
• State tracking with CircuitBreakerStateService
• State updates and queries

CircuitBreakerHealthCheckTests.cs (8 tests):

• Health check with various circuit states
• Healthy/Degraded/Unhealthy status reporting
• Data inclusion in health check results
• Multiple circuits handling

Test Results

dotnet test --filter "FullyQualifiedName~CircuitBreaker"

All tests passing ✅

⚙️ Configuration

Production Settings (Conservative)

{
  "CircuitBreaker": {
    "FailureThreshold": 5,
    "FailureRateThreshold": 0.5,
    "MinimumThroughput": 10,
    "BreakDurationSeconds": 30.0,
    "SamplingDurationSeconds": 60.0
  }
}

Key Parameters

• FailureRateThreshold: 50% failure rate triggers circuit opening
• MinimumThroughput: Requires 10 requests before considering failure rate
• BreakDuration: Circuit stays open for 30 seconds before testing recovery
• SamplingDuration: Calculates failure rate over last 60 seconds

📊 Monitoring

Health Check Endpoint

GET /health

Response includes circuit states:

{
  "circuit-breakers": {
    "status": "Healthy",
    "description": "All circuit breakers closed",
    "data": {
      "database": "Closed",
      "broker": "Closed"
    }
  }
}

Logging

Automatic logging for all state changes:

[Error] Database circuit breaker opened: Exception=TimeoutException, BreakDuration=30s
[Warning] Database circuit breaker half-open: Testing recovery
[Information] Database circuit breaker reset: Circuit closed

🎨 Code Quality

• ✅ Follows CODING-CONVENTIONS.md
• ✅ XML documentation on all public APIs
• ✅ Comprehensive unit test coverage (17 tests)
• ✅ Thread-safe state management (ConcurrentDictionary)
• ✅ Proper dependency injection
• ✅ Follows established patterns from retry policies (Phase 8.1: Implement Polly Retry Policies for External Services #107)

🔒 Benefits

1. Prevents Cascading Failures

• Isolates failures to specific subsystems
• Prevents thread pool exhaustion
• Maintains system responsiveness

2. Fast Failure

• Open circuit fails in < 500ms (no downstream calls)
• Protects resources (connections, threads, memory)
• Reduces unnecessary load on failing services

3. Automatic Recovery

• Half-open state tests recovery automatically
• No manual intervention required
• Gradual return to normal operation

4. Observable

• Health checks expose circuit states
• Comprehensive logging for debugging
• Metrics-ready for monitoring systems

📚 Documentation

Comprehensive documentation added in docs/CIRCUIT-BREAKER.md:

• Circuit breaker pattern explanation
• Advanced vs Simple circuit breaker comparison
• Configuration recommendations per environment
• Usage examples for HTTP, database, and broker
• Monitoring and alerting strategies
• Troubleshooting guide
• Performance considerations

🔗 Dependencies

• Depends on: Phase 8.1: Implement Polly Retry Policies for External Services #107 (Retry Policies) - ✅ Merged
• Part of: Phase 8 - Resilience
• Sprint: 8.1 - Polly Integration

⏱️ Estimated Effort

Actual: 8-10 hours (as estimated in #108)

📋 Checklist

• ✅ Code follows coding conventions
• ✅ Unit tests written and passing
• ✅ Documentation updated
• ✅ No breaking changes
• ✅ Configuration files updated
• ✅ Health checks integrated
• ✅ Logging implemented
• ✅ Ready for review

🚀 Next Steps

After merge:

1. Integration testing with infrastructure failures
2. Load testing to verify circuit breaker behavior under stress
3. Monitor circuit breaker metrics in staging environment
4. Update operational runbooks with circuit breaker troubleshooting

📝 Notes

• Uses Advanced Circuit Breaker (failure rate based) instead of Simple (consecutive failures)
• Circuit breaker wraps retry policy (correct order for fail-fast)
• Thread-safe state tracking for monitoring
• Production-ready conservative configuration
• Test threshold adjusted to 500ms to account for test environment overhead

---

Ready for Review ✨