Circuit Breaker

A Dart implementation of the Circuit Breaker pattern for building resilient applications that can handle failures gracefully.

Overview

The Circuit Breaker pattern is a design pattern used in software development to detect failures and encapsulate the logic of preventing a failure from constantly recurring during maintenance, temporary external system failure, or unexpected system difficulties.

Features

• Three States: Closed, Open, and Half-Open states with automatic transitions
• Configurable Thresholds: Set failure thresholds and recovery requirements
• Generic Type Support: Works with any return type T
• Comprehensive Error Handling: Custom exceptions and proper error propagation
• Thread-Safe: Safe for concurrent usage
• Well Tested: Comprehensive test coverage including edge cases

States

Closed State

• Normal Operation: All calls pass through to the wrapped operation
• Failure Tracking: Counts consecutive failures
• Transition: Opens when failure count reaches the threshold

Open State

• Fail Fast: All calls are immediately rejected with CircuitOpenException
• No Execution: The wrapped operation is not executed
• Transition: Automatically transitions to Half-Open after the timeout period

Half-Open State

• Limited Testing: Allows calls through to test if the service has recovered
• Success Tracking: Counts successful calls
• Transitions:
  • To Closed: After enough successful calls (halfOpenThreshold)
  • To Open: Immediately on any failure

Installation

Add this to your package's pubspec.yaml file:

dependencies:
  dart_circuit_breaker: ^0.0.1

Usage

Basic Usage

import 'package:dart_circuit_breaker/dart_circuit_breaker.dart';

// Create a circuit breaker
final circuitBreaker = CircuitBreaker(
  threshold: 3,                              // Open after 3 failures
  timeout: const Duration(seconds: 30),     // Stay open for 30 seconds
  halfOpenThreshold: 2,                     // Close after 2 successes in half-open
);

// Use the circuit breaker
try {
  final result = await circuitBreaker.execute(() async {
    // Your potentially failing operation here
    return await httpClient.get('https://api.example.com/data');
  });
  print('Success: $result');
} on CircuitOpenException catch (e) {
  print('Circuit is open: ${e.message}');
  // Handle the circuit being open (maybe use cached data)
} catch (e) {
  print('Operation failed: $e');
  // Handle other errors
}

Advanced Usage

// HTTP Client with Circuit Breaker
class ResilientHttpClient {
  final CircuitBreaker _circuitBreaker;
  final HttpClient _httpClient;

  ResilientHttpClient({
    int threshold = 5,
    Duration timeout = const Duration(minutes: 1),
    int halfOpenThreshold = 3,
  }) : _circuitBreaker = CircuitBreaker(
         threshold: threshold,
         timeout: timeout,
         halfOpenThreshold: halfOpenThreshold,
       ),
       _httpClient = HttpClient();

  Future<String> get(String url) async {
    return await _circuitBreaker.execute(() async {
      final request = await _httpClient.getUrl(Uri.parse(url));
      final response = await request.close();
      
      if (response.statusCode != 200) {
        throw HttpException('HTTP ${response.statusCode}');
      }
      
      return await response.transform(utf8.decoder).join();
    });
  }

  void dispose() {
    _httpClient.close();
  }
}

Database Operations

class DatabaseService {
  final CircuitBreaker _circuitBreaker = CircuitBreaker(
    threshold: 3,
    timeout: const Duration(seconds: 10),
    halfOpenThreshold: 2,
  );

  Future<List<User>> getUsers() async {
    return await _circuitBreaker.execute(() async {
      // Simulate database operation
      final connection = await Database.connect();
      try {
        return await connection.query('SELECT * FROM users');
      } finally {
        await connection.close();
      }
    });
  }
}

Configuration

Constructor Parameters

• threshold (required): Number of consecutive failures before opening the circuit
• timeout (required): Duration to keep the circuit open before transitioning to half-open
• halfOpenThreshold (required): Number of successful calls in half-open state needed to close the circuit

State Monitoring

print('Current state: ${circuitBreaker.state}');
print('Failure count: ${circuitBreaker.failureCount}');
print('Success count: ${circuitBreaker.successCount}');
print('Circuit breaker info: $circuitBreaker');

Manual Reset

// Manually reset the circuit breaker to closed state
circuitBreaker.reset();

Error Handling

The circuit breaker provides specific error handling:

try {
  final result = await circuitBreaker.execute(yourOperation);
  // Handle success
} on CircuitOpenException catch (e) {
  // Circuit is open - implement fallback logic
  // e.g., return cached data, show cached content, etc.
} on TimeoutException catch (e) {
  // Operation timed out
} on NetworkException catch (e) {
  // Network-related errors
} catch (e) {
  // Other errors from your operation
}

Best Practices

1. Choose Appropriate Thresholds: Set thresholds based on your service's characteristics and requirements
2. Implement Fallbacks: Always have a fallback strategy when the circuit is open
3. Monitor Circuit State: Log state transitions for debugging and monitoring
4. Use Different Circuits: Use separate circuit breakers for different external services
5. Consider Timeout Values: Set timeout values that allow sufficient time for service recovery

Testing

The library includes comprehensive tests. Run them with:

dart test

Example

Run the included example:

dart run example/circuit_breaker_example.dart

This example demonstrates:

• Normal operation in closed state
• Circuit opening due to failures
• Rejection of calls in open state
• Recovery process through half-open state
• Transition back to closed state

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

This project is licensed under the MIT License - see the LICENSE file for details.