Circuit Breaker Pattern

Prevent cascade failures by failing fast when a service is unhealthy.

When to Use This Skill

• Adding resilience to external API calls
• Protecting against slow or failing downstream services
• Implementing graceful degradation
• Building fault-tolerant microservices

Core Concepts

A circuit breaker has three states:

1. CLOSED: Normal operation, requests pass through
2. OPEN: Service is failing, requests fail immediately
3. HALF_OPEN: Testing if service recovered

TypeScript Implementation

// circuit-breaker.ts
type CircuitState = 'CLOSED' | 'OPEN' | 'HALF_OPEN';

interface CircuitBreakerConfig {
  failureThreshold: number;      // Failures before opening
  successThreshold: number;      // Successes to close from half-open
  timeout: number;               // Ms before trying half-open
  fallback?: () => Promise<any>; // Optional fallback
}

class CircuitBreaker {
  private state: CircuitState = 'CLOSED';
  private failures = 0;
  private successes = 0;
  private lastFailureTime = 0;
  private readonly config: CircuitBreakerConfig;

  constructor(config: Partial<CircuitBreakerConfig> = {}) {
    this.config = {
      failureThreshold: 5,
      successThreshold: 2,
      timeout: 30000,
      ...config,
    };
  }

  async execute<T>(fn: () => Promise<T>): Promise<T> {
    if (this.state === 'OPEN') {
      if (Date.now() - this.lastFailureTime >= this.config.timeout) {
        this.state = 'HALF_OPEN';
      } else {
        if (this.config.fallback) {
          return this.config.fallback();
        }
        throw new CircuitOpenError('Circuit is OPEN');
      }
    }

    try {
      const result = await fn();
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure();
      throw error;
    }
  }

  private onSuccess(): void {
    this.failures = 0;
    if (this.state === 'HALF_OPEN') {
      this.successes++;
      if (this.successes >= this.config.successThreshold) {
        this.state = 'CLOSED';
        this.successes = 0;
      }
    }
  }

  private onFailure(): void {
    this.failures++;
    this.lastFailureTime = Date.now();
    if (this.failures >= this.config.failureThreshold) {
      this.state = 'OPEN';
    }
  }

  getState(): CircuitState {
    return this.state;
  }
}

class CircuitOpenError extends Error {
  constructor(message: string) {
    super(message);
    this.name = 'CircuitOpenError';
  }
}

export { CircuitBreaker, CircuitBreakerConfig, CircuitOpenError };

Python Implementation

# circuit_breaker.py
import time
from enum import Enum
from typing import Callable, TypeVar, Optional
from functools import wraps

T = TypeVar('T')

class CircuitState(Enum):
    CLOSED = "closed"
    OPEN = "open"
    HALF_OPEN = "half_open"

class CircuitOpenError(Exception):
    pass

class CircuitBreaker:
    def __init__(
        self,
        failure_threshold: int = 5,
        success_threshold: int = 2,
        timeout: float = 30.0,
        fallback: Optional[Callable] = None,
    ):
        self.failure_threshold = failure_threshold
        self.success_threshold = success_threshold
        self.timeout = timeout
        self.fallback = fallback
        
        self._state = CircuitState.CLOSED
        self._failures = 0
        self._successes = 0
        self._last_failure_time = 0.0

    @property
    def state(self) -> CircuitState:
        return self._state

    def __call__(self, fn: Callable[..., T]) -> Callable[..., T]:
        @wraps(fn)
        def wrapper(*args, **kwargs) -> T:
            return self.execute(lambda: fn(*args, **kwargs))
        return wrapper

    def execute(self, fn: Callable[[], T]) -> T:
        if self._state == CircuitState.OPEN:
            if time.time() - self._last_failure_time >= self.timeout:
                self._state = CircuitState.HALF_OPEN
            else:
                if self.fallback:
                    return self.fallback()
                raise CircuitOpenError("Circuit is OPEN")

        try:
            result = fn()
            self._on_success()
            return result
        except Exception as e:
            self._on_failure()
            raise

    def _on_success(self) -> None:
        self._failures = 0
        if self._state == CircuitState.HALF_OPEN:
            self._successes += 1
            if self._successes >= self.success_threshold:
                self._state = CircuitState.CLOSED
                self._successes = 0

    def _on_failure(self) -> None:
        self._failures += 1
        self._last_failure_time = time.time()
        if self._failures >= self.failure_threshold:
            self._state = CircuitState.OPEN

Usage Examples

TypeScript with API Client

const paymentCircuit = new CircuitBreaker({
  failureThreshold: 3,
  timeout: 60000,
  fallback: async () => ({ status: 'pending', message: 'Payment service unavailable' }),
});

async function processPayment(amount: number) {
  return paymentCircuit.execute(async () => {
    const response = await fetch('https://api.stripe.com/v1/charges', {
      method: 'POST',
      body: JSON.stringify({ amount }),
    });
    if (!response.ok) throw new Error('Payment failed');
    return response.json();
  });
}

Python Decorator Style

circuit = CircuitBreaker(failure_threshold=3, timeout=60)

@circuit
def call_external_api(endpoint: str) -> dict:
    response = requests.get(endpoint, timeout=5)
    response.raise_for_status()
    return response.json()

Integration with Observability

Add metrics to track circuit state:

// With Prometheus
import { Gauge, Counter } from 'prom-client';

const circuitState = new Gauge({
  name: 'circuit_breaker_state',
  help: 'Current circuit breaker state (0=closed, 1=open, 2=half-open)',
  labelNames: ['service'],
});

const circuitTrips = new Counter({
  name: 'circuit_breaker_trips_total',
  help: 'Total circuit breaker trips',
  labelNames: ['service'],
});

Best Practices

1. Tune thresholds per service: Critical services need lower thresholds
2. Always provide fallbacks: Graceful degradation > hard failures
3. Monitor circuit state: Alert when circuits open frequently
4. Use separate circuits: One per external dependency
5. Consider bulkheads: Combine with connection pooling

Common Mistakes

• Setting timeout too short (service never recovers)
• Sharing circuits across unrelated services
• Not handling CircuitOpenError in calling code
• Forgetting to add observability

Related Patterns

• Retry with exponential backoff
• Bulkhead isolation
• Timeout pattern
• Fallback pattern