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Circuit Breaker

A circuit breaker is a fault tolerance pattern borrowed from electrical engineering. When a downstream service fails repeatedly, the circuit breaker “opens” and blocks requests for a cooldown period. This prevents your application from wasting resources on doomed calls and shields the struggling service from further overload.

How It Works

A circuit breaker has three states:

  1. Closed (normal): Requests pass through. Failures are counted.
  2. Open (tripped): Requests are immediately rejected without calling the service. A timer starts.
  3. Half-open (testing): After the cooldown, one request is allowed through. If it succeeds, the circuit closes. If it fails, the circuit reopens.
Closed ──(failure threshold reached)──→ Open
  ↑                                       │
  │                              (cooldown expires)
  │                                       ↓
  └───────(test request succeeds)─── Half-Open

Example

class CircuitBreaker {
  constructor({ threshold = 5, cooldown = 30000 }) {
    this.failures = 0;
    this.threshold = threshold;
    this.cooldown = cooldown;
    this.state = 'closed';
    this.nextAttempt = 0;
  }


  async call(fn) {
    if (this.state === 'open') {
      if (Date.now() < this.nextAttempt) {
        throw new Error('Circuit breaker is open');
      }
      this.state = 'half-open';
    }


    try {
      const result = await fn();
      this.failures = 0;
      this.state = 'closed';
      return result;
    } catch (error) {
      this.failures++;
      if (this.failures >= this.threshold) {
        this.state = 'open';
        this.nextAttempt = Date.now() + this.cooldown;
      }
      throw error;
    }
  }
}


const breaker = new CircuitBreaker({ threshold: 3, cooldown: 10000 });
const data = await breaker.call(() => fetch('https://api.example.com/data'));

When to Use Circuit Breakers

  • Calling external APIs that may become unavailable
  • Connecting to databases or caches that can go down
  • Any dependency where repeated failures would cascade to callers

Circuit Breakers vs. Retries

Retries re-attempt a single failed request. Circuit breakers block all requests to a service after a pattern of failure. They work best together: retry individual failures, and open the circuit when retries fail consistently.