System Design Essentials

				
					/**
 * A robust Circuit Breaker implementation for an API Gateway.
 * It tracks failures and prevents system-wide hanging when downstream services fail.
 */
class CircuitBreaker {
  constructor(requestFunc, { threshold = 3, timeout = 30000 }) {
    this.requestFunc = requestFunc; // The API call to wrap
    this.threshold = threshold;    // Max failures before opening circuit
    this.timeout = timeout;        // Time to wait before retrying (ms)
    
    this.failures = 0;
    this.state = 'CLOSED';
    this.nextAttempt = Date.now();
  }

  async execute(...args) {
    if (this.state === 'OPEN') {
      if (Date.now() > this.nextAttempt) {
        this.state = 'HALF_OPEN';
      } else {
        throw new Error("Service unavailable: Circuit is currently OPEN.");
      }
    }

    try {
      const result = await this.requestFunc(...args);
      this.reset();
      return result;
    } catch (error) {
      this.recordFailure();
      throw error;
    }
  }

  recordFailure() {
    this.failures++;
    if (this.failures >= this.threshold || this.state === 'HALF_OPEN') {
      this.state = 'OPEN';
      this.nextAttempt = Date.now() + this.timeout;
    }
  }

  reset() {
    this.failures = 0;
    this.state = 'CLOSED';
  }
}