Circuit Breaker Pattern

type State = 'CLOSED' | 'OPEN' | 'HALF_OPEN';

class CircuitBreaker {
  private state: State = 'CLOSED';
  private failureCount = 0;
  private lastFailureTime = 0;

  constructor(
    private readonly threshold: number = 5,
    private readonly resetTimeout: number = 30_000
  ) {}

  async call<T>(fn: () => Promise<T>): Promise<T> {
    if (this.state === 'OPEN') {
      if (Date.now() - this.lastFailureTime > this.resetTimeout) {
        this.state = 'HALF_OPEN';
      } else {
        throw new Error('Circuit breaker is OPEN');
      }
    }
    try {
      const result = await fn();
      this.onSuccess();
      return result;
    } catch (err) {
      this.onFailure();
      throw err;
    }
  }

  private onSuccess() {
    this.failureCount = 0;
    this.state = 'CLOSED';
  }

  private onFailure() {
    this.failureCount++;
    this.lastFailureTime = Date.now();
    if (this.failureCount >= this.threshold) {
      this.state = 'OPEN';
    }
  }
}

export { CircuitBreaker };