Circuit Breaker

Circuit Breaker

When a downstream service is failing, continuously retrying just makes things worse — it wastes resources and prevents recovery. The Circuit Breaker pattern stops sending requests to a failing service and lets it recover.

The Three States

  success     too many failures    timeout elapsed
CLOSED  ──────────────────────►  OPEN  ──────────►  HALF-OPEN
  ◄─────────────────────────────────────────────────
               success                    │  failure
                                          ▼
                                         OPEN

• CLOSED — normal operation. Requests pass through. Track failures.
• OPEN — service is down. Reject all requests immediately. Start a timeout.
• HALF-OPEN — tentatively try one request. If it succeeds, go CLOSED. If it fails, go back OPEN.

class CircuitBreaker {
  constructor(threshold, timeout) {
    this.threshold = threshold;  // failures before opening
    this.timeout = timeout;      // ms before trying again
    this.failures = 0;
    this.state = "CLOSED";
    this.openedAt = null;
  }

  call(fn) {
    if (this.state === "OPEN") {
      if (Date.now() - this.openedAt >= this.timeout) {
        this.state = "HALF-OPEN";
      } else {
        throw new Error("Circuit OPEN");
      }
    }
    try {
      const result = fn();
      this.onSuccess();
      return result;
    } catch (err) {
      this.onFailure();
      throw err;
    }
  }

  onSuccess() {
    this.failures = 0;
    this.state = "CLOSED";
  }

  onFailure() {
    this.failures++;
    if (this.failures >= this.threshold) {
      this.state = "OPEN";
      this.openedAt = Date.now();
    }
  }
}

Used By

• Netflix Hystrix — the original circuit breaker library for microservices.
• Resilience4j — modern Java circuit breaker.
• AWS SDK — built-in retries with exponential backoff and jitter.
• Envoy/Istio — service mesh with circuit breaking at the proxy level.

Your Task

Implement a CircuitBreaker class. For testing purposes, use a callsAttempted counter instead of real time — the circuit opens after threshold failures and allows a retry after timeout additional calls.