How do you implement a custom circuit breaker with metrics?

Question

Accepted Answer

**Circuit breaker with metrics** — track failure rates and latency.

```javascript
class CircuitBreaker {
    constructor(options) {
        this.state = "CLOSED";
        this.failureCount = 0;
        this.successCount = 0;
        this.totalRequests = 0;
        this.failureThreshold = options.failureThreshold || 5;
        this.resetTimeout = options.resetTimeout || 30000;
        this.nextAttempt = 0;
        this.latencies = [];
    }

async call(fn) {
        if (this.state === "OPEN") {
            if (Date.now() < this.nextAttempt) {
                throw new Error("Circuit breaker is OPEN");
            }
            this.state = "HALF_OPEN";
        }

const start = Date.now();
        this.totalRequests++;

try {
            const result = await fn();
            const latency = Date.now() - start;
            this.latencies.push(latency);
            if (this.latencies.length > 100) this.latencies.shift();
            this.onSuccess();
            return result;
        } catch (err) {
            this.onFailure();
            throw err;
        }
    }

onSuccess() {
        this.successCount++;
        this.failureCount = 0;
        this.state = "CLOSED";
    }

onFailure() {
        this.failureCount++;
        if (this.failureCount >= this.failureThreshold) {
            this.state = "OPEN";
            this.nextAttempt = Date.now() + this.resetTimeout;
        }
    }

getMetrics() {
        const sorted = [...this.latencies].sort((a, b) => a - b);
        return {
            state: this.state,
            totalRequests: this.totalRequests,
            successCount: this.successCount,
            failureCount: this.failureCount,
            avgLatency: this.latencies.length
                ? this.latencies.reduce((a, b) => a + b, 0) / this.latencies.length
                : 0,
            p95Latency: sorted[Math.floor(sorted.length * 0.95)] || 0,
        };
    }
}

// Usage
const breaker = new CircuitBreaker({ failureThreshold: 5 });
const result = await breaker.call(() => fetch("https://api.example.com"));
console.log(breaker.getMetrics());
```

Node.js: How do you implement a custom circuit breaker with metrics?