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Prometheus Metrics for Your Node.js Circuit Breakers (opossum-prom)

#prometheus #monitoring #node #devops

Prometheus Metrics for Your Node.js Circuit Breakers (opossum-prom)

You've added opossum circuit breakers to protect your Node.js app from cascading failures. Good. But do you know when they're open? How often they're failing? What your 95th percentile fallback latency is?

Without metrics, a circuit breaker is a black box. It's protecting you, but you can't see what's happening. Production debugging becomes guesswork.

Today I'm releasing opossum-prom — zero-boilerplate Prometheus metrics for opossum circuit breakers. One line to add. Everything you need to know, surfaced to Grafana.

The Problem With Unobserved Circuit Breakers

Here's a typical production scenario: your payment service is intermittently failing. Orders are going through but your error rate is climbing. Is the circuit breaker helping? Is it stuck open? Is it in half-open state and rejecting valid requests?

Without Prometheus metrics, you're flying blind. With them, you can write alerts like:

# Fire a warning if any circuit breaker has been open for > 1 minute
circuit_breaker_state == 1
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And you'll know exactly which service is failing, when it tripped, and how long it's been open.

Install 

npm install opossum-prom opossum prom-client
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Usage: One Line 

const CircuitBreaker = require('opossum');
const { instrument } = require('opossum-prom');

const breaker = new CircuitBreaker(callPaymentAPI, {
  timeout: 3000,
  errorThresholdPercentage: 50,
  resetTimeout: 30000,
});

// This is the only line you need to add
instrument(breaker, { name: 'payment_service' });

// Your /metrics endpoint now includes circuit breaker metrics automatically
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That's it. Your existing prom-client metrics endpoint will now include circuit breaker data alongside your other Node.js metrics.

What Gets Measured

opossum-prom registers six metrics per circuit breaker:

Metric Type Labels What It Tells You
circuit_breaker_state Gauge name 0=closed, 1=open, 2=half-open
circuit_breaker_requests_total Counter name, result All calls, by outcome
circuit_breaker_failures_total Counter name Function threw or rejected
circuit_breaker_fallbacks_total Counter name Times fallback was called
circuit_breaker_timeouts_total Counter name Calls that timed out
circuit_breaker_duration_seconds Histogram name Execution latency

The result label on circuit_breaker_requests_total has five values: success, failure, reject, timeout, fallback. This single counter gives you a complete picture of what's happening at the breaker.

Full Express + prom-client Setup

Here's a complete production setup:

const express = require('express');
const CircuitBreaker = require('opossum');
const client = require('prom-client');
const { instrument } = require('opossum-prom');

const app = express();

// Collect default Node.js metrics (CPU, memory, event loop lag)
client.collectDefaultMetrics();

// Create your circuit breakers
const dbBreaker = new CircuitBreaker(queryDatabase, {
  timeout: 5000,
  errorThresholdPercentage: 50,
  resetTimeout: 30000,
  name: 'database', // opossum name (optional, we set our own label)
});

const stripeBreaker = new CircuitBreaker(callStripe, {
  timeout: 3000,
  errorThresholdPercentage: 30, // Payments: lower tolerance
  resetTimeout: 60000,
});

const emailBreaker = new CircuitBreaker(sendEmail, {
  timeout: 10000,
  errorThresholdPercentage: 50,
  resetTimeout: 30000,
});

// Instrument all three — each gets its own `name` label
instrument(dbBreaker,     { name: 'database' });
instrument(stripeBreaker, { name: 'stripe' });
instrument(emailBreaker,  { name: 'email' });

// Optional: add fallbacks AFTER instrumenting (fallback events are still tracked)
stripeBreaker.fallback(() => ({ queued: true }));

// Prometheus metrics endpoint
app.get('/metrics', async (req, res) => {
  res.set('Content-Type', client.register.contentType);
  res.end(await client.register.metrics());
});

app.listen(3000, () => console.log('Server up. Metrics at /metrics'));
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Multiple Breakers: instrumentAll

If you have many breakers, use instrumentAll to set them all up at once:

const { instrumentAll } = require('opossum-prom');

instrumentAll([
  { breaker: dbBreaker,     name: 'database' },
  { breaker: stripeBreaker, name: 'stripe' },
  { breaker: emailBreaker,  name: 'email' },
  { breaker: cacheBreaker,  name: 'redis' },
  { breaker: searchBreaker, name: 'elasticsearch' },
]);

// Returns a handle with .deregister() to clean up all of them at once
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Private Registry (Microservices)

If you're running multiple services in the same process, or want to isolate circuit breaker metrics:

const client = require('prom-client');
const { instrumentAll } = require('opossum-prom');

const circuitRegistry = new client.Registry();

instrumentAll([
  { breaker: authBreaker,    name: 'auth' },
  { breaker: billingBreaker, name: 'billing' },
], { registry: circuitRegistry });

// Expose only circuit breaker metrics on a separate port
app.get('/circuit-metrics', async (req, res) => {
  res.set('Content-Type', circuitRegistry.contentType);
  res.end(await circuitRegistry.metrics());
});
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Grafana PromQL Queries

Once your metrics are flowing, here are the queries you'll actually use:

Is any circuit breaker open right now? 

circuit_breaker_state == 1
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Create a Grafana panel with a threshold: green when all zeros, red when any value is 1. This is your circuit breaker dashboard hero metric.

Request rate by outcome 

sum by (name, result) (
  rate(circuit_breaker_requests_total[5m])
)
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Shows you success vs failure vs reject vs timeout per service, over the last 5 minutes.

Failure rate percentage 

rate(circuit_breaker_failures_total[5m])
  / rate(circuit_breaker_requests_total[5m])
  * 100
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Alert on this when it crosses 10% for more than 2 minutes.

95th percentile latency 

histogram_quantile(
  0.95,
  sum by (name, le) (
    rate(circuit_breaker_duration_seconds_bucket[5m])
  )
)
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Spot performance degradation before the circuit trips.

Fallback rate (dependency health proxy) 

rate(circuit_breaker_fallbacks_total[5m])
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Rising fallback rate = your dependency is degrading. This is an early warning before failures spike.

Alert Rules

Copy these into your Prometheus alerting config:

groups:
  - name: circuit_breakers
    rules:
      - alert: CircuitBreakerOpen
        expr: circuit_breaker_state == 1
        for: 1m
        labels:
          severity: warning
        annotations:
          summary: "Circuit breaker {{ $labels.name }} is OPEN"
          description: "{{ $labels.name }} has been in OPEN state for > 1 minute. Requests are being rejected."

      - alert: CircuitBreakerHighFailureRate
        expr: |
          rate(circuit_breaker_failures_total[5m])
          / rate(circuit_breaker_requests_total[5m]) > 0.10
        for: 2m
        labels:
          severity: critical
        annotations:
          summary: "Circuit breaker {{ $labels.name }} failure rate > 10%"
          description: "{{ $labels.name }} has a {{ $value | humanizePercentage }} failure rate."

      - alert: CircuitBreakerHighLatency
        expr: |
          histogram_quantile(0.95,
            sum by (name, le) (
              rate(circuit_breaker_duration_seconds_bucket[5m])
            )
          ) > 2
        for: 3m
        labels:
          severity: warning
        annotations:
          summary: "Circuit breaker {{ $labels.name }} p95 latency > 2s"
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Clean Shutdown

When you shut down gracefully, clean up circuit breaker listeners to prevent memory leaks:

const handle = instrument(breaker, { name: 'database' });

// In your shutdown handler
process.on('SIGTERM', async () => {
  handle.deregister(); // Removes listeners + unregisters metrics from prom-client
  await server.close();
  process.exit(0);
});
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deregister() is smart: it only unregisters the Prometheus metrics when the last circuit breaker on that registry deregisters. If you have five breakers on the same registry, the metrics stay until all five are deregistered.

How It Works Internally

opossum-prom hooks into opossum's event system. Opossum fires events for every state change and request outcome:

breaker.on('fire', ...)      → timer started
breaker.on('success', ...)   → timer stopped, success counter
breaker.on('failure', ...)   → timer stopped, failure counter
breaker.on('reject', ...)    → reject counter (no timer — request was rejected before execution)
breaker.on('timeout', ...)   → timeout counter
breaker.on('fallback', ...)  → fallback counter
breaker.on('open', ...)      → state gauge → 1
breaker.on('halfOpen', ...)  → state gauge → 2
breaker.on('close', ...)     → state gauge → 0
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The metrics are shared per registry using a WeakMap cache. This means ten circuit breakers all write to the same circuit_breaker_requests_total Counter, differentiated by their name label — no "metric already registered" errors.

Why Another opossum Metrics Package?

There's opossum-prometheus from the NodeShift team. It's solid and battle-tested. But:

  • It uses a class-based API (new PrometheusMetrics({ circuits: [...] }))
  • It requires all circuits at construction time (you can add later, but the API is different)
  • It doesn't support custom histogram buckets per-breaker
  • TypeScript types aren't included

opossum-prom uses a functional API (instrument(breaker, options)), includes TypeScript definitions, supports custom buckets, and has a clean deregister story. Pick the one that fits your style.

TypeScript

Full TypeScript support is included:

import { instrument, instrumentAll, STATE } from 'opossum-prom';
import type { Registry } from 'prom-client';
import CircuitBreaker from 'opossum';

const breaker = new CircuitBreaker(myAsyncFn, { timeout: 3000 });

const handle = instrument(breaker, {
  name: 'my_service',
  registry: myRegistry as Registry,
  buckets: [0.01, 0.05, 0.1, 0.5, 1, 5],
});

// handle.deregister() is typed
handle.deregister();

// STATE is typed as a const enum
console.log(STATE.CLOSED);   // 0
console.log(STATE.OPEN);     // 1
console.log(STATE.HALF_OPEN); // 2
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The Bigger Picture: Observability Stack

opossum-prom pairs well with the rest of your Node.js observability stack:

  • pino-correlation-id — inject correlation IDs into pino via AsyncLocalStorage. When a circuit breaker fires, your logs automatically include the request ID that triggered it.
  • prom-client — the de facto Prometheus client for Node.js
  • Grafana + Prometheus — visualize everything in dashboards
  • OpenTelemetry — distributed tracing that complements your metrics 
// Correlation ID in logs + circuit breaker metrics = full request visibility
const { expressMiddleware, getLogger } = require('pino-correlation-id');
const { instrument } = require('opossum-prom');

app.use(expressMiddleware({ logger }));

const paymentBreaker = new CircuitBreaker(async (orderId) => {
  const log = getLogger(logger);
  log.info({ orderId }, 'Calling payment API'); // Includes reqId automatically
  return await stripe.paymentIntents.create(...);
});

instrument(paymentBreaker, { name: 'stripe' });
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Install and Star 

npm install opossum-prom
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If this is useful, a GitHub star helps others find it. If you use it in production and find an edge case, open an issue — I'm actively maintaining this.

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