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Opossum is a Node.js circuit breaker that executes asynchronous functions and monitors their execution status. When things start failing, opossum plays dead and fails fast. If you want, you can provide a fallback function to be executed when in the failure state.

For more about the circuit breaker pattern, there are lots of resources on the web - search it! Fowler's blog post is one place to start reading.

Project Info
License: Apache-2.0
Build: make
Issue tracker:
Engines: Node.js 8.x, 10.x, 11.x, 12.x


Let's say you've got an API that depends on something that might fail - a network operation, or disk read, for example. Wrap those functions up in a CircuitBreaker and you have control over your destiny.

const CircuitBreaker = require('opossum');
function asyncFunctionThatCouldFail (x, y) {
  return new Promise((resolve, reject) => {
    // Do something, maybe on the network or a disk
const options = {
  timeout: 3000, // If our function takes longer than 3 seconds, trigger a failure
  errorThresholdPercentage: 50, // When 50% of requests fail, trip the circuit
  resetTimeout: 30000 // After 30 seconds, try again.
const breaker = new CircuitBreaker(asyncFunctionThatCouldFail, options);


You can also provide a fallback function that will be executed in the event of failure. To take some action when the fallback is performed, listen for the fallback event.

const breaker = new CircuitBreaker(asyncFunctionThatCouldFail, options);
// if asyncFunctionThatCouldFail starts to fail, firing the breaker
// will trigger our fallback function
breaker.fallback(() => 'Sorry, out of service right now');
breaker.on('fallback', (result) => reportFallbackEvent(result));

Once the circuit has opened, a timeout is set based on options.resetTimeout. When the resetTimeout expires, opossum will enter the halfOpen state. Once in the halfOpen state, the next time the circuit is fired, the circuit's action will be executed again. If successful, the circuit will close and emit the close event. If the action fails or times out, it immediately re-enters the open state.

When a fallback function is triggered, it's considered a failure, and the fallback function will continue to be executed until the breaker is closed.


Opossum really shines in a browser. You can use it to guard against network failures in your AJAX calls.

We recommend using webpack to bundle your applications, since it does not have the effect of polluting the window object with a global. However, if you need it, you can access a circuitBreaker function in the global namespace by doing something similar to what is shown in the below example.

Here is an example using hapi.js. See the opossum-examples repository for more detail.

Include opossum.js in your HTML file.

  <title>My Super App</title>
  <script type='text/javascript' src="/jquery.js"></script> 
  <script type='text/javascript' src="/opossum.js"></script> 
  <script type='text/javascript' src="/app.js"></script> 

In your application, set a route to the file, pointing to node_modules/opossum/dist/opossum-min.js.

// server.js
const server = new Hapi.Server();
server.register(require('inert', (err) => possibleError(err)));
  method: 'GET',
  path: '/opossum.js',
  handler: {
    file: {
      path: path.join(__dirname, 'node_modules', 'opossum', 'dist', 'opossum-min.js'),

In the browser's global scope will be a CircuitBreaker constructor. Use it to create circuit breakers, guarding against network failures in your REST API calls.

// app.js
const route = '';
const circuitBreakerOptions = {
  timeout: 500,
  maxFailures: 3,
  resetTimeout: 5000
const circuit = new CircuitBreaker(() => $.get(route), circuitBreakerOptions);
circuit.fallback(() => `${route} unavailable right now. Try later.`));
circuit.on('success', (result) => $(element).append(JSON.stringify(result)}));
$(() => {
  $('#serviceButton').click(() => => console.error(e)));


A CircuitBreaker will emit events for important things that occur. Here are the events you can listen for.

  • fire - emitted when the breaker is fired.
  • reject - emitted when the breaker is open (or halfOpen).
  • timeout - emitted when the breaker action times out.
  • success - emitted when the breaker action completes successfully
  • failure - emitted when the breaker action fails, called with the error
  • open - emitted when the breaker state changes to open
  • close - emitted when the breaker state changes to closed
  • halfOpen - emitted when the breaker state changes to halfOpen
  • fallback - emitted when the breaker has a fallback function and executes it
  • semaphoreLocked - emitted when the breaker is at capacity and cannot execute the request
  • healthCheckFailed - emitted when a user-supplied health check function returns a rejected promise

Handling events gives a greater level of control over your application behavior.

const circuit = new CircuitBreaker(() => $.get(route), circuitBreakerOptions);
circuit.fallback(() => ({ body: `${route} unavailable right now. Try later.` }));
  (result) => $(element).append(
    makeNode(`SUCCESS: ${JSON.stringify(result)}`)));
  () => $(element).append(
    makeNode(`TIMEOUT: ${route} is taking too long to respond.`)));
  () => $(element).append(
    makeNode(`REJECTED: The breaker for ${route} is open. Failing fast.`)));
  () => $(element).append(
    makeNode(`OPEN: The breaker for ${route} just opened.`)));
  () => $(element).append(
    makeNode(`HALF_OPEN: The breaker for ${route} is half open.`)));
  () => $(element).append(
    makeNode(`CLOSE: The breaker for ${route} has closed. Service OK.`)));
  (data) => $(element).append(
    makeNode(`FALLBACK: ${JSON.stringify(data)}`)));

Promises vs. Callbacks

The opossum API returns a Promise from But your circuit action - the async function that might fail - doesn't have to return a promise. You can easily turn Node.js style callback functions into something opossum understands by using the built in Node core utility function util.promisify() .

const fs = require('fs');
const { promisify } = require('util');
const CircuitBreaker = require('opossum');
const readFile = promisify(fs.readFile);
const breaker = new CircuitBreaker(readFile, options);'./package.json', 'utf-8')

And just for fun, your circuit doesn't even really have to be a function. Not sure when you'd use this - but you could if you wanted to.

const breaker = new CircuitBreaker('foo', options);
  .then(console.log) // logs 'foo'


Typings are available here.

If you'd like to add them, run npm install @types/opossum in your project.



The opossum-prometheus module can be used to produce metrics that are consumable by Prometheus. These metrics include information about the circuit itself, for example how many times it has opened, as well as general Node.js statistics, for example event loop lag.


The opossum-hystrix module can be used to produce metrics that are consumable by the Hystrix Dashboard.


You may run into issues related to too many listeners on an EventEmitter like this.

(node:25619) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 10 unpipe listeners added. Use emitter.setMaxListeners() to increase limit
(node:25619) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 drain listeners added. Use emitter.setMaxListeners() to increase limit
(node:25619) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 error listeners added. Use emitter.setMaxListeners() to increase limit
(node:25619) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 close listeners added. Use emitter.setMaxListeners() to increase limit
(node:25619) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 finish listeners added. Use emitter.setMaxListeners() to increase limit

In some cases, seeing this error might indicate a bug in client code, where many CircuitBreakers are inadvertently being created. But there are legitimate scenarios where this may not be the case. For example, it could just be that you need more than 10 CircuitBreakers in your app. That's ok.

To get around the error, you can set the number of listeners on the stream.


Or it could be that you have a large test suite which exercises some code that creates CircuitBreakers and does so repeatedly. If the CircuitBreaker being created is only needed for the duration of the test, use circuit.shutdown() when the circuit is no longer in use to clean up all listeners.


npm i opossum

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