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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
Documentation: https://bucharest-gold.github.io/opossum/
Issue tracker: https://github.com/bucharest-gold/opossum/issues
Engines: Node.js 6.x, 8.x, 9.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 = 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 = 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 and try the action again. If successful, the circuit will close and emit the close event.

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. A browserified version of the module is available as a compressed file, or exploded in the dist folder.

Here is an example using hapi.js. See the examples folder 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 function. Use it to create circuit breakers, guarding against network failures in your REST API calls.

// app.js
const route = 'https://example-service.com/rest/route';
const circuitBreakerOptions = {
  timeout: 500,
  maxFailures: 3,
  resetTimeout: 5000

const circuit = circuitBreaker(() => $.get(route), circuitBreakerOptions);
circuit.fallback(() => `${route} unavailable right now. Try later.`));
circuit.on('success', (result) => $(element).append(JSON.stringify(result)}));

$(() => {
  $('#serviceButton').click(() => circuit.fire().catch((e) => 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
  • semaphore-locked - emitted when the breaker is at capacity and cannot execute the request
  • health-check-failed - 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 = 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 CircuitBreaker.fire(). 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 circuitBreaker.promisify().

const fs = require('fs');
const circuitBreaker = require('opossum');

const readFile = circuitBreaker.promisify(fs.readFile);
const breaker = circuitBreaker(readFile, options);

breaker.fire('./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 = circuitBreaker('foo', options);

  .then(console.log) // logs 'foo'

Hystrix Metrics

A Hystrix Stream is available for use with a Hystrix Dashboard using the circuitBreaker.hystrixStats.getHystrixStream method.

This method returns a Node.js Stream, which makes it easy to create an SSE stream that will be compliant with a Hystrix Dashboard.

Additional Reading: Hystrix Metrics Event Stream, Turbine, Hystrix Dashboard