opossum

What is opossum?

Opossum is a Node.js package that implements the circuit breaker pattern. It helps in making your application more resilient by preventing cascading failures and providing fallback mechanisms when external services fail or become unresponsive.

What are opossum's main functionalities?

Circuit Breaker

This feature allows you to wrap a function that might fail with a circuit breaker. If the function fails too many times, the circuit breaker will open and prevent further calls to the failing function until it recovers.

const CircuitBreaker = require('opossum');

async function asyncFunctionThatCouldFail() {
  // Simulate a function that could fail
  return 'Success!';
}

const breaker = new CircuitBreaker(asyncFunctionThatCouldFail);

breaker.fire()
  .then(console.log)
  .catch(console.error);

Fallback

This feature allows you to specify a fallback function that will be called when the main function fails. This can be useful for providing a default response or alternative behavior when the primary function is unavailable.

const CircuitBreaker = require('opossum');

async function asyncFunctionThatCouldFail() {
  throw new Error('Failed!');
}

const breaker = new CircuitBreaker(asyncFunctionThatCouldFail, {
  fallback: () => 'Fallback response'
});

breaker.fire()
  .then(console.log)
  .catch(console.error);

Status Monitoring

This feature allows you to monitor the status of the circuit breaker. You can listen for events such as 'open', 'halfOpen', and 'close' to take appropriate actions based on the state of the circuit breaker.

const CircuitBreaker = require('opossum');

async function asyncFunctionThatCouldFail() {
  return 'Success!';
}

const breaker = new CircuitBreaker(asyncFunctionThatCouldFail);

breaker.on('open', () => console.log('Circuit breaker opened!'));
breaker.on('halfOpen', () => console.log('Circuit breaker half-open!'));
breaker.on('close', () => console.log('Circuit breaker closed!'));

breaker.fire()
  .then(console.log)
  .catch(console.error);

Other packages similar to opossum

cockatiel

Cockatiel is a resilience library for JavaScript and TypeScript that provides policies for retries, timeouts, and circuit breakers. Compared to Opossum, Cockatiel offers a more comprehensive set of resilience policies and is designed to work seamlessly with TypeScript.

promise-breaker

Promise-breaker is a lightweight library that provides a simple implementation of the circuit breaker pattern for promises. It is less feature-rich compared to Opossum but can be a good choice for simpler use cases where only basic circuit breaker functionality is needed.

brakes

Brakes is a Node.js library that provides a robust implementation of the circuit breaker pattern along with additional features like bulkheading and fallback mechanisms. It is similar to Opossum in terms of functionality but offers more advanced features for handling complex failure scenarios.

README

opossum

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 4.x, 6.x, 8.x

Usage

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
  maxFailures: 5, // Once we fail this many times in a row, start failing fast
  resetTimeout: 30000 // After 30 seconds, try again.
};
const breaker = circuitBreaker(asyncFunctionThatCouldFail, options);

breaker.fire(params)
  .then(console.log)
  .catch(console.error);

Fallback

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.

Browser

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.

<html>
<head>
  <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>
<body>
...
</body>
</head>
</html>

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)));
server.route({
  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)));
});

Events

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

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.` }));

circuit.on('success',
  (result) => $(element).append(
    makeNode(`SUCCESS: ${JSON.stringify(result)}`)));

circuit.on('timeout',
  () => $(element).append(
    makeNode(`TIMEOUT: ${route} is taking too long to respond.`)));

circuit.on('reject',
  () => $(element).append(
    makeNode(`REJECTED: The breaker for ${route} is open. Failing fast.`)));

circuit.on('open',
  () => $(element).append(
    makeNode(`OPEN: The breaker for ${route} just opened.`)));

circuit.on('halfOpen',
  () => $(element).append(
    makeNode(`HALF_OPEN: The breaker for ${route} is half open.`)));

circuit.on('close',
  () => $(element).append(
    makeNode(`CLOSE: The breaker for ${route} has closed. Service OK.`)));

circuit.on('fallback',
  (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')
  .then(console.log)
  .catch(console.error);

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);

breaker.fire()
  .then(console.log) // logs 'foo'
  .catch(console.error);

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