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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
Issue tracker:
Engines: Node.js >= 16


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.

AbortController support

You can provide an AbortController (, for aborting on going request upon reaching Opossum timeout.


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.

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.

The fallback function accepts the same parameters as the fire function:

Breaker State Initialization

There may be times where you will need to initialize the state of a Circuit Breaker. Primary use cases for this are in a serverless environment such as Knative or AWS Lambda, or any container based platform, where the container being deployed is ephemeral.

The toJSON method is a helper function to get the current state and status of a breaker:

const breakerState = breaker.toJSON();

This will return an object that might look similar to this:

  state: {
    enabled: true,
    name: 'functionName'
    closed: true,
    open: false,
    halfOpen: false,
    warmUp: false,
    shutdown: false
  status: {

A new circuit breaker instance can be created with this state by passing this object in:

const breaker = new CircuitBreaker({state: state});

Status Initialization

There may also be times where you will need to pre-populate the stats of the Circuit Breaker Status Object. Primary use cases for this are also in a serverless environment such as Knative or AWS Lambda, or any container based platform, where the container being deployed is ephemeral.

Getting the existing cumalative stats for a breaker can be done like this:

const stats = breaker.stats;

stats will be an object that might look similar to this:

  failures: 11,
  fallbacks: 0,
  successes: 5,
  rejects: 0,
  fires: 16,
  timeouts: 0,
  cacheHits: 0,
  cacheMisses: 0,
  semaphoreRejections: 0,
  percentiles: {
    '0': 0,
    '1': 0,
    '0.25': 0,
    '0.5': 0,
    '0.75': 0,
    '0.9': 0,
    '0.95': 0,
    '0.99': 0,
    '0.995': 0
  latencyTimes: [ 0 ],
  latencyMean: 0

To then re-import those stats, first create a new Status object with the previous stats and then pass that as an option to the CircuitBreaker constructor:

const statusOptions = {
  stats: {....}

const newStatus = CircuitBreaker.newStatus(statusOptions);

const breaker = new CircuitBreaker({status: newStatus});


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.

In your application, set a route to the file, pointing to 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.


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
  • shutdown - emitted when the breaker shuts down

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

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() .

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.

Calculating errorThresholdPercentage

The errorThresholdPercentage value is compared to the error rate. That rate is determined by dividing the number of failures by the number of times the circuit has been fired. You can see this comparison here:

The numbers for fires and failures come from the stats that are indeed governed by rollingCountTimeout and rollingCountBuckets. The timeout value is the total number of seconds for which the stats are being maintained, and the buckets value is the number of slots in the window. The defaults are 10 seconds and 10 buckets. So, the statistics that are being compared against errorThresholdPercentage are based on 10 samples, one per second over the last 10 seconds.

Example: a circuit is fired 24 times over 10 seconds with a somewhat bursty pattern, failing three times.

| fires: 2 | fires: 1 | fires: 3 | fires: 0 | fires: 9 | fires: 3 | fires: 2 | fires: 0 | fires: 4 | fires: 0 |
| fails: 0 | fails: 0 | fails: 0 | fails: 0 | fails: 0 | fails: 3 | fails: 0 | fails: 0 | fails: 0 | fails: 0 |

The failure rate here is 3/24 or 1/8 or 12.5%. The default error threshold is 50%, so in this case, the circuit would not open. However, if you modified the rollingCountTimeout to 3 seconds, and the rollingCountBuckets to 3 (not recommended), then the stats array might look like these three seconds from above.

| fires: 3 | fires: 2 | fires: 0 |
| fails: 3 | fails: 0 | fails: 0 |

Now, without changing errorThresholdPercentage our circuit will open because our error rate is now 3/5 or 60%. It's tricky to test this stuff because the array of statistics is a rolling count. Every second the oldest bucket is removed and a new one is added, so the totals change constantly in a way that may not be intuitive.

For example, if the first example is shifted right, dropping the first bucket and adding another with fires: 3 the total number of fires now in the stats is not 27 (24+3) but 25 (24-2+3).

The code that is summing the stats samples is here:


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.

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 breaker.shutdown() when the circuit is no longer in use to clean up all listeners.


Constructs a CircuitBreaker.

new CircuitBreaker(action: Function, options: Object)

Extends EventEmitter

action (Function) The action to fire for this CircuitBreaker
options (Object) Options for the CircuitBreaker
Name Description
options.status Status A Status object that might have pre-prime stats
options.timeout Number The time in milliseconds that action should be allowed to execute before timing out. Timeout can be disabled by setting this to false . Default 10000 (10 seconds)
options.maxFailures Number (Deprecated) The number of times the circuit can fail before opening. Default 10.
options.resetTimeout Number The time in milliseconds to wait before setting the breaker to halfOpen state, and trying the action again. Default: 30000 (30 seconds)
options.rollingCountTimeout Number Sets the duration of the statistical rolling window, in milliseconds. This is how long Opossum keeps metrics for the circuit breaker to use and for publishing. Default: 10000
options.rollingCountBuckets Number Sets the number of buckets the rolling statistical window is divided into. So, if options.rollingCountTimeout is 10000, and options.rollingCountBuckets is 10, then the statistical window will be 1000/1 second snapshots in the statistical window. Default: 10 String the circuit name to use when reporting stats. Default: the name of the function this circuit controls.
options.rollingPercentilesEnabled boolean This property indicates whether execution latencies should be tracked and calculated as percentiles. If they are disabled, all summary statistics (mean, percentiles) are returned as -1. Default: true
options.capacity Number the number of concurrent requests allowed. If the number currently executing function calls is equal to options.capacity, further calls to fire() are rejected until at least one of the current requests completes. Default: Number.MAX_SAFE_INTEGER .
options.errorThresholdPercentage Number the error percentage at which to open the circuit and start short-circuiting requests to fallback. Default: 50
options.enabled boolean whether this circuit is enabled upon construction. Default: true
options.allowWarmUp boolean determines whether to allow failures without opening the circuit during a brief warmup period (this is the rollingCountTimeout property). Default: false This can help in situations where no matter what your errorThresholdPercentage is, if the first execution times out or fails, the circuit immediately opens.
options.volumeThreshold Number the minimum number of requests within the rolling statistical window that must exist before the circuit breaker can open. This is similar to options.allowWarmUp in that no matter how many failures there are, if the number of requests within the statistical window does not exceed this threshold, the circuit will remain closed. Default: 0
options.errorFilter Function an optional function that will be called when the circuit's function fails (returns a rejected Promise). If this function returns truthy, the circuit's failPure statistics will not be incremented. This is useful, for example, when you don't want HTTP 404 to trip the circuit, but still want to handle it as a failure case.
options.cache boolean whether the return value of the first successful execution of the circuit's function will be cached. Once a value has been cached that value will be returned for every subsequent execution: the cache can be cleared using clearCache . (The metrics cacheHit and cacheMiss reflect cache activity.) Default: false
options.cacheTTL Number the time to live for the cache in milliseconds. Set 0 for infinity cache. Default: 0 (no TTL)
options.cacheGetKey Function function that returns the key to use when caching the result of the circuit's fire. Better to use custom one, because JSON.stringify is not good from performance perspective. Default: (...args) => JSON.stringify(args)
options.cacheTransport CacheTransport custom cache transport should implement get , set and flush methods.
options.abortController AbortController this allows Opossum to signal upon timeout and properly abort your on going requests instead of leaving it in the background
options.enableSnapshots boolean whether to enable the rolling stats snapshots that opossum emits at the bucketInterval. Disable this as an optimization if you don't listen to the 'snapshot' event to reduce the number of timers opossum initiates.
options.rotateBucketController EventEmitter if you have multiple breakers in your app, the number of timers across breakers can get costly. This option allows you to provide an EventEmitter that rotates the buckets so you can have one global timer in your app. Make sure that you are emitting a 'rotate' event from this EventEmitter
Static Members
Instance Members
call(context, rest)
healthCheck(func, interval?)


Emitted after options.resetTimeout has elapsed, allowing for a single attempt to call the service again. If that attempt is successful, the circuit will be closed. Otherwise it remains open.


Type: Number


Emitted when the breaker is reset allowing the action to execute again



Emitted when the breaker opens because the action has failure percentage greater than options.errorThresholdPercentage.



Emitted when the circuit breaker has been shut down.



Emitted when the circuit breaker action is executed


Type: any


Emitted when the circuit breaker is using the cache and finds a value.



Emitted when the circuit breaker does not find a value in the cache, but the cache option is enabled.



Emitted when the circuit breaker is open and failing fast


Type: Error


Emitted when the circuit breaker action takes longer than options.timeout


Type: Error


Emitted when the circuit breaker action succeeds


Type: any


Emitted when the rate limit has been reached and there are no more locks to be obtained.


Type: Error


Emitted with the user-supplied health check function returns a rejected promise.


Type: Error


Emitted when the circuit breaker executes a fallback function


Type: any


Emitted when the circuit breaker action fails


Type: Error


Tracks execution status for a given CircuitBreaker. A Status instance is created for every CircuitBreaker and does not typically need to be created by a user.

A Status instance will listen for all events on the CircuitBreaker and track them in a rolling statistical window. The window duration is determined by the rollingCountTimeout option provided to the CircuitBreaker. The window consists of an array of Objects, each representing the counts for a CircuitBreaker's events.

The array's length is determined by the CircuitBreaker's rollingCountBuckets option. The duration of each slice of the window is determined by dividing the rollingCountTimeout by rollingCountBuckets.

new Status(options: Object)

Extends EventEmitter

options (Object) for the status window
Name Description
options.rollingCountBuckets Number number of buckets in the window
options.rollingCountTimeout Number the duration of the window
options.rollingPercentilesEnabled Boolean whether to calculate percentiles
options.stats Object object of previous stats
// Creates a 1 second window consisting of ten time slices,
// each 100ms long.
const circuit = circuitBreaker(fs.readFile,
 { rollingCountBuckets: 10, rollingCountTimeout: 1000});

// get the cumulative statistics for the last second

// get the array of 10, 1 second time slices for the last second
Instance Members


Emitted at each time-slice. Listeners for this event will receive a cumulative snapshot of the current status window.


Type: Object


Simple in-memory cache implementation

new MemoryCache()
cache (Map) : Cache map
Instance Members
set(key, value, ttl)