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3.1.1 • Public • Published


This is a semaphore implementation for use with async and await. The implementation follows the traditional definition of a semaphore rather than the definition of an asynchronous semaphore seen in some js community examples. Where as the latter one generally allows every defined task to proceed immediately and synchronizes at the end, async-sema allows only a selected number of tasks to proceed at once while the rest will remain waiting.

Async-sema manages the semaphore count as a list of tokens instead of a single variable containing the number of available resources. This enables an interesting application of managing the actual resources with the semaphore object itself. To make it practical the constructor for Sema includes an option for providing an init function for the semaphore tokens. Use of a custom token initializer is demonstrated in examples/pooling.js.


Firstly, add the package to your project's dependencies:

npm install --save async-sema


yarn add async-sema

Then start using it like shown in the following example. Check more use case examples here.


const { Sema } = require('async-sema');
const s = new Sema(
  4, // Allow 4 concurrent async calls
    capacity: 100 // Prealloc space for 100 tokens

async function fetchData(x) {
  await s.acquire()
  try {
    console.log(s.nrWaiting() + ' calls to fetch are waiting')
    // ... do some async stuff with x
  } finally {

const data = await Promise.all(;

The package also offers a simple rate limiter utilizing the semaphore implementation.

const { RateLimit } = require('async-sema');

async function f() {
  const lim = RateLimit(5); // rps

  for (let i = 0; i < n; i++) {
    await lim();
    // ... do something async



Constructor(nr, { initFn, pauseFn, resumeFn, capacity })

Creates a semaphore object. The first argument is mandatory and the second argument is optional.

  • nr The maximum number of callers allowed to acquire the semaphore concurrently.
  • initFn Function that is used to initialize the tokens used to manage the semaphore. The default is () => '1'.
  • pauseFn An optional fuction that is called to opportunistically request pausing the the incoming stream of data, instead of piling up waiting promises and possibly running out of memory. See examples/pausing.js.
  • resumeFn An optional function that is called when there is room again to accept new waiters on the semaphore. This function must be declared if a pauseFn is declared.
  • capacity Sets the size of the preallocated waiting list inside the semaphore. This is typically used by high performance where the developer can make a rough estimate of the number of concurrent users of a semaphore.

async drain()

Drains the semaphore and returns all the initialized tokens in an array. Draining is an ideal way to ensure there are no pending async tasks, for example before a process will terminate.


Returns the number of callers waiting on the semaphore, i.e. the number of pending promises.


Attempt to acquire a token from the semaphore, if one is available immediately. Otherwise, return undefined.

async acquire()

Acquire a token from the semaphore, thus decrement the number of available execution slots. If initFn is not used then the return value of the function can be discarded.


Release the semaphore, thus increment the number of free execution slots. If initFn is used then the token returned by acquire() should be given as an argument when calling this function.

RateLimit(rps, { timeUnit, uniformDistribution })

Creates a rate limiter function that blocks with a promise whenever the rate limit is hit and resolves the promise once the call rate is within the limit set by rps. The second argument is optional.

The timeUnit is an optional argument setting the width of the rate limiting window in milliseconds. The default timeUnit is 1000 ms, therefore making the rps argument act as requests per second limit.

The uniformDistribution argument enforces a discrete uniform distribution over time, instead of the default that allows hitting the function rps time and then pausing for timeWindow milliseconds. Setting the uniformDistribution option is mainly useful in a situation where the flow of rate limit function calls is continuous and and occuring faster than timeUnit (e.g. reading a file) and not enabling it would cause the maximum number of calls to resolve immediately (thus exhaust the limit immediately) and therefore the next bunch calls would need to wait for timeWindow milliseconds. However if the flow is sparse then this option may make the code run slower with no advantages.


  1. Fork this repository to your own GitHub account and then clone it to your local device
  2. Move into the directory of the clone: cd async-sema
  3. Link it to the global module directory of Node.js: npm link

Inside the project where you want to test your clone of the package, you can now either use npm link async-sema to link the clone to the local dependencies.


Olli Vanhoja (@OVanhoja)



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