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. 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 example1.js
.
Firstly, add the package to your project's dependencies
:
npm install --save async-sema
Then start using it like shown here.
See /examples for more use cases.
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 {
s.release();
}
}
const data = await Promise.all(array.map(fetchData));
The package also offers a simple rate limiter utilizing the semaphore implementation.
const RateLimit = require('async-sema/rate-limit');
async function f() {
const lim = RateLimit(5); // rps
for (let i = 0; i < n; i++) {
await lim();
// ... do something async
}
}
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 apauseFn
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.
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.
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.
- Fork this repository to your own GitHub account and then clone it to your local device
- Move into the directory of the clone:
cd async-sema
- 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.