@pocke/await-semaphore

await-semaphore

Awaitable semaphore/mutex

A semaphore implementation using ES6 promises and supporting 3 styles:

• async/await style (needs typescript)
• thunk style (automatic acquire/release)
• promise style

Also includes Mutex as a convenience for new Semaphore(1).

API

new Semaphore(count: number)

Create a new semaphore with the given count.

import {Semaphore} from 'await-semaphore';

var semaphore = new Semaphore(10);

semaphore.acquire(): Promise<() => void>

Acquire the semaphore and returns a promise for the release function. Be sure to handle release for exception case.

semaphore.acquire()
.then(release => {
    //critical section...
    doSomething()
    .then(res => {
        //...
        release();
    })
    .catch(err => {
        //...
        release();
    });
});

semaphore.use(thunk: () => Promise): Promise

Alternate method for using the semaphore by providing a thunk. This automatically handles acquire/release.

semaphore.use(() => {
    //critical section...
});

new Mutex()

An alias for new Semaphore(1). Mutex has the same methods as Semaphore.

import {Mutex} from 'await-semaphore';

var mutex = new Mutex();

Examples

Create a version of fetch() with concurrency limited to 10.

async/await style (typescript)

var semaphore = new Semaphore(10);

async function niceFetch(url) {
    var release = await semaphore.acquire();
    var result = await fetch(url);
    release();
    return result;
}

thunk style (javascript)

var semaphore = new Semaphore(10);

function niceFetch(url) {
    return semaphore.use(() => fetch(url));
}

promise style (javascript)

var semaphore = new Semaphore(10);

function niceFetch(url) {
    return semaphore.acquire()
    .then(release => {
        return fetch(url)
        .then(result => {
            release();
            return result;
        });
    });
}