What is it?

This package implements primitives for synchronizing asynchronous operations in Javascript.

Mutex

The term "mutex" usually refers to a data structure used to synchronize concurrent processes running on different threads. For example, before accessing a non-threadsafe resource, a thread will lock the mutex. This is guaranteed to block the thread until no other thread holds a lock on the mutex and thus enforces exclusive access to the resource. Once the operation is complete, the thread releases the lock, allowing other threads to aquire a lock and access the resource.

While Javascript is strictly single-threaded, the asynchronous nature of its execution model allows for race conditions that require similar synchronization primitives. Consider for example a library communicating with a web worker that needs to exchange several subsequent messages with the worker in order to achieve a task. As these messages are exchanged in an asynchronous manner, it is perfectly possible that the library is called again during this process. Depending on the way state is handled during the async process, this will lead to race conditions that are hard to fix and even harder to track down.

This library solves the problem by applying the concept of mutexes to Javascript. Locking the mutex will return a promise that resolves once the mutex becomes available. Once the async process is complete (usually taking multiple spins of the event loop), a callback supplied to the caller is called in order to release the mutex, allowing the next scheduled worker to execute.

Semaphore

Imagine a situation where you need to control access to several instances of a shared resource. For example, you might want to distribute images between several worker processes that perform transformations, or you might want to create a web crawler that performs a defined number of requests in parallel.

A semaphore is a data structure that is initialized to a positive integer value and that can be locked multiple times. As long as the semaphore value is positive, locking it will return the current value and the locking process will continue execution immediately; the semaphore will be decremented upon locking. Releasing the lock will increment the semaphore again.

Once the semaphore has reached zero, the next process that attempts to acquire a lock will be suspended until another process releases its lock and this increments the semaphore again.

This library provides a semaphore implementation for Javascript that is similar to the mutex implementation described above.

How to use it?

Installation

You can install the library into your project via npm

npm install async-mutex

The library is written in TypeScript and will work in any environment that supports ES5, ES6 promises and Array.isArray. On ancient browsers, a shim can be used (e.g. core-js). No external typings are required for using this library with TypeScript (version >= 2).

Starting with Node 12, native ES6 style imports are supported.

WARNING: Node 13 versions < 13.2.0 fail to import this package correctly. Node 12 and earlier are fine, as are newer versions of node 13.

Importing

CommonJS:

var Mutex = require('async-mutex').Mutex;
var Semaphore = require('async-mutex').Semaphore;
var withTimeout = require('async-mutex').withTimeout;

ES6:

import {Mutex, Semaphore, withTimeout} from 'async-mutex';

Starting with Node 12, native ES6 style imports are supported.

TypeScript:

import {Mutex, MutexInterface, Semaphore, SemaphoreInterface, withTimeout} from 'async-mutex';

Mutex API

Creating

const mutex = new Mutex();

Create a new mutex.

Locking

Promise style:

mutex
    .acquire()
    .then(function(release) {
        // ...
    });

async/await:

const release = await mutex.acquire();
try {
    // ...
} finally {
    release();
}

acquire returns an (ES6) promise that will resolve as soon as the mutex is available and ready to be accessed. The promise resolves with a function release that must be called once the mutex should be released again.

IMPORTANT: Failure to call release will hold the mutex locked and will lilely deadlock the application. Make sure to call release under all circumstances and handle exceptions accordingly.

Synchronized code execution

Promise style:

mutex
    .runExclusive(function() {
        // ...
    })
    .then(function(result) {
        // ...
    });

async/await:

await mutex.runExclusive(async () => {
    // ...
});

runExclusive schedules the supplied callback to be run once the mutex is unlocked. The function may return a promise. Once the promise is resolved or rejected (or immediately after execution if an immediate value was returned), the mutex is released. runExclusive returns a promise that adopts the state of the function result.

The mutex is released and the result rejected if an exception occurs during execution if the callback.

Checking whether the mutex is locked

mutex.isLocked();

Semaphore API

Creating

const semaphore = new Semaphore(initialValue);

Creates a new semaphore. initialValue is a positive integer that defines the initial value of the semaphore (aka the maximum number of concurrent consumers)

Locking

Promise style:

semaphore
    .acquire()
    .then(function([value, release]) {
        // ...
    });

async/await:

const [value, release] = await semaphore.acquire();
try {
    // ...
} finally {
    release();
}

acquire returns an (ES6) promise that will resolve as soon as the semaphore is available and ready to be accessed. The promise resolves to an array with the first entry being the current value of the semaphore, and the second value a function that must be called to release the semaphore once the critical operation has completed.

IMPORTANT: Failure to call release will hold the semaphore locked and will lilely deadlock the application. Make sure to call release under all circumstances and handle exceptions accordingly.

Synchronized code execution

Promise style:

semaphore
    .runExclusive(function(value) {
        // ...
    })
    .then(function(result) {
        // ...
    });

async/await:

await semaphore.runExclusive(async (value) => {
    // ...
});

runExclusive schedules the supplied callback to be run once the semaphore is available. The callback will receive the current value of the semaphore as its argument. The function may return a promise. Once the promise is resolved or rejected (or immediately after execution if an immediate value was returned), the semaphore is released. runExclusive returns a promise that adopts the state of the function result.

The semaphore is released and the result rejected if an exception occurs during execution if the callback.

Checking whether the semaphore is locked

semaphore.isLocked();

The semaphore is considered to be locked if it has a value of zero.

Limiting the time waiting for a mutex or semaphore to become available

Sometimes it is desirable to limit the time a program waits for a mutex or semaphore to become available. The withTimeout decorator can be applied to both semaphores and mutexes and changes the behavior of acquire and runExclusive accordingly.

    const mutexWithTimeout = withTimeout(new Mutex(), 100, new Error('timeout'));
    const semaphoreWithTimeout = withTimeout(new Semaphore(5), 100, new Error('timeout'));

The API of the decorated mutex or semaphore is unchanged.

The second argument of withTimeout is the timout in milliseconds. After the timeout is exceeded, the promsie returned by acquire and runExclusive will reject. The latter will not run the provided callback in case of an timeout.

The third argument of withTimeout is optional and can be used to customize the error with which the promise is rejected.

License

Feel free to use this library under the conditions of the MIT license.