This package implements a mutex for synchronizing asynchronous operations in Javascript.

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 guranteed 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. A mutex is locked by providing a worker callback that will be called once no other locks are held on the mutex. Once the async process is complete (usually taking multiple spins of the event loop), a callback supplied to the worker is called in order to release the mutex, allowing the next scheduled worker to execute.

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 and ES6 promises. If ES6 promises are not supported natively, a shim can be used (e.g. core-js). No external typings are required for using this library with TypeScript (version >= 2).

ES5 / CommonJS

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

ES6

import {Mutex} from 'async-mutex';

TypeScript

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

ES5/ES6/TypeScript

const mutex = new Mutex();

Create a new mutex.

ES5/ES6/TypeScript

mutex
    .acquire()
    .then(function(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.

const release = await mutex.acquire();
try {
    const i = await store.get();
    await store.put(i + 1);
} finally {
    release();
}

ES5/ES6/TypeScript

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

This example is equivalent to the async/await example that locks the mutex directly:

await mutex.runExclusive(async () => {
    const i = await store.get();
    await store.put(i + 1);
});

runExclusive schedules the supplied callback to be run once the mutex is unlocked. The function is expected to return a Promises/A+ compliant promise. Once the promise is resolved (or rejected), the mutex is released. runExclusive returns a promise that adops the state of the function result.

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

ES5/ES6/TypeScript

mutex.isLocked();

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