What is it?

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 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 acquire 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 read more motivation in this blog post.

How to use it?

Installation

You can install the library into your project via npm

npm install async-timeout-mutex

Importing

CommonJS

var createMutex = require('async-timeout-mutex').createMutex;

Browser

Globally/CommonJS/RequireJS from the UMD bundle

<!-- global install -->
<script src="dist/async-timeout-mutex.js"></script>
<script>
var createMutex = AsyncTimeoutMutex.createMutex;
// ...
</script>

ESM module

<script type="module">
import {createMutex} from 'dist/async-timeout-mutex.mjs';
//...
</script>

API

Creating

const mutex = createMutex({timeout: 500});

Creates a new mutex. The configuration object with the timeout setting is optional.

Locking

mutex.acquire()
.then(release =>
   doAsyncStuff()  // worker returns a promise
   .finally(release)
);

acquire returns a 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 likely deadlock the application. Make sure to call release under all circumstances and handle exceptions accordingly.

If a timeout duration was specified when creating the Mutex with createMutex, the .acquire promise will reject with a MutexTimeoutError when the waiting period had elapsed before a lock on the mutex could be obtained.

Timeout example

// blocks the mutex for a minute
mutex.acquire()
.then(release => 
   new Promise(resolve => setTimeout(() => { resolve("X"); }, 60000))
   .finally(release)
);

mutex.acquire()
.then(release =>
   doAsyncStuff()  // worker returns a promise
   .finally(release)
)
.catch(error => {
   if (error.name === 'MutexTimeoutError') {
      console.warn("Resource Busy! Please retry later!");
      return defaultValue;
   }
   // ... other exceptions here
});

Async function example

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

Synchronized code execution

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

Async function example

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 promise. Once the promise is resolved (or rejected), 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();

Browser Demo

Run in browser

License

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