event-loop-mutex

This module acts similar to a traditional mutex, but within the JavaScrip event loop. The lock/unlock functionality can be used to guard async execution that is interrupted by an await. There are 3 ways to use this module: with async/await, with callbacks, and with for-await.

Construction

To create or access a mutex instance, construct it with new Mutex(handle). All mutexes with the same handle refer to the same instance.

To create an instance guaranteed to be unique and reuse the instance itself, construct the mutex with new Mutex(Symbol()).

Async/await usage

const Mutex = require("event-loop-mutex")
// ...
const unlock = await new Mutex("unique_handle").lock()
// sensitive code here
unlock()

Callback usage

const Mutex = require("event-loop-mutex")
// ...
new Mutex("unique_handle").lock().then((unlock) => {
    // sensitive code here
    unlock()
    // more code, potentially
})

For-await usage

const Mutex = require("event-loop-mutex")
// ...
for await (const unlock of new Mutex("unique_handle")) {
    // sensitive code here
    unlock()
    // more code, potentially
}

Error handling

If the code guarded by the mutex could potentially throw an error, include an unlock() in a finally clause.

const Mutex = require("event-loop-mutex")
// ...
const unlock = await new Mutex("unique_handle").lock()
try {
    // sensitive code here
    unlock()
    // more code, potentially
} catch (error) {
    // error handling
} finally {
    unlock()
}

Mutex.defer()

Mutex.defer() is a static method that simply returns a promise that resolves at the end of the current event queue. It's equivalent to new Promise(setImmediate) or new Promise(setTimeout) depending on whether the code is running in Node or a browser.

Notes

• It is extremely important that .lock() is not called in such a way that it could deadlock the application. If that's unavoidable, a timeout mechanism may need to be implemented in the application to unlock all acquired locks as a last resort.
• This package does not have anything to do with threads, and will not work with Node's cluster functionality or any other method of using threads in Node.

Compatibility

This module requires the following features:

• Symbol
• Symbol.asyncIterator
• Symbol.toStringTag
• Promise
• classes
• class members
• static class members
• getters
• async/await
• const
• CommonJS module.exports