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Lock on asynchronous code

Build Status

  • ES6 promise supported
  • Multiple keys lock supported
  • Timeout supported
  • Pending task limit supported
  • Domain reentrant supported
  • 100% code coverage


I did not create this package, and I will not add any features to it myself. I was granted the ownership because it was no longer being maintained, and I volunteered to fix a bug.

If you have a new feature you would like to have incorporated, please send me a PR and I will be happy to work with you and get it merged. For any bugs, PRs are most welcome but when possible I will try to get them resolved as soon as possible.

Why do you need locking on single threaded nodejs?

Nodejs is single threaded, and the code execution never gets interrupted inside an event loop, so locking is unnecessary? This is true ONLY IF your critical section can be executed inside a single event loop. However, if you have any async code inside your critical section (it can be simply triggered by any I/O operation, or timer), your critical logic will across multiple event loops, therefore it's not concurrency safe!

Consider the following code

redis.get('key', function(err, value) {
	redis.set('key', value * 2);

The above code simply multiply a redis key by 2. However, if two users run concurrently, the execution order may like this

user1: redis.get('key') -> 1
user2: redis.get('key') -> 1
user1: redis.set('key', 1 x 2) -> 2
user2: redis.set('key', 1 x 2) -> 2

Obviously it's not what you expected

With asyncLock, you can easily write your async critical section

lock.acquire('key', function(cb) {
	// Concurrency safe
	redis.get('key', function(err, value) {
		redis.set('key', value * 2, cb);
}, function(err, ret) {

Get Started

var AsyncLock = require('async-lock');
var lock = new AsyncLock();

 * @param {String|Array} key 	resource key or keys to lock
 * @param {function} fn 	execute function
 * @param {function} cb 	(optional) callback function, otherwise will return a promise
 * @param {Object} opts 	(optional) options
lock.acquire(key, function(done) {
	// async work
	done(err, ret);
}, function(err, ret) {
	// lock released
}, opts);

// Promise mode
lock.acquire(key, function() {
	// return value or promise
}, opts).then(function() {
	// lock released

Error Handling

// Callback mode
lock.acquire(key, function(done) {
	done(new Error('error'));
}, function(err, ret) {
	console.log(err.message) // output: error

// Promise mode
lock.acquire(key, function() {
	throw new Error('error');
}).catch(function(err) {
	console.log(err.message) // output: error

Acquire multiple keys

lock.acquire([key1, key2], fn, cb);

Domain reentrant lock

Lock is reentrant in the same domain

var domain = require('domain');
var lock = new AsyncLock({domainReentrant : true});

var d = domain.create(); {
	lock.acquire('key', function() {
		//Enter lock
		return lock.acquire('key', function() {
			//Enter same lock twice


// Specify timeout
var lock = new AsyncLock({timeout: 5000});
lock.acquire(key, fn, function(err, ret) {
	// timed out error will be returned here if lock not acquired in given time

// Set max pending tasks
var lock = new AsyncLock({maxPending: 1000});
lock.acquire(key, fn, function(err, ret) {
	// Handle too much pending error

// Whether there is any running or pending async function

// Use your own promise library instead of the global Promise variable
var lock = new AsyncLock({Promise: require('bluebird')}); // Bluebird
var lock = new AsyncLock({Promise: require('q')}); // Q

// Add a task to the front of the queue waiting for a given lock
lock.acquire(key, fn1, cb); // runs immediately
lock.acquire(key, fn2, cb); // added to queue
lock.acquire(key, priorityFn, cb, {skipQueue: true}); // jumps queue and runs before fn2


See Changelog


See issue tracker.




Lock on asynchronous code for Nodejs




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