rwlock

Asynchronous read/write lock implementation for Node.js.

Main rules:

• there may be zero or more readers at a time,
• there may be only one writer at a time,
• there may be no writer if there are one or more readers already.

Installation

It's on npmjs:

$ npm install rwlock

Basic usage

Requiring the package, creating an instance:

var ReadWriteLock = require('rwlock');

var lock = new ReadWriteLock();

Acquiring a read lock:

lock.readLock(function (release) {
	// do stuff

	release();
});

Acquiring a write lock:

lock.writeLock(function (release) {
	// do stuff

	release();
});

Locks can be released later:

lock.readLock(function (release) {
	// not ready to release yet

	setTimeout(function () {
		// ok, now I'm ready
		release();
	}, 1000);
});

Upgrading to a write lock

ReadWriteLock does not explicitly support upgrading but you can take advantage of the asynchronous-ness:

lock.readLock(function (release) {
	// read stuff here

	// ok, I now realize I need to write

	// this will be queued
	lock.writeLock(function (release) {
		// you can write here

		release();

		// everything is now released.
	});

	// release the read lock, this will activate the writer
	release();
});

Downgrading to a read lock

Similar to upgrading:

lock.writeLock(function (release) {
	lock.readLock(function (release) {
		// ...
		release();
	});
	release();
});

Keys

Every ReadWriteLock instance allows you to work on a virtually unlimited number of completely independent read/write locks.

Locks are identified by names called "keys". Every exposed method has an optional "key" first argument indicating the lock to work on; if you don't specify a key, the default lock is used.

Example:

lock.writeLock('lock1', function (release) {
	console.log('writing 1...');
	lock.writeLock('lock2', function (release) {
		console.log('writing 2...');
		release();
		console.log('done 2.');
	});
	release();
	console.log('done 1.');
});

The previous example logs:

writing 1...
writing 2...
done 2.
done 1.

async compatibility

The ReadWriteLock class does not return errors to your callbacks, but many APIs in Node do. The async module uses that as a convention: callbacks usually receive two arguments, a possibly null error object and the actual result in case there is no error.

To aid async compatibility, ReadWriteLock sends null errors if you specify the async flag like in the following example:

lock.async.readLock(function (error, release) {
	// no need to check on error, it will always be null

	// do stuff here

	release();
});

You can use rwlock and async together like in this example:

var releaseLock = null;

async.waterfall([function (next) {
	lock.async.writeLock(next);
}, function (release, next) {
	releaseLock = release;
	fs.writeFile('file', 'content', next);
}, function (next) {
	releaseLock();
	next(null);
}], function (error) {
	if (error) {
		if (releaseLock) {
			releaseLock();
		}
		console.dir(error);
	} else {
		console.log('done.');
	}
});

Building from source and testing

You don't need this, but in case you want:

$ sudo npm install -g grunt-cli
$ cd
$ git clone https://github.com/71104/rwlock.git
$ cd rwlock
$ npm install
$ grunt all

The following folders will be generated:

• lib, containing the minified ReadWriteLock class to require in Node.js;
• doc, containing the API reference documentation in HTML format.

License

MIT. Copyright 2013 Alberto La Rocca