rwlock

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:

``bash
$ npm install rwlock
`

Basic usage
-----------

Requiring the package, creating an instance:

`javascript
var ReadWriteLock = require('rwlock');

var lock = new ReadWriteLock();
`

Acquiring a read lock:

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

release();
});
`

Acquiring a write lock:

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

release();
});
`

Locks can be released later:

`javascript
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:

`javascript
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:

`javascript
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:

`javascript
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:

`javascript
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:

`javascript
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:

`bash
$ 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