Locks

Locks implements locking/synchronization mechanisms that have traditionally been used for protecting shared memory between multiple threads. JavaScript is inherently single threaded and does not suffer from these security and stability issues. However, because of its asynchronous eventy nature JavaScript can still benefit from making particular operations wait for the completion of others.

Installation

Node.js users:

npm install locks

Component users:

component install Wizcorp/locks

API

Accessing the locks module:

var locks = require('locks');

Please note that all the examples below will also demonstrate how to unlock each time. But in general, this matters:

// unlocking will give waiting processes a chance to get the lock and continue
myLock.unlock();

Mutex locks

Mutex locks are the most basic locks which aim to prevent the simultaneous access to a resource by more than one actor at a time. more info

Creating a Mutex Lock:

var mutex = locks.createMutex();

Waiting to lock:

mutex.lock(function () {
	console.log('We got the lock!');
	// do stuff
	mutex.unlock();
});

Waiting to lock, with timeout:

mutex.timedLock(5000, function (error) {
	if (error) {
		console.log('Could not get the lock within 5 seconds, so gave up');
	} else {
		console.log('We got the lock!');
		// do stuff
		mutex.unlock();
	}
});

Checking if a lock is held:

if (mutex.isLocked) {
	console.log('Something has the lock.');
}

Optimistic attempt to lock:

if (mutex.tryLock()) {
	console.log('We got the lock!');
	// do stuff
	mutex.unlock();
} else {
	console.log('Could not get the lock at this time');
}

Read/Write locks

Read/Write Locks are used to allow many actors to read from a resource, as long as nothing is writing to it. That also means that only one actor may be writing at any given time. more info

Creating a Read/Write Lock:

var rwlock = locks.createReadWriteLock();

Waiting to read lock:

rwlock.readLock(function () {
	console.log('We may now read from a resource!');
	// do stuff
	rwlock.unlock();
});

Waiting to write lock:

rwlock.writeLock(function () {
	console.log('We may now write to a resource!');
	// do stuff
	rwlock.unlock();
});

Waiting to read lock, with timeout:

rwlock.timedReadLock(5000, function (error) {
	if (error) {
		console.log('Could not get the lock within 5 seconds, so gave up');
	} else {
		console.log('We may now read from a resource!');
		// do stuff
		rwlock.unlock();
	}
});

Waiting to write lock, with timeout:

rwlock.timedWriteLock(5000, function (error) {
	if (error) {
		console.log('Could not get the lock within 5 seconds, so gave up');
	} else {
		console.log('We may now write to a resource!');
		// do stuff
		rwlock.unlock();
	}
});

Checking if a lock is held:

if (rwlock.isWriteLocked) {
	console.log('Something has the write lock.');
}
else if (rwlock.isReadLocked) {
	console.log('The read lock is held one or more times.');
}

Optimistic attempt to read lock:

if (rwlock.tryReadLock()) {
	console.log('We may now read from a resource!');
	// do stuff
	rwlock.unlock();
} else {
	console.log('Could not get the lock at this time');
}

Optimistic attempt to write lock:

if (rwlock.tryWriteLock()) {
	console.log('We may now write to a resource!');
	// do stuff
	rwlock.unlock();
} else {
	console.log('Could not get the lock at this time');
}

Condition variables

Condition variables allow synchronization between processes based on values.

Creating a Condition Variable:

var initialValue = 'hello world';
var cond = locks.createCondVariable(initialValue);

Waiting for a condition to be met:

cond.wait(
	function conditionTest(value) {
		return value.indexOf('こんにちは') !== -1;
	},
	function whenConditionMet() {
		console.log('Our welcome message is in Japanese!');
	}
);

Setting the value on a Condition Variable:

cond.set('こんにちは世界！');

Semaphores

Semaphores solve the problem of sharing a limited set of resources. more info

Creating a Semaphore:

var initialValue = 3;  // amount of resources available
var sem = locks.createSemaphore(initialValue);

Claiming and releasing a resource:

sem.wait(function () {
	console.log('We may now access one resource!');
	// do stuff
	// release the resource
	sem.signal();
});

License

MIT