Valvekit

Valvekit is a locking/synchronization mechanisms library forked from locks. This library 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 --save valvekit

Accessing the valvekit module:

var valvekit = require('valvekit');

Barrier locks

Barriers allow synchronization between processes based on tickets.

Creating a Barrier:

var defaultTicket = 'foo-bar';
var barrier = valvekit.createBarrier(defaultTicket);

Waiting for a provided ticket that matching a value of accepted array:

barrier.wait(['th3-s3cr3t-k3y', 'invalid'], function matched() {
		console.log('Provided ticket value is matched!');
	}
);

or a condition test function to be met:

barrier.wait(
	function conditionTest(value) {
		return value.indexOf('s3cr3t') !== -1;
	},
	function whenConditionMet() {
		console.log('Provided ticket value is matched!');
	}
);

Setting the ticket on a Barrier:

barrier.set('th3-s3cr3t-k3y');

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 = valvekit.createMutex();

Waiting to lock:

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

Waiting to lock, with timeout:

mutex.lock(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();
	}
}, 5000);

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 = valvekit.createLock();

Waiting to read lock:

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

Waiting to read lock, with timeout:

rwlock.readLock(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();
	}
}, 1000);

Waiting to write lock:

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

Waiting to write lock, with timeout:

rwlock.writeLock(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();
	}
}, 5000);

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');
}

Semaphores

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

Creating a Semaphore:

var resourceLimit = 3;  // amount of resources available
var sem = valvekit.createSemaphore(resourceLimit);

Claiming and releasing a resource:

sem.take(function () {
	// a resource is available, do something
	// ...
	// done, release the resource
	sem.release();
});

Or using an all-in-one function:

sem.sandbox(function () {
	// a resource is available, do something
	// ...
	// automatic release the resource
});

Example:

var sem = require('valvekit').createSemaphore(5);

function demo(i) {
  return sem.sandbox(function() {
    if (Math.random() > 0.7) {
      throw new Error("Item[" + i + "] - accident");
    }
    return new Promise(function(resolved, rejected) {
      setTimeout(function() {
        if (Math.random() > 0.5) 
					rejected("Item[" + i + "] - rejected");
				else
					resolved("Item[" + i + "]");
      }, 100)
    })
  })
}

function show(msg) {
	console.log(typeof msg === 'string' ? msg : msg.message);
}

var count = 0;
for(var i=0; i<100; i++) {
  demo(i).then(show, show).finally(function() {
    count++;
    console.log("current: %d", count);
  });
}

License

MIT