Semlocks

Mind-blowingly simple local resource management

Sponsored by Leadnomics.

Installation

In your project folder, type:

npm install semlocks --save

Usage

Getting Started

var locks = require('semlocks');

locks.acquire('hello', function(err, release) {
	console.log('Hello');
	setTimeout(release, 1000);
});

locks.acquire('hello', function(err, release) {
	// Prints a second after 'Hello'
	console.log('World!');
	release();
});

Locks can be acquired for one or many semaphores at once; just pass an array!

locks.acquire(['hello', 'there'], function(err, release) {
	console.log('Hello');
	setTimeout(release, 1000);
});

locks.acquire('hello', function(err, release) {
	// Prints a second after 'Hello'
	console.log('there,');
	release();
});

locks.acquire('there', function(err, release) {
	// Prints a second after 'Hello' and immediately after 'there,'
	console.log('World!');
	release();
});

Release some locks earlier than others if you're finished with them:

locks.acquire(['hello', 'there'], function(err, release) {
	console.log('Hello');
	setTimeout(function() { release('there'); }, 1000);
	// Calling release() now is the same as release('hello')
	setTimeout(release, 2000);
});

locks.acquire('there', function(err, release) {
	// Prints a second after 'Hello'
	console.log('there,');
	release();
});

locks.acquire('hello', function(err, release) {
	// Prints two seconds after 'Hello'
	console.log('World!');
	release();
});

Specify when you can't wait for a lock:

locks.acquire('hello', function(err, release) {
	console.log('Hello');
	setTimeout(release, 1000);
});

locks.acquire('hello', {instant: true}, function(err, release) {
	// Output: Could not acquire all locks instantly
	console.log(err.message);
});

Avoid deadlocks with wait limits and TTLs:

locks.acquire('hello', {wait: 1000, ttl: 2000}, function(err, release) {
	// This will give us an 'err' if we waited more than 1 second and couldn't
	// get a lock.

	// If we do get a lock and this is called without an 'err', we have 2
	// seconds to finish before 'hello' is forcibly unlocked.  That does not
	// impede the execution of this function, however.
});

Allow semaphores to be held multiple times simultaneously:

locks.setMaxLocks('hello', 2);
locks.acquire('hello', function(err, release) {
	console.log('Hello');
	setTimeout(release, 1000);
});

locks.acquire('hello', function(err, release) {
	// Prints immediately after 'Hello' with no wait. 'hello' is held twice!
	console.log('World!');
	setTimeout(release, 1000);
});

Jump in line with priorities:

locks.acquire('hello', function(err, release) {
	console.log('Hello');
	locks.acquire('hello', {priority: 2}, function(err, release) {
    	console.log('World!');
    	release();
    });
    locks.acquire('hello', {priority: 1}, function(err, release) {
    	// Prints between "Hello" and "World!". The priority 1 jumps ahead of
    	// the priority 2 in line!
    	console.log('there,');
    	release();
    });
	release();
});

Cancel pending locks by grabbing the handle:

var handle = locks.acquire(['hello', 'world'], function(err, release) {
	console.log('This will never be seen.');
});

locks.cancel(handle);

You can use the handle to release locks, too:

var handle = locks.acquire('hello', function(err) {
    // Do something
});

setTimeout(function() {
	locks.release(handle);
}, 1000);

New Instances

Need more than one independent group of semaphores?

var Semlocks = require('semlocks').Semlocks;

var locks = new Semlocks();
var moreLocks = new Semlocks();

API

acquire(semaphore, [options], callback)

Requests the provided semaphore(s) and calls the callback when they're all locked. Returns a numeric handle for this request, which can be used to cancel it before the locks are obtained. Note that the callback is called in the tick immediately following the tick in which the last semaphore was locked, in order to ensure the handle is returned and available when the callback executes.

• semaphore string|array: A string or array of strings indicating which semaphores to lock before executing the callback
• [options] object: Optionally, an object specifying any of the following key/value pairs:
  • wait number: The number of milliseconds to wait for a lock. If this time limit is reached and the locks have not all been obtained, the callback is executed with an error argument. Default: unbounded
  • instant boolean: If true, the callback will be called with an error argument if the locks cannot be immediately obtained. Default: false
  • ttl number: The 'time-to-live': a number of milliseconds that a callback can take to release all its locks before they are forcibly released and a 'killed' event is fired. Note that this does not halt the execution of the callback. Default: unbounded
  • priority number: The priority of this request. Locks are awarded in priority order, with lower values coming first and higher values coming only after lower values have been serviced. Equal priorities are awarded in the order in which they were requested. Default: 2
• [callback] function: A callback to be executed when all the locks are obtained, or when the locks cannot be obtained due to the wait or instant options above. The callback is called in the tick immediately after the acquisition of the last semaphore, with the following arguments:
  • err Error|null: An error object, if locks could not be obtained due to the wait or instant options.
  • release([semaphore]) function: Releases the specified semaphore or array of semaphores. If no semaphore is provided, all semaphores belonging to this lock request are released.

cancel(handle, [err])

Forcibly releases any currently held locks for the specified handle, and removes it from the waiting lists for any locks it requested and has not yet acquired. This can be called for handles regardless of whether their callback has been executed or not.

• handle number: The handle of the request to be canceled, as it was returned by the acquire function.
• [err] Error: An error object. If specified, this will be passed to the callback function if it's not yet been executed. Otherwise, the callback function will not be called.

forceRelease(semaphore)

Forcibly releases all locks for the given semaphore by cycling through all handles currently holding a lock and calling release() for each. Note that, at the end of the call, the lock may be held by other requests that were waiting for this semaphore. To prevent the lock from being held again, call setMaxLocks() to set the available locks to 0 first.

• semaphore string: A string representing the semaphore to be forcibly released.

getLocks()

Gets a object mapping of all currently held semaphore names to the number of currently held locks on that semaphore. Note that this object is a snapshot only, and will not auto-update as more locks are acquired and released.

getMaxLocks([semaphore])

Gets the currently set max locks for a given semaphore. If semaphore is omitted, the default max locks will be returned.

• semaphore string: A string representing the semaphore whose max should be retrieved.

release(handle, [semaphore])

Releases all or some of a request's currently held semaphores.

• handle number: The handle of the request that owns the locks to be released.
• [semaphore] string|array: A semaphore or array of semaphores to be released. If omitted, all semaphores owned by this handle will be released.

setDefaultMaxLocks(max)

Sets the maximum number of times that semaphores without an explicitly defined max (see setMaxLocks below) can be simultaneously locked. By default, all semaphores are exclusive (max of 1). Note that 0 can be used to prevent any locks without explicit maximums from being granted.

• max number|null: The number of simultaneous locks to allow on all semaphores by default. If null, the max will revert to 1.

setMaxLocks(semaphore, max)

Sets the maximum number of times the specified semaphore can be simultaneously locked. By default, all semaphores are exclusive (max of 1). Note that 0 can be used to prevent a semaphore from being acquired until the max is changed. Raising the maximum will automatically trigger locks to be granted to the appropriate number of queued requests for this semaphore.

• semaphore string: A string representing the semaphore whose max should be changed.
• max number|null: The number of simultaneous locks for this semaphore. If null, the max will revert to following the default setting (see setDefaultMaxLocks above).

Events

Semlocks is an EventEmitter that fires the following events:

acquire (semaphore, handle)

Fires every time a lock is acquired by a new handle.

• semaphore string: The name of the semaphore that was locked
• handle number: The handle of the request that was granted the lock

acquire:SEMAPHORE (handle)

Fires every time "SEMAPHORE" (replaced with the name of the semaphore) is granted to a new handle.

• handle number: The handle of the request that was granted the lock

release (semaphore, handle)

Fires every time a lock is released by a handle.

• semaphore string: The name of the semaphore that was released
• handle number: The handle of the request that released the lock

release:SEMAPHORE (handle)

Fires every time "SEMAPHORE" (replaced with the name of the semaphore) is released.

• handle number: The handle of the request that released the lock

killed (handle)

Fires when locks are forcibly released from a handle because that request's TTL was reached.

• handle number: The handle of the request that was killed

Testing

Testing is easy! Just run the following from the project root:

npm test

License

Semlocks is distributed under the MIT license.

Credits

Semlocks was created by Tom Frost at Leadnomics in 2014.