zk-lock

Distributed locking using node-zookeeper-client, following the recipe in the zookeeper documentation.

The library exposes a single class, ZookeeperLock which is both the lock object, and provides static methods for initializing the configuration and acquiring locks which are already locked.

All locks are stored in a top level zookeeper folder locks/, can have an optional path prefix, and take a key argument which indicates the resource that should be locked.

Locks must be explicitly released with the unlock method in order to allow other processes to obtain the lock. unlock will also close the zookeeper connection.

Configuration

serverLocator: () => Promise<any> serverLocator is a service discovery/location resolving library such as locators, or anything which implements a function which takes no arguments and can return a promise of a Location object that has properties host and port which specify where the zookeeper server can be discovered.

pathPrefix: string This is a prefix to add to any key which is to be locked. Note: all locks will start with the path locks/, so with a prefix of foo/bar, when locking the key baz the zookeeper path will be of the form locks/foo/bar/baz.

sessionTimeout: number node-zookeeper-client sessionTimeout parameter that is passed down to the zookeeper client.

spinDelay: number node-zookeeper-client spinDelay parameter that is passed down to the zookeeper client.

retries: number node-zookeeper-client retries parameter that is passed down to the zookeeper client.

failImmediate: boolean when set to true, instead of a blocking call to .lock, it will fail immediately throwing a ZookeeperLockAlreadyLockedError

maxConcurrentHolders: number allow multiple calls to .lock to grab a lock, allowing the lock to control the number of participants in an action

ZookeeperLock

constructor

usage:

	var lock = new ZookeeperLock(config);
  

Static methods

initialize

usage: ZookeeperLock.initialize(config);

Initialize a global configuration for zookeeper locks.

lockFactory

usage:

	var lock = ZookeeperLock.lockFactory();

lock

usage:

	ZookeeperLock.lock('key').then(function (lock) {
    	... do stuff
  	});

Instance methods

lock

usage:

    var lock = ZookeeperLock.lockFactory();
    lock.lock('key').then(function() {
    	... do stuff
  	});

unlock

usage;

    var lock = ZookeeperLock.lockFactory();
    lock.lock('key').then(function() {
    	... do stuff
    	lock.unlock().then(function () {
    	    ... 
    	});
    });

Session timeouts

Due to the nature of zookeeper, this locking strategy is susceptible to a situation when sessionTimeout occurs that a lock holder can still think that they own the lock, but the key has been lost on the zookeeper server. To handle this, ZookeeperLock exposes a signal property that emits a lost event when a sessionTimeout occurs, which MUST be handled by the lock holder to terminate execution.

usage:

    var lock = ZookeeperLock.lockFactory();
    lock.lock('key').then(function() {
    	lock.on('lost', function () {
    		... handle losing the lock, i.e. restart locking cycle.
    	});
    });

Development

npm run build to compile.

The tests currently require a local installation of zookeeper to run successfully, specified by the zkServerCommandPath variable which is defined near the beginning of the tests.