hurt-locker

General

Having a pessimistic lock mechanism isn't a pessimistic thing. It's awesome. You can keep other threads out of your business with a nice critical section. You just have to be careful to unlock before you return. And be careful about deadlocks. And giant mice. With great big pointy teeth.

OK, maybe not so much those but it's nice to have a critical section capability. I hear you saying "but Node is single threaded! I don't need no steengkeeng creeteecal sections!" Well, you do if you need to keep other processes at bay. This library allows you to do that but only on a single machine. It's compatible with app-cluster and any other Node processes running on the same box. Think of it as a synchronized block in Java or a lock block in C#.

Here's how you can use it to obtain and release a lock:

var LockManager = require('hurt-locker').LockManager;
.
.
.
var lockManager = new LockManager();

// Ask for a lock called 'my-lock'.  Supply an owner object and a timeout of one second.  If we don't get the lock
// in the timeout period, we'll get an error.
var lockResult = lockManager.obtainExclusiveLock('my-lock', {name: 'bob'}, 1000);

// Now wait for the lock.
lockResult.when(function(err, lockName, owner, elapsedTimeMillis)
{
    // err -> undefined if everything went well, otherwise and Error object that contains a description of the problem.
    // lockName -> The name of the lock.
    // owner ->  The supplied owner object if you got the lock.  The actual owner if you didn't.
    // elapsedTimeMillis -> The amount of time since you originally requested the lock.  Approximate.
    if (err)
    {
        // We didn't get the lock!  We must do our best to handle our disappointment!
        return;
    }

    // Do something in the critical section, like drink tea or eat cucumber sandwiches or something.

    // Unlock.
    var releaseResult = lockManager.releaseExclusiveLock(lockName, owner);
    releaseResult.when(function(err, lockName, owner)
    {
        // err -> undefined if the release worked.  An error if not.
        // lockName -> The name of the lock that was released (or failed to release).
        // owner -> If the release worked, will be the same as the owner you passed to the release method.
        //          If the release failed, will be the actual owner or undefined if there is no actual owner.
        if (err)
        {
            // Something went wrong when releasing the lock!
        }
    });
});

The lock semantics are pretty straight forward: You either get the lock in the supplied timeout period or you don't. The 'owner' object is an arbitrary object. It can have any format, be anything at all (well, anything that can be translated to JSON using JSON.stringify, which means that you can't have cyclic properties). Best to pick something that looks good in the logs. The timeout period is in milliseconds and is, like all Javascript timings, approximate.

The release semantics are not so simple because the release call will fail if you try to release a lock with the wrong owner object. That's why the 'owner' parameter can change based on the error and may even be undefined if a low-level problem occurs.

Under The Covers

The lock manager uses files to obtain locks. Each lock corresponds to a file descriptor open for read-exclusive access. When something gets a lock and something else asks for it, the lock manager internally kicks off a setTimeout call to re-check for the lock. After enough of these have gone by with no success, the LockManager fulfills the Future result with an error. Lock requests are not served in sequential order. If you wait long enough, and the previous holders are sensible and release the lock you're waiting for, you'll get it. Eventually.

Configuration

The lock manager uses a config object to allow configuration. Here's the rundown:

// Get the config module
var lmConfig = require('hurt-locker').config;

createDefaultConfig()

This method creates the default config object. So:

var config = lmConfig.createDefaultConfig();

generates an object that looks like this:

{
    lockDir: './locks',
    lockFileSuffix: '.lock',
    lockRetryTimeMillis: 100
}

lockDir: The directory in which the lock files should be written. lockFileSuffix: The file extension to use for each lock file. lockRetryTimeMillis: The amount of time to wait between successive lock retry attempts.

This:

var lockManager = new LockManager();

is the equivalent of this:

var lockManager = new LockManager(lmConfig.createDefaultConfig());

Customizing Config

The LockManager always uses a default configuration and overlays supplied configuration information. You can customize the LockManager configuration by sparsely populating a JSON object upon construction:

var lmConfig = require('hurt-locker').config;
var LockManager = require('hurt-locker').LockManager;

var lockManager = new LockManager({
    lockRetryTimeMillis: 1000
});

You can, of course, just construct a LockManager and then tweak its settings:

lockManager.settings.lockDir = './another-lock-dir';

Just be sure not to do that while the lock manager is being used. Bad things will happen then. They'll probably involve dragons.

Known Issues

o There seems to be a problem obtaining a lock when the lock directory path contains spaces. Looks like it may be a node.js or OSX issue.