proper-lockfile

A lockfile utility based on fs that works cross process and machine (network file systems).

Installation

$ npm install proper-lockfile --save

Design

There are various ways to achieve file locking.

This library utilizes the mkdir strategy which works atomically on any kind of file system, even network based ones. The lockfile path is based on the file path you are trying to lock by suffixing it with .lock.

When a lock is successfully acquired, the lockfile's mtime (modified time) is periodically updated to prevent staleness. This allows to effectively check if a lock is stale by checking its mtime against a stale threshold. If the update of the mtime fails several times, the lock might be compromised.

Comparison

This library is similar to lockfile but the later has some drawbacks:

• It relies on open with O_EXCL flag which has problems in network file systems. proper-lockfile uses mkdir which doesn't have this issue.

O_EXCL is broken on NFS file systems; programs which rely on it for performing locking tasks will contain a race condition.

• The lockfile staleness check is done via creation time, which is unsuitable for long running processes. proper-lockfile constantly updates lockfiles mtime to do proper staleness check.

• It does not check if the lockfile was compromised, which can led to undesirable situations. proper-lockfile checks the lockfile when updating the mtime.

Usage

.lock(file, [options], callback, [compromised])

Tries to acquire a lock on file.

If the lock succeeds, an unlock function is given that should be called when you want to release the lock.
If the lock get compromised, the provided compromised function will be called (optionally).

Available options:

• stale: Duration in milliseconds in which the lock is considered stale, defaults to 10000 (false to disable)
• update: The interval in which the lockfile's mtime will be updated, defaults to 5000
• retries: The number of retries or a retry options object, defaults to 0
• resolve: Resolve to a canonical path to handle relative paths & symlinks properly, defaults to true
• fs: A custom fs to use, defaults to graceful-fs

var lockfile = require('proper-lockfile');

lockfile.lock('some/file', function (err, unlock) {
    if (err) {
        throw err;      // Lock failed
    }

    // Do something while the file is locked

    // Call the provided unlock function when you're done
    // Note that you can optionally handle unlock errors
    unlock(/* function (err) {
        if (err) {
            throw err;  // Unlock failed
        }

        // Lock is released
    }*/)
}, function (err) {
    // If we get here, the lock has been compromised
    // e.g.: the lock has been manually deleted
});

.remove(file, [options], callback)

Removes a lock.

You should NOT call this function to unlock a lockfile that isn't owned by you. This function is an alternative to the provided unlock function (as explained above) and you should ONLY call it if you own the lock.

Available options:

• resolve: Resolve to a canonical path to handle relative paths & symlinks properly, defaults to true
• fs: A custom fs to use, defaults to graceful-fs

var lockfile = require('proper-lockfile');

lockfile.remove('some/file', function (err, unlock) {
    if (err) {
        throw err;  // Removal failed
    }
});

Tests

Simply run the test suite with $ npm test

The test suite is very extensive. We even have a stress test to guarantee exclusiveness of locks.

License

Released under the MIT License.