locknix - NFS-safe file locking with timeouts for POSIX systems
Copyright (C) 2007-2010 by Barry A. Warsaw
Licensed under the GNU Lesser General Public License

Introduction
============

This code implements an NFS-safe file-based locking algorithm influenced by
the GNU/Linux open(2) manpage, under the description of the O_EXCL option:

        [...] O_EXCL is broken on NFS file systems, programs which rely on it
        for performing locking tasks will contain a race condition.  The
        solution for performing atomic file locking using a lockfile is to
        create a unique file on the same fs (e.g., incorporating hostname and
        pid), use link(2) to make a link to the lockfile.  If link() returns
        0, the lock is successful.  Otherwise, use stat(2) on the unique file
        to check if its link count has increased to 2, in which case the lock
        is also successful.

The assumption made here is that there will be no 'outside interference',
e.g. no agent external to this code will ever link() to the specific lock
files used.

Lock objects support lock-breaking so that you can't wedge a process forever.
This is especially helpful in a web environment, but may not be appropriate
for all applications.

Locks have a 'lifetime', which is the maximum length of time the process
expects to retain the lock.  It is important to pick a good number here
because other processes will not break an existing lock until the expected
lifetime has expired.  Too long and other processes will hang; too short and
you'll end up trampling on existing process locks -- and possibly corrupting
data.  In a distributed (NFS) environment, you also need to make sure that
your clocks are properly synchronized.


Requirements
============

locknix requires Python 2.6 or newer.


Project details
===============

You may download and the latest version of the package from the Python
Package Index.  See http://pypi.python.org for details.

Or, if you have ez_install installed, you can just do the following:

    % sudo easy_install locknix

You can grab the latest development copy of the code using Bazaar.  See
http://bazaar-vcs.org for details on the Bazaar distributed revision control
system.  If you have Bazaar installed, you can grab your own branch of the
code like this:

     bzr branch sftp://bazaar.launchpad.net/~barry/locknix/trunk locknix

You may contact the author via barry@python.org.
