*************************
Basic inter-process locks
*************************

The zc.lockfile package provides a basic portable implementation of
interprocess locks using lock files.  The purpose if not specifically
to lock files, but to simply provide locks with an implementation
based on file-locking primitives.  Of course, these locks could be
used to mediate access to *other* files.  For example, the ZODB file
storage implementation uses file locks to mediate access to
file-storage database files.  The database files and lock file files
are separate files.

.. contents::

Detailed Documentation
**********************

Lock file support
=================

The ZODB lock_file module provides support for creating file system
locks.  These are locks that are implemented with lock files and
OS-provided locking facilities.  To create a lock, instantiate a
LockFile object with a file name:

    >>> import zc.lockfile
    >>> lock = zc.lockfile.LockFile('lock')

If we try to lock the same name, we'll get a lock error:

    >>> import zope.testing.loggingsupport
    >>> handler = zope.testing.loggingsupport.InstalledHandler('zc.lockfile')
    >>> try:
    ...     zc.lockfile.LockFile('lock')
    ... except zc.lockfile.LockError:
    ...     print("Can't lock file")
    Can't lock file

    >>> for record in handler.records: # doctest: +ELLIPSIS
    ...     print(record.levelname+' '+record.getMessage())
    ERROR Error locking file lock; pid=...

To release the lock, use it's close method:

    >>> lock.close()

The lock file is not removed.  It is left behind:

    >>> import os
    >>> os.path.exists('lock')
    True

Of course, now that we've released the lock, we can create it again:

    >>> lock = zc.lockfile.LockFile('lock')
    >>> lock.close()

.. Cleanup

    >>> import os
    >>> os.remove('lock')

Hostname in lock file
=====================

In a container environment (e.g. Docker), the PID is typically always
identical even if multiple containers are running under the same operating
system instance.

Clearly, inspecting lock files doesn't then help much in debugging. To identify
the container which created the lock file, we need information about the
container in the lock file. Since Docker uses the container identifier or name
as the hostname, this information can be stored in the lock file in addition to
or instead of the PID.

Use the ``content_template`` keyword argument to ``LockFile`` to specify a
custom lock file content format:

    >>> lock = zc.lockfile.LockFile('lock', content_template='{pid};{hostname}')
    >>> lock.close()

If you now inspected the lock file, you would see e.g.:

    $ cat lock
     123;myhostname


Change History
***************

1.2.0 (2016-06-09)
==================

- Added the ability to include the hostname in the lock file content.

- Code and ReST markup cosmetics.
  [alecghica]

1.1.0 (2013-02-12)
==================

- Added Trove classifiers and made setup.py zest.releaser friendly.

- Added Python 3.2, 3.3 and PyPy 1.9 support.

- Removed Python 2.4 and Python 2.5 support.

1.0.2 (2012-12-02)
==================

- Fixed: the fix included in 1.0.1 caused multiple pids to be written
  to the lock file

1.0.1 (2012-11-30)
==================

- Fixed: when there was lock contention, the pid in the lock file was
  lost.

  Thanks to Daniel Moisset reporting the problem and providing a fix
  with tests.

- Added test extra to declare test dependency on ``zope.testing``.

- Using Python's ``doctest`` module instead of depreacted
  ``zope.testing.doctest``.

1.0.0 (2008-10-18)
==================

- Fixed a small bug in error logging.

1.0.0b1 (2007-07-18)
====================

- Initial release

Download
**********************
