sherlock 0.3.0

Sherlock Distributed Locks with a choice of backend
===================================================

Sherlock is a library that provides easy-to-use distributed inter-process
locks and also allows you to choose a backend of your choice for lock
synchronization.

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Overview
--------

When you are working with resources which are accessed by multiple services or
distributed services, more than often you need some kind of locking mechanism
to make it possible to access some resources at a time.

Distributed Locks or Mutexes can help you with this. Sherlock provides
the exact same facility, with some extra goodies. It provides an easy-to-use API
that resembles standard library's `threading.Lock` semantics.

Apart from this, Sherlock gives you the flexibilty of using a backend of
your choice for managing locks.

Sherlock also makes it simple for you to extend Sherlock to use
backends that are not supported.

Features
++++++++

* API similar to standard library's `threading.Lock`.
* Support for With statement, to cleanly acquire and release locks.
* Backend agnostic: supports `Redis`_, `Memcached`_ and `Etcd`_ as choice of
backends.
* Extendable: can be easily extended to work with any other of backend of
choice by extending base lock class. Read :ref:`extending`.

.. _Redis: http://redis.io
.. _Memcached: http://memcached.org
.. _Etcd: http://github.com/coreos/etcd

Supported Backends and Client Libraries
+++++++++++++++++++++++++++++++++++++++

Following client libraries are supported for every supported backend:

* Redis: `redis-py`_
* Memcached: `pylibmc`_
* Etcd: `python-etcd`_

.. _redis-py: http://github.com
.. _pylibmc: http://github.com
.. _python-etcd: https://github.com/jplana/python-etcd

As of now, only the above mentioned libraries are supported. Although
Sherlock takes custom client objects so that you can easily provide
settings that you want to use for that backend store, but Sherlock also
checks if the provided client object is an instance of the supported clients
and accepts client objects which pass this check, even if the APIs are the
same. Sherlock might get rid of this issue later, if need be and if
there is a demand for that.

Installation
------------

Installation is simple.

.. code:: bash

pip install sherlock

.. note:: Sherlock will install all the client libraries for all the
supported backends.

Basic Usage
-----------

Sherlock is simple to use as at the API and semantics level, it tries to
conform to standard library's :mod:`threading.Lock` APIs.

.. code-block:: python

import sherlock
from sherlock import Lock

# Configure Sherlock's locks to use Redis as the backend,
# never expire locks and retry acquiring an acquired lock after an
# interval of 0.1 second.
sherlock.configure(backend=sherlock.backends.REDIS,
expire=None,
retry_interval=0.1)

# Note: configuring sherlock to use a backend does not limit you
# another backend at the same time. You can import backend specific locks
# like RedisLock, MCLock and EtcdLock and use them just the same way you
# use a generic lock (see below). In fact, the generic Lock provided by
# sherlock is just a proxy that uses these specific locks under the hood.

# acquire a lock called my_lock
lock = Lock('my_lock')

# acquire a blocking lock
lock.acquire()

# check if the lock has been acquired or not
lock.locked() == True

# release the lock
lock.release()

Support for ``with`` statement
++++++++++++++++++++++++++++++

.. code-block:: python

# using with statement
with Lock('my_lock'):
# do something constructive with your locked resource here
pass

Blocking and Non-blocking API
+++++++++++++++++++++++++++++

.. code-block:: python

# acquire non-blocking lock
lock1 = Lock('my_lock')
lock2 = Lock('my_lock')

# successfully acquire lock1
lock1.acquire()

# try to acquire lock in a non-blocking way
lock2.acquire(False) == True # returns False

# try to acquire lock in a blocking way
lock2.acquire() # blocks until lock is acquired to timeout happens

Using two backends at the same time
+++++++++++++++++++++++++++++++++++

Configuring Sherlock to use a backend does not limit you from using
another backend at the same time. You can import backend specific locks like
RedisLock, MCLock and EtcdLock and use them just the same way you use a generic
lock (see below). In fact, the generic Lock provided by Sherlock is just
a proxy that uses these specific locks under the hood.

.. code-block:: python

import sherlock
from sherlock import Lock

# Configure Sherlock's locks to use Redis as the backend
sherlock.configure(backend=sherlock.backends.REDIS)

# Acquire a lock called my_lock, this lock uses Redis
lock = Lock('my_lock')

# Now acquire locks in Memcached
from sherlock import MCLock
mclock = MCLock('my_mc_lock')
mclock.acquire()

Tests
-----

To run all the tests (including integration), you have to make sure that all
the databases are running. Make sure all the services are running:

.. code:: bash

# memcached
memcached

# redis-server
redis-server

# etcd (etcd is probably not available as package, here is the simplest way
# to run it).
wget https://github.com/coreos/etcd/releases/download/<version>/etcd-<version>-<platform>.tar.gz
tar -zxvf etcd-<version>-<platform>.gz
./etcd-<version>-<platform>/etcd

Run tests like so:

.. code:: bash

python setup.py test

Documentation
-------------

Available `here`_.

.. _here: http://sher-lock.readthedocs.org

Roadmap
-------

* Support for `Zookeeper`_ as backend.
* Support for `Gevent`_, `Multithreading`_ and `Multiprocessing`_.

.. _Zookeeper: http://zookeeper.apache.org/
.. _Gevent: http://www.gevent.org/
.. _Multithreading: http://docs.python.org/2/library/multithreading.html
.. _Multiprocessing: http://docs.python.org/2/library/multiprocessing.html

License
-------

See `LICENSE`_.

.. _LICENSE: http://github.com/vaidik/sherlock/blob/master/LICENSE.rst