Add docs for Zookeeper Data Store.

Author: GaneshChandrasekaran-zz

components/lock.rst

@@ -175,6 +175,7 @@ Store                                         Scope   Blocking  Expiring
 :ref:`MemcachedStore <lock-store-memcached>`  remote  no        yes
 :ref:`RedisStore <lock-store-redis>`          remote  no        yes
 :ref:`SemaphoreStore <lock-store-semaphore>`  local   yes       no
+:ref:`ZookeeperStore <lock-store-zookeeper>`  remote  no        no
 ============================================  ======  ========  ========
 
 .. _lock-store-flock:
@@ -281,6 +282,28 @@ the stores.
     working when a single server fails (because this strategy requires that the
     lock is acquired in more than half of the servers).
 
+.. _lock-store-zookeeper:
+
+ZookeeperStore
+~~~~~~~~~~~~~~
+
+The ZookeeperStore saves locks on a Zookeeper server, it requires a Zookeeper
+connection implementing the ``\Zookeeper`` class. This store does not
+support blocking and expiration but the lock is automatically released when the
+PHP process is terminated
+
+    use Symfony\Component\Lock\Store\ZookeeperStore;
+
+    $zookeeper_server = 'localhost:2181'; // For High Availablity Cluster you can pass 'localhost1:2181,localhost2:2181,localhost3:2181'
+    $zookeeper = new \Zookeeper($zookeeper_server);
+
+    $store = new ZookeeperStore($zookeeper);
+
+.. note::
+
+    Zookeeper does not require a TTL as the nodes used for locking are ephemeral and die when the PHP process is terminated.
+
+
 Reliability
 -----------
 
@@ -290,8 +313,8 @@ the component is used in the following way.
 Remote Stores
 ~~~~~~~~~~~~~
 
-Remote stores (:ref:`MemcachedStore <lock-store-memcached>` and
-:ref:`RedisStore <lock-store-redis>`) use an unique token to recognize the true
+Remote stores (:ref:`MemcachedStore <lock-store-memcached>`,
+:ref:`RedisStore <lock-store-redis>` and :ref:`RedisStore <lock-store-zookeeper>`) use an unique token to recognize the true
 owner of the lock. This token is stored in the
 :class:`Symfony\\Component\\Lock\\Key` object and is used internally by the
 ``Lock``, therefore this key must not be shared between processes (session,
@@ -493,6 +516,22 @@ can be two running containers in parallel.
     concurrent process on a new machine, check that other process are stopped
     on the old one.
 
+ZookeeperStore
+~~~~~~~~~~
+
+The way ZookeeperStore works is by maintaining locks in ephemeral nodes within PHP_LOCK_NODE on the server. That means that by using
+the :ref:`ZookeeperStore <lock-store-zookeeper>` the locks will be automatically released at the end of the session
+in case the client cannot unlock for any reason.
+
+If the Zookeeper service or the machine hosting it restarts, every lock would
+be lost without notifying the running processes.
+
+.. tip::
+
+    To use Zookeeper's High Availability feature, you can setup a cluster of multiple servers so that in case one of
+    the server goes down, the majority will still be up and serving the requests. All the available servers in the
+    cluster will see the same state.
+
 Overall
 ~~~~~~~