To begin using Kazoo, a
KazooClient object must be
created and a connection established:
from kazoo.client import KazooClient zk = KazooClient(hosts='127.0.0.1:2181') zk.start()
By default, the client will connect to a local Zookeeper server on the default
port (2181). You should make sure Zookeeper is actually running there first,
start command will be waiting until its default timeout.
Once connected, the client will attempt to stay connected regardless of intermittent connection loss or Zookeeper session expiration. The client can be instructed to drop a connection by calling stop:
If logging is not setup for your application, you can get following message:
No handlers could be found for logger "kazoo.client"
To avoid this issue you can at the very minimum do the following:
import logging logging.basicConfig()
Read Python’s logging tutorial for more details.
Listening for Connection Events¶
It can be useful to know when the connection has been dropped, restored, or when the Zookeeper session has expired. To simplify this process Kazoo uses a state system and lets you register listener functions to be called when the state changes.
from kazoo.client import KazooState def my_listener(state): if state == KazooState.LOST: # Register somewhere that the session was lost elif state == KazooState.SUSPENDED: # Handle being disconnected from Zookeeper else: # Handle being connected/reconnected to Zookeeper zk.add_listener(my_listener)
When using the
kazoo.recipe.lock.Lock or creating ephemeral nodes, its
highly recommended to add a state listener so that your program can properly
deal with connection interruptions or a Zookeeper session loss.
Understanding Kazoo States¶
KazooState object represents several states
the client transitions through. The current state of the client can always be
determined by viewing the
state property. The
possible states are:
KazooClient instance is first created, it is in
the LOST state. After a connection is established it transitions to the
CONNECTED state. If any connection issues come up or if it needs to connect
to a different Zookeeper cluster node, it will transition to SUSPENDED to let
you know that commands cannot currently be run. The connection will also be
lost if the Zookeeper node is no longer part of the quorum, resulting in a
Upon re-establishing a connection the client could transition to LOST if the session has expired, or CONNECTED if the session is still valid.
These states should be monitored using a listener as described previously so that the client behaves properly depending on the state of the connection.
When a connection transitions to SUSPENDED, if the client is performing an action that requires agreement with other systems (using the Lock recipe for example), it should pause what it’s doing. When the connection has been re-established the client can continue depending on if the state is LOST or transitions directly to CONNECTED again.
When a connection transitions to LOST, any ephemeral nodes that have been created will be removed by Zookeeper. This affects all recipes that create ephemeral nodes, such as the Lock recipe. Lock’s will need to be re-acquired after the state transitions to CONNECTED again. This transition occurs when a session expires or when you stop the clients connection.
Valid State Transitions
LOST -> CONNECTED
New connection, or previously lost one becoming connected.
CONNECTED -> SUSPENDED
Connection loss to server occurred on a connection.
CONNECTED -> LOST
Only occurs if invalid authentication credentials are provided after the connection was established.
SUSPENDED -> LOST
Connection resumed to server, but then lost as the session was expired.
SUSPENDED -> CONNECTED
Connection that was lost has been restored.
New in version 0.6.
Zookeeper 3.4 and above supports a read-only mode. This mode
must be turned on for the servers in the Zookeeper cluster for the
client to utilize it. To use this mode with Kazoo, the
KazooClient should be called with the
read_only option set to True. This will let the client connect to
a Zookeeper node that has gone read-only, and the client will continue
to scan for other nodes that are read-write.
from kazoo.client import KazooClient zk = KazooClient(hosts='127.0.0.1:2181', read_only=True) zk.start()
A new attribute on
KeeperState has been
added, CONNECTED_RO. The connection states above are still valid,
however upon CONNECTED, you will need to check the clients non-
simplified state to see if the connection is CONNECTED_RO. For
from kazoo.client import KazooState from kazoo.client import KeeperState @zk.add_listener def watch_for_ro(state): if state == KazooState.CONNECTED: if zk.client_state == KeeperState.CONNECTED_RO: print("Read only mode!") else: print("Read/Write mode!")
It’s important to note that a KazooState is passed in to the listener but the read-only information is only available by comparing the non-simplified client state to the KeeperState object.
A client using read-only mode should not use any of the recipes.
Zookeeper includes several functions for creating, reading, updating, and deleting Zookeeper nodes (called znodes or nodes here). Kazoo adds several convenience methods and a more Pythonic API.
ensure_path() will recursively create the node
and any nodes in the path necessary along the way, but can not set the data for
the node, only the ACL.
create() creates a node and can set the data on
the node along with a watch function. It requires the path to it to exist
first, unless the makepath option is set to True.
# Ensure a path, create if necessary zk.ensure_path("/my/favorite") # Create a node with data zk.create("/my/favorite/node", b"a value")
exists() checks to see if a node exists.
get() fetches the data of the node along with
detailed node information in a
get_children() gets a list of the children of
a given node.
# Determine if a node exists if zk.exists("/my/favorite"): # Do something # Print the version of a node and its data data, stat = zk.get("/my/favorite") print("Version: %s, data: %s" % (stat.version, data.decode("utf-8"))) # List the children children = zk.get_children("/my/favorite") print("There are %s children with names %s" % (len(children), children))
set() updates the data for a given node. A
version for the node can be supplied, which will be required to match before
updating the data, or a
BadVersionError will be
raised instead of updating.
zk.set("/my/favorite", b"some data")
delete() deletes a node, and can optionally
recursively delete all children of the node as well. A version can be
supplied when deleting a node which will be required to match the version of
the node before deleting it or a
will be raised instead of deleting.
Connections to Zookeeper may get interrupted if the Zookeeper server goes down
or becomes unreachable. By default, kazoo does not retry commands, so these
failures will result in an exception being raised. To assist with failures
kazoo comes with a
retry() helper that will
retry a function should one of the Zookeeper connection exceptions get raised.
result = zk.retry(zk.get, "/path/to/node")
Some commands may have unique behavior that doesn’t warrant automatic retries
on a per command basis. For example, if one creates a node a connection might
be lost before the command returns successfully but the node actually got
created. This results in a
raised when it runs again. A similar unique situation arises when a node is
created with ephemeral and sequence options set,
documented here on the Zookeeper site.
retry() method takes a function to
call and its arguments, a function that runs multiple Zookeeper commands could
be passed to it so that the entire function will be retried if the connection
This snippet from the lock implementation shows how it uses retry to re-run the function acquiring a lock, and checks to see if it was already created to handle this condition:
# kazoo.recipe.lock snippet def acquire(self): """Acquire the mutex, blocking until it is obtained""" try: self.client.retry(self._inner_acquire) self.is_acquired = True except KazooException: # if we did ultimately fail, attempt to clean up self._best_effort_cleanup() self.cancelled = False raise def _inner_acquire(self): self.wake_event.clear() # make sure our election parent node exists if not self.assured_path: self.client.ensure_path(self.path) node = None if self.create_tried: node = self._find_node() else: self.create_tried = True if not node: node = self.client.create(self.create_path, self.data, ephemeral=True, sequence=True) # strip off path to node node = node[len(self.path) + 1:]
create_tried records whether it has tried to create the node already in the event the connection is lost before the node name is returned.
Sometimes you may wish to have specific retry policies for a command or
set of commands that differs from the
retry() method. You can manually create
KazooRetry instance with the specific retry
policy you prefer:
from kazoo.retry import KazooRetry kr = KazooRetry(max_tries=3, ignore_expire=False) result = kr(client.get, "/some/path")
This will retry the
client.get command up to 3 times, and raise a
session expiration if it occurs. You can also make an instance with the
default behavior that ignores session expiration during a retry.
Kazoo can set watch functions on a node that can be triggered either when the node has changed or when the children of the node change. This change to the node or children can also be the node or its children being deleted.
Watchers can be set in two different ways, the first is the style that Zookeeper supports by default for one-time watch events. These watch functions will be called once by kazoo, and do not receive session events, unlike the native Zookeeper watches. Using this style requires the watch function to be passed to one of these methods:
A watch function passed to
exists() will be called when the data on the
node changes or the node itself is deleted. It will be passed a
def my_func(event): # check to see what the children are now # Call my_func when the children change children = zk.get_children("/my/favorite/node", watch=my_func)
Kazoo includes a higher level API that watches for data and children
modifications that’s easier to use as it doesn’t require re-setting the watch
every time the event is triggered. It also passes in the data and
ZnodeStat when watching a node or the list of
children when watching a nodes children. Watch functions registered with this
API will be called immediately and every time there’s a change, or until the
function returns False. If allow_session_lost is set to True, then the
function will no longer be called if the session is lost.
The following methods provide this functionality:
These classes are available directly on the
instance and don’t require the client object to be passed in when used in this
manner. The instance returned by instantiating either of the classes can be
called directly allowing them to be used as decorators:
@zk.ChildrenWatch("/my/favorite/node") def watch_children(children): print("Children are now: %s" % children) # Above function called immediately, and from then on @zk.DataWatch("/my/favorite") def watch_node(data, stat): print("Version: %s, data: %s" % (stat.version, data.decode("utf-8")))
New in version 0.6.
Zookeeper 3.4 and above supports the sending of multiple commands at once that will be committed as a single atomic unit. Either they will all succeed or they will all fail. The result of a transaction will be a list of the success/failure results for each command in the transaction.
transaction = zk.transaction() transaction.check('/node/a', version=3) transaction.create('/node/b', b"a value") results = transaction.commit()
transaction() method returns a
TransactionRequest instance. It’s methods may be
called to queue commands to be completed in the transaction. When the
transaction is ready to be sent, the
commit() method on it is called.
In the example above, there’s a command not available unless a transaction is being used, check. This can check nodes for a specific version, which could be used to make the transaction fail if a node doesn’t match a version that it should be at. In this case the node /node/a must be at version 3 or /node/b will not be created.