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"""Zookeeper Locking Implementations
Error Handling
==============
It's highly recommended to add a state listener with
:meth:`~KazooClient.add_listener` and watch for
:attr:`~KazooState.LOST` and :attr:`~KazooState.SUSPENDED` state
changes and re-act appropriately. In the event that a
:attr:`~KazooState.LOST` state occurs, its certain that the lock
has been lost.
"""
import uuid
from kazoo.retry import ForceRetryError
from kazoo.exceptions import CancelledError
from kazoo.exceptions import NoNodeException
class Lock(object):
"""Kazoo Basic Lock
Example usage with a :class:`~kazoo.client.KazooClient` instance::
zk = KazooClient()
lock = zk.Lock("/lockpath", "my-identifier")
with lock: # blocks waiting for lock acquisition
# do something with the lock
"""
_NODE_NAME = '__lock__'
def __init__(self, client, path, identifier=None):
"""Create a Kazoo Lock
:param client: A :class:`~kazoo.client.KazooClient` instance
:param path: The lock path to use
:param identifier: Name to use for this lock contender. This
can be useful for querying to see who the
current lock contenders are.
"""
self.client = client
self.path = path
# some data is written to the node. this can be queried via
# contenders() to see who is contending for the lock
self.data = str(identifier or "")
self.wake_event = client.handler.event_object()
# props to Netflix Curator for this trick. It is possible for our
# create request to succeed on the server, but for a failure to
# prevent us from getting back the full path name. We prefix our
# lock name with a uuid and can check for its presence on retry.
self.prefix = uuid.uuid4().hex + self._NODE_NAME
self.create_path = self.path + "/" + self.prefix
self.create_tried = False
self.is_acquired = False
self.assured_path = False
self.cancelled = False
def cancel(self):
"""Cancel a pending lock acquire"""
self.cancelled = True
self.wake_event.set()
def acquire(self):
"""Acquire the mutex, blocking until it is obtained"""
try:
self.client.retry(self._inner_acquire)
self.is_acquired = True
except Exception:
# 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:]
self.node = node
while True:
# bail out with an exception if cancellation has been requested
if self.cancelled:
raise CancelledError()
children = self._get_sorted_children()
try:
our_index = children.index(node)
except ValueError:
# somehow we aren't in the children -- probably we are
# recovering from a session failure and our ephemeral
# node was removed
raise ForceRetryError()
#noinspection PySimplifyBooleanCheck
if our_index == 0:
# we have the lock
return True
# otherwise we are in the mix. watch predecessor and bide our time
predecessor = self.path + "/" + children[our_index - 1]
if self.client.exists(predecessor, self._watch_predecessor):
self.wake_event.wait()
def _watch_predecessor(self, event):
self.wake_event.set()
def _get_sorted_children(self):
children = self.client.get_children(self.path)
# can't just sort directly: the node names are prefixed by uuids
lockname = self._NODE_NAME
children.sort(key=lambda c: c[c.find(lockname) + len(lockname):])
return children
def _find_node(self):
children = self.client.get_children(self.path)
for child in children:
if child.startswith(self.prefix):
return child
return None
def _best_effort_cleanup(self):
try:
node = self._find_node()
if node:
self.client.delete(self.path + "/" + node)
except Exception:
pass
def release(self):
"""Release the mutex immediately"""
return self.client.retry(self._inner_release)
def _inner_release(self):
if not self.is_acquired:
return False
self.client.delete(self.path + "/" + self.node)
self.is_acquired = False
self.node = None
return True
def contenders(self):
"""Return an ordered list of the current contenders for the lock
.. note::
If the contenders did not set an identifier, it will appear
as a blank string.
"""
# make sure our election parent node exists
if not self.assured_path:
self.client.ensure_path(self.path)
children = self._get_sorted_children()
contenders = []
for child in children:
try:
data, stat = self.client.get(self.path + "/" + child)
contenders.append(data)
except NoNodeException:
pass
return contenders
def __enter__(self):
self.acquire()
def __exit__(self, exc_type, exc_value, traceback):
self.release()
|
