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"""Zookeeper Barriers
:Maintainer: None
:Status: Unknown
"""
import os
import socket
import uuid
from kazoo.exceptions import KazooException, NoNodeError, NodeExistsError
from kazoo.protocol.states import EventType
class Barrier(object):
"""Kazoo Barrier
Implements a barrier to block processing of a set of nodes until
a condition is met at which point the nodes will be allowed to
proceed. The barrier is in place if its node exists.
.. warning::
The :meth:`wait` function does not handle connection loss and
may raise :exc:`~kazoo.exceptions.ConnectionLossException` if
the connection is lost while waiting.
"""
def __init__(self, client, path):
"""Create a Kazoo Barrier
:param client: A :class:`~kazoo.client.KazooClient` instance.
:param path: The barrier path to use.
"""
self.client = client
self.path = path
def create(self):
"""Establish the barrier if it doesn't exist already"""
self.client.retry(self.client.ensure_path, self.path)
def remove(self):
"""Remove the barrier
:returns: Whether the barrier actually needed to be removed.
:rtype: bool
"""
try:
self.client.retry(self.client.delete, self.path)
return True
except NoNodeError:
return False
def wait(self, timeout=None):
"""Wait on the barrier to be cleared
:returns: True if the barrier has been cleared, otherwise
False.
:rtype: bool
"""
cleared = self.client.handler.event_object()
def wait_for_clear(event):
if event.type == EventType.DELETED:
cleared.set()
exists = self.client.exists(self.path, watch=wait_for_clear)
if not exists:
return True
cleared.wait(timeout)
return cleared.is_set()
class DoubleBarrier(object):
"""Kazoo Double Barrier
Double barriers are used to synchronize the beginning and end of
a distributed task. The barrier blocks when entering it until all
the members have joined, and blocks when leaving until all the
members have left.
.. note::
You should register a listener for session loss as the process
will no longer be part of the barrier once the session is
gone. Connection losses will be retried with the default retry
policy.
"""
def __init__(self, client, path, num_clients, identifier=None):
"""Create a Double Barrier
:param client: A :class:`~kazoo.client.KazooClient` instance.
:param path: The barrier path to use.
:param num_clients: How many clients must enter the barrier to
proceed.
:type num_clients: int
:param identifier: An identifier to use for this member of the
barrier when participating. Defaults to the
hostname + process id.
"""
self.client = client
self.path = path
self.num_clients = num_clients
self._identifier = identifier or '%s-%s' % (
socket.getfqdn(), os.getpid())
self.participating = False
self.assured_path = False
self.node_name = uuid.uuid4().hex
self.create_path = self.path + "/" + self.node_name
def enter(self):
"""Enter the barrier, blocks until all nodes have entered"""
try:
self.client.retry(self._inner_enter)
self.participating = True
except KazooException:
# We failed to enter, best effort cleanup
self._best_effort_cleanup()
self.participating = False
def _inner_enter(self):
# make sure our barrier parent node exists
if not self.assured_path:
self.client.ensure_path(self.path)
self.assured_path = True
ready = self.client.handler.event_object()
try:
self.client.create(
self.create_path,
self._identifier.encode('utf-8'), ephemeral=True)
except NodeExistsError:
pass
def created(event):
if event.type == EventType.CREATED:
ready.set()
self.client.exists(self.path + '/' + 'ready', watch=created)
children = self.client.get_children(self.path)
if len(children) < self.num_clients:
ready.wait()
else:
self.client.ensure_path(self.path + '/ready')
return True
def leave(self):
"""Leave the barrier, blocks until all nodes have left"""
try:
self.client.retry(self._inner_leave)
except KazooException: # pragma: nocover
# Failed to cleanly leave
self._best_effort_cleanup()
self.participating = False
def _inner_leave(self):
# Delete the ready node if its around
try:
self.client.delete(self.path + '/ready')
except NoNodeError:
pass
while True:
children = self.client.get_children(self.path)
if not children:
return True
if len(children) == 1 and children[0] == self.node_name:
self.client.delete(self.create_path)
return True
children.sort()
ready = self.client.handler.event_object()
def deleted(event):
if event.type == EventType.DELETED:
ready.set()
if self.node_name == children[0]:
# We're first, wait on the highest to leave
if not self.client.exists(self.path + '/' + children[-1],
watch=deleted):
continue
ready.wait()
continue
# Delete our node
self.client.delete(self.create_path)
# Wait on the first
if not self.client.exists(self.path + '/' + children[0],
watch=deleted):
continue
# Wait for the lowest to be deleted
ready.wait()
def _best_effort_cleanup(self):
try:
self.client.retry(self.client.delete, self.create_path)
except NoNodeError:
pass
|
