tooz.drivers.memcached

Source code for tooz.drivers.memcached

# -*- coding: utf-8 -*-
#
# Copyright © 2014 eNovance
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.

import contextlib
import errno
import functools
import logging
import socket

from oslo_utils import encodeutils
from pymemcache import client as pymemcache_client
import six

import tooz
from tooz import _retry
from tooz import coordination
from tooz import locking
from tooz import utils


LOG = logging.getLogger(__name__)


@contextlib.contextmanager
def _failure_translator():
    """Translates common pymemcache exceptions into tooz exceptions.

    https://github.com/pinterest/pymemcache/blob/d995/pymemcache/client.py#L202
    """
    try:
        yield
    except pymemcache_client.MemcacheUnexpectedCloseError as e:
        utils.raise_with_cause(coordination.ToozConnectionError,
                               encodeutils.exception_to_unicode(e),
                               cause=e)
    except (socket.timeout, socket.error,
            socket.gaierror, socket.herror) as e:
        # TODO(harlowja): get upstream pymemcache to produce a better
        # exception for these, using socket (vs. a memcache specific
        # error) seems sorta not right and/or the best approach...
        msg = encodeutils.exception_to_unicode(e)
        if e.errno is not None:
            msg += " (with errno %s [%s])" % (errno.errorcode[e.errno],
                                              e.errno)
        utils.raise_with_cause(coordination.ToozConnectionError,
                               msg, cause=e)
    except pymemcache_client.MemcacheError as e:
        utils.raise_with_cause(tooz.ToozError,
                               encodeutils.exception_to_unicode(e),
                               cause=e)


def _translate_failures(func):

    @six.wraps(func)
    def wrapper(*args, **kwargs):
        with _failure_translator():
            return func(*args, **kwargs)

    return wrapper


class MemcachedLock(locking.Lock):
    _LOCK_PREFIX = b'__TOOZ_LOCK_'

    def __init__(self, coord, name, timeout):
        super(MemcachedLock, self).__init__(self._LOCK_PREFIX + name)
        self.coord = coord
        self.timeout = timeout

    def is_still_owner(self):
        if not self.acquired:
            return False
        else:
            owner = self.get_owner()
            if owner is None:
                return False
            return owner == self.coord._member_id

    def acquire(self, blocking=True, shared=False):
        if shared:
            raise tooz.NotImplemented

        @_retry.retry(stop_max_delay=blocking)
        @_translate_failures
        def _acquire():
            if self.coord.client.add(
                    self.name,
                    self.coord._member_id,
                    expire=self.timeout,
                    noreply=False):
                self.coord._acquired_locks.append(self)
                return True
            if blocking is False:
                return False
            raise _retry.TryAgain

        return _acquire()

    @_translate_failures
    def break_(self):
        return bool(self.coord.client.delete(self.name, noreply=False))

    @_translate_failures
    def release(self):
        if not self.acquired:
            return False
        # NOTE(harlowja): this has the potential to delete others locks
        # especially if this key expired before the delete/release call is
        # triggered.
        #
        # For example:
        #
        # 1. App #1 with coordinator 'A' acquires lock "b"
        # 2. App #1 heartbeats every 10 seconds, expiry for lock let's
        #    say is 11 seconds.
        # 3. App #2 with coordinator also named 'A' blocks trying to get
        #    lock "b" (let's say it retries attempts every 0.5 seconds)
        # 4. App #1 is running behind a little bit, tries to heartbeat but
        #    key has expired (log message is written); at this point app #1
        #    doesn't own the lock anymore but it doesn't know that.
        # 5. App #2 now retries and adds the key, and now it believes it
        #    has the lock.
        # 6. App #1 (still believing it has the lock) calls release, and
        #    deletes app #2 lock, app #2 now doesn't own the lock anymore
        #    but it doesn't know that and now app #(X + 1) can get it.
        # 7. App #2 calls release (repeat #6 as many times as desired)
        #
        # Sadly I don't think memcache has the primitives to actually make
        # this work, redis does because it has lua which can check a session
        # id and then do the delete and bail out if the session id is not
        # as expected but memcache doesn't seem to have any equivalent
        # capability.
        if self not in self.coord._acquired_locks:
            return False
        # Do a ghetto test to see what the value is... (see above note),
        # and how this really can't be done safely with memcache due to
        # it being done in the client side (non-atomic).
        value = self.coord.client.get(self.name)
        if value != self.coord._member_id:
            return False
        else:
            was_deleted = self.coord.client.delete(self.name, noreply=False)
            if was_deleted:
                self.coord._acquired_locks.remove(self)
            return was_deleted

    @_translate_failures
    def heartbeat(self):
        """Keep the lock alive."""
        if self.acquired:
            poked = self.coord.client.touch(self.name,
                                            expire=self.timeout,
                                            noreply=False)
            if poked:
                return True
            LOG.warning("Unable to heartbeat by updating key '%s' with "
                        "extended expiry of %s seconds", self.name,
                        self.timeout)
        return False

    @_translate_failures
    def get_owner(self):
        return self.coord.client.get(self.name)

    @property
    def acquired(self):
        return self in self.coord._acquired_locks


[docs]class MemcachedDriver(coordination.CoordinationDriverCachedRunWatchers, coordination.CoordinationDriverWithExecutor): """A `memcached`_ based driver. This driver users `memcached`_ concepts to provide the coordination driver semantics and required API(s). It **is** fully functional and implements all of the coordination driver API(s). It stores data into memcache using expiries and `msgpack`_ encoded values. The Memcached driver connection URI should look like:: memcached://[HOST[:PORT]][?OPTION1=VALUE1[&OPTION2=VALUE2[&...]]] If not specified, HOST defaults to localhost and PORT defaults to 11211. Available options are: ================== ======= Name Default ================== ======= timeout 30 membership_timeout 30 lock_timeout 30 leader_timeout 30 max_pool_size None ================== ======= General recommendations/usage considerations: - Memcache (without different backend technology) is a **cache** enough said. .. _memcached: http://memcached.org/ .. _msgpack: http://msgpack.org/ """ CHARACTERISTICS = ( coordination.Characteristics.DISTRIBUTED_ACROSS_THREADS, coordination.Characteristics.DISTRIBUTED_ACROSS_PROCESSES, coordination.Characteristics.DISTRIBUTED_ACROSS_HOSTS, coordination.Characteristics.CAUSAL, ) """ Tuple of :py:class:`~tooz.coordination.Characteristics` introspectable enum member(s) that can be used to interogate how this driver works. """ #: Key prefix attached to groups (used in name-spacing keys) GROUP_PREFIX = b'_TOOZ_GROUP_' #: Key prefix attached to leaders of groups (used in name-spacing keys) GROUP_LEADER_PREFIX = b'_TOOZ_GROUP_LEADER_' #: Key prefix attached to members of groups (used in name-spacing keys) MEMBER_PREFIX = b'_TOOZ_MEMBER_' #: Key where all groups 'known' are stored. GROUP_LIST_KEY = b'_TOOZ_GROUP_LIST' #: Default socket/lock/member/leader timeout used when none is provided. DEFAULT_TIMEOUT = 30 #: String used to keep a key/member alive (until it next expires). STILL_ALIVE = b"It's alive!"
[docs] def __init__(self, member_id, parsed_url, options): super(MemcachedDriver, self).__init__(member_id, parsed_url, options) self.host = (parsed_url.hostname or "localhost", parsed_url.port or 11211) default_timeout = self._options.get('timeout', self.DEFAULT_TIMEOUT) self.timeout = int(default_timeout) self.membership_timeout = int(self._options.get( 'membership_timeout', default_timeout)) self.lock_timeout = int(self._options.get( 'lock_timeout', default_timeout)) self.leader_timeout = int(self._options.get( 'leader_timeout', default_timeout)) max_pool_size = self._options.get('max_pool_size', None) if max_pool_size is not None: self.max_pool_size = int(max_pool_size) else: self.max_pool_size = None self._acquired_locks = []
@staticmethod def _msgpack_serializer(key, value): if isinstance(value, six.binary_type): return value, 1 return utils.dumps(value), 2 @staticmethod def _msgpack_deserializer(key, value, flags): if flags == 1: return value if flags == 2: return utils.loads(value) raise coordination.SerializationError("Unknown serialization" " format '%s'" % flags) @_translate_failures def _start(self): super(MemcachedDriver, self)._start() self.client = pymemcache_client.PooledClient( self.host, serializer=self._msgpack_serializer, deserializer=self._msgpack_deserializer, timeout=self.timeout, connect_timeout=self.timeout, max_pool_size=self.max_pool_size) # Run heartbeat here because pymemcache use a lazy connection # method and only connect once you do an operation. self.heartbeat() @_translate_failures def _stop(self): super(MemcachedDriver, self)._stop() for lock in list(self._acquired_locks): lock.release() self.client.delete(self._encode_member_id(self._member_id)) self.client.close() def _encode_group_id(self, group_id): return self.GROUP_PREFIX + utils.to_binary(group_id) def _encode_member_id(self, member_id): return self.MEMBER_PREFIX + utils.to_binary(member_id) def _encode_group_leader(self, group_id): return self.GROUP_LEADER_PREFIX + utils.to_binary(group_id) @_retry.retry() def _add_group_to_group_list(self, group_id): """Add group to the group list. :param group_id: The group id """ group_list, cas = self.client.gets(self.GROUP_LIST_KEY) if cas: group_list = set(group_list) group_list.add(group_id) if not self.client.cas(self.GROUP_LIST_KEY, list(group_list), cas): # Someone updated the group list before us, try again! raise _retry.TryAgain else: if not self.client.add(self.GROUP_LIST_KEY, [group_id], noreply=False): # Someone updated the group list before us, try again! raise _retry.TryAgain @_retry.retry() def _remove_from_group_list(self, group_id): """Remove group from the group list. :param group_id: The group id """ group_list, cas = self.client.gets(self.GROUP_LIST_KEY) group_list = set(group_list) group_list.remove(group_id) if not self.client.cas(self.GROUP_LIST_KEY, list(group_list), cas): # Someone updated the group list before us, try again! raise _retry.TryAgain
[docs] def create_group(self, group_id): encoded_group = self._encode_group_id(group_id) @_translate_failures def _create_group(): if not self.client.add(encoded_group, {}, noreply=False): raise coordination.GroupAlreadyExist(group_id) self._add_group_to_group_list(group_id) return MemcachedFutureResult(self._executor.submit(_create_group))
[docs] def get_groups(self): @_translate_failures def _get_groups(): return self.client.get(self.GROUP_LIST_KEY) or [] return MemcachedFutureResult(self._executor.submit(_get_groups))
[docs] def join_group(self, group_id, capabilities=b""): encoded_group = self._encode_group_id(group_id) @_retry.retry() @_translate_failures def _join_group(): group_members, cas = self.client.gets(encoded_group) if group_members is None: raise coordination.GroupNotCreated(group_id) if self._member_id in group_members: raise coordination.MemberAlreadyExist(group_id, self._member_id) group_members[self._member_id] = { b"capabilities": capabilities, } if not self.client.cas(encoded_group, group_members, cas): # It changed, let's try again raise _retry.TryAgain self._joined_groups.add(group_id) return MemcachedFutureResult(self._executor.submit(_join_group))
[docs] def leave_group(self, group_id): encoded_group = self._encode_group_id(group_id) @_retry.retry() @_translate_failures def _leave_group(): group_members, cas = self.client.gets(encoded_group) if group_members is None: raise coordination.GroupNotCreated(group_id) if self._member_id not in group_members: raise coordination.MemberNotJoined(group_id, self._member_id) del group_members[self._member_id] if not self.client.cas(encoded_group, group_members, cas): # It changed, let's try again raise _retry.TryAgain self._joined_groups.discard(group_id) return MemcachedFutureResult(self._executor.submit(_leave_group))
def _destroy_group(self, group_id): self.client.delete(self._encode_group_id(group_id))
[docs] def delete_group(self, group_id): encoded_group = self._encode_group_id(group_id) @_retry.retry() @_translate_failures def _delete_group(): group_members, cas = self.client.gets(encoded_group) if group_members is None: raise coordination.GroupNotCreated(group_id) if group_members != {}: raise coordination.GroupNotEmpty(group_id) # Delete is not atomic, so we first set the group to # using CAS, and then we delete it, to avoid race conditions. if not self.client.cas(encoded_group, None, cas): raise _retry.TryAgain self.client.delete(encoded_group) self._remove_from_group_list(group_id) return MemcachedFutureResult(self._executor.submit(_delete_group))
@_retry.retry() @_translate_failures def _get_members(self, group_id): encoded_group = self._encode_group_id(group_id) group_members, cas = self.client.gets(encoded_group) if group_members is None: raise coordination.GroupNotCreated(group_id) actual_group_members = {} for m, v in six.iteritems(group_members): # Never kick self from the group, we know we're alive if (m == self._member_id or self.client.get(self._encode_member_id(m))): actual_group_members[m] = v if group_members != actual_group_members: # There are some dead members, update the group if not self.client.cas(encoded_group, actual_group_members, cas): # It changed, let's try again raise _retry.TryAgain return actual_group_members
[docs] def get_members(self, group_id): def _get_members(): return set(self._get_members(group_id).keys()) return MemcachedFutureResult(self._executor.submit(_get_members))
[docs] def get_member_capabilities(self, group_id, member_id): def _get_member_capabilities(): group_members = self._get_members(group_id) if member_id not in group_members: raise coordination.MemberNotJoined(group_id, member_id) return group_members[member_id][b'capabilities'] return MemcachedFutureResult( self._executor.submit(_get_member_capabilities))
[docs] def update_capabilities(self, group_id, capabilities): encoded_group = self._encode_group_id(group_id) @_retry.retry() @_translate_failures def _update_capabilities(): group_members, cas = self.client.gets(encoded_group) if group_members is None: raise coordination.GroupNotCreated(group_id) if self._member_id not in group_members: raise coordination.MemberNotJoined(group_id, self._member_id) group_members[self._member_id][b'capabilities'] = capabilities if not self.client.cas(encoded_group, group_members, cas): # It changed, try again raise _retry.TryAgain return MemcachedFutureResult( self._executor.submit(_update_capabilities))
[docs] def get_leader(self, group_id): def _get_leader(): return self._get_leader_lock(group_id).get_owner() return MemcachedFutureResult(self._executor.submit(_get_leader))
[docs] @_translate_failures def heartbeat(self): self.client.set(self._encode_member_id(self._member_id), self.STILL_ALIVE, expire=self.membership_timeout) # Reset the acquired locks for lock in self._acquired_locks: lock.heartbeat() return min(self.membership_timeout, self.leader_timeout, self.lock_timeout)
[docs] def get_lock(self, name): return MemcachedLock(self, name, self.lock_timeout)
def _get_leader_lock(self, group_id): return MemcachedLock(self, self._encode_group_leader(group_id), self.leader_timeout)
[docs] @_translate_failures def run_elect_coordinator(self): for group_id, hooks in six.iteritems(self._hooks_elected_leader): # Try to grab the lock, if that fails, that means someone has it # already. leader_lock = self._get_leader_lock(group_id) if leader_lock.acquire(blocking=False): # We got the lock hooks.run(coordination.LeaderElected( group_id, self._member_id))
[docs] def run_watchers(self, timeout=None): result = super(MemcachedDriver, self).run_watchers(timeout=timeout) self.run_elect_coordinator() return result
MemcachedFutureResult = functools.partial( coordination.CoordinatorResult, failure_translator=_failure_translator)
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