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read_preferences.py
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read_preferences.py
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# Copyright 2012-2015 MongoDB, Inc.
#
# 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.
"""Utilities for choosing which member of a replica set to read from."""
import random
from pymongo.errors import ConfigurationError
_PRIMARY = 0
_PRIMARY_PREFERRED = 1
_SECONDARY = 2
_SECONDARY_PREFERRED = 3
_NEAREST = 4
class ReadPreference:
"""An enum that defines the read preference modes supported by PyMongo.
Used in three cases:
:class:`~pymongo.mongo_client.MongoClient` connected to a single host:
* `PRIMARY`: Queries are allowed if the host is standalone or the replica
set primary.
* All other modes allow queries to standalone servers, to the primary, or
to secondaries.
:class:`~pymongo.mongo_client.MongoClient` connected to a mongos, with a
sharded cluster of replica sets:
* `PRIMARY`: Queries are sent to the primary of a shard.
* `PRIMARY_PREFERRED`: Queries are sent to the primary if available,
otherwise a secondary.
* `SECONDARY`: Queries are distributed among shard secondaries. An error
is raised if no secondaries are available.
* `SECONDARY_PREFERRED`: Queries are distributed among shard secondaries,
or the primary if no secondary is available.
* `NEAREST`: Queries are distributed among all members of a shard.
:class:`~pymongo.mongo_replica_set_client.MongoReplicaSetClient`:
* `PRIMARY`: Queries are sent to the primary of the replica set.
* `PRIMARY_PREFERRED`: Queries are sent to the primary if available,
otherwise a secondary.
* `SECONDARY`: Queries are distributed among secondaries. An error
is raised if no secondaries are available.
* `SECONDARY_PREFERRED`: Queries are distributed among secondaries,
or the primary if no secondary is available.
* `NEAREST`: Queries are distributed among all members.
"""
PRIMARY = _PRIMARY
PRIMARY_PREFERRED = _PRIMARY_PREFERRED
SECONDARY = _SECONDARY
SECONDARY_ONLY = _SECONDARY
SECONDARY_PREFERRED = _SECONDARY_PREFERRED
NEAREST = _NEAREST
# For formatting error messages
modes = {
_PRIMARY: 'PRIMARY',
_PRIMARY_PREFERRED: 'PRIMARY_PREFERRED',
_SECONDARY: 'SECONDARY',
_SECONDARY_PREFERRED: 'SECONDARY_PREFERRED',
_NEAREST: 'NEAREST',
}
_mongos_modes = [
'primary',
'primaryPreferred',
'secondary',
'secondaryPreferred',
'nearest',
]
def mongos_mode(mode):
return _mongos_modes[mode]
def mongos_enum(enum):
return _mongos_modes.index(enum)
def select_primary(members):
for member in members:
if member.is_primary:
return member
return None
def select_member_with_tags(members, tags, secondary_only, latency):
candidates = []
for candidate in members:
if secondary_only and candidate.is_primary:
continue
if not (candidate.is_primary or candidate.is_secondary):
# In RECOVERING or similar state
continue
if candidate.matches_tags(tags):
candidates.append(candidate)
if not candidates:
return None
# ping_time is in seconds
fastest = min([candidate.get_avg_ping_time() for candidate in candidates])
near_candidates = [
candidate for candidate in candidates
if candidate.get_avg_ping_time() - fastest <= latency / 1000.]
return random.choice(near_candidates)
def select_member(members,
mode=ReadPreference.PRIMARY,
tag_sets=None,
latency=15):
"""Return a Member or None.
"""
if tag_sets is None:
tag_sets = [{}]
if mode == _PRIMARY:
if tag_sets != [{}]:
raise ConfigurationError("PRIMARY cannot be combined with tags")
return select_primary(members)
elif mode == _PRIMARY_PREFERRED:
# Recurse.
candidate_primary = select_member(members, _PRIMARY, [{}], latency)
if candidate_primary:
return candidate_primary
else:
return select_member(members, _SECONDARY, tag_sets, latency)
elif mode == _SECONDARY:
for tags in tag_sets:
candidate = select_member_with_tags(members, tags, True, latency)
if candidate:
return candidate
return None
elif mode == _SECONDARY_PREFERRED:
# Recurse.
candidate_secondary = select_member(
members, _SECONDARY, tag_sets, latency)
if candidate_secondary:
return candidate_secondary
else:
return select_member(members, _PRIMARY, [{}], latency)
elif mode == _NEAREST:
for tags in tag_sets:
candidate = select_member_with_tags(members, tags, False, latency)
if candidate:
return candidate
# Ran out of tags.
return None
else:
raise ConfigurationError("Invalid mode %s" % repr(mode))
"""Commands that may be sent to replica-set secondaries, depending on
ReadPreference and tags. All other commands are always run on the primary.
"""
secondary_ok_commands = frozenset([
"group", "aggregate", "collstats", "dbstats", "count", "distinct",
"geonear", "geosearch", "geowalk", "mapreduce", "getnonce", "authenticate",
"text", "parallelcollectionscan"
])
class MovingAverage(object):
def __init__(self, samples):
"""Immutable structure to track a 5-sample moving average.
"""
self.samples = samples[-5:]
assert self.samples
self.average = sum(self.samples) / float(len(self.samples))
def clone_with(self, sample):
"""Get a copy of this instance plus a new sample"""
return MovingAverage(self.samples + [sample])
def get(self):
return self.average
def _validate_tag_sets(tag_sets):
"""Validate tag sets for a MongoReplicaSetClient.
"""
if tag_sets is None:
return tag_sets
if not isinstance(tag_sets, list):
raise TypeError((
"Tag sets %r invalid, must be a list") % (tag_sets,))
if len(tag_sets) == 0:
raise ValueError((
"Tag sets %r invalid, must be None or contain at least one set of"
" tags") % (tag_sets,))
for tags in tag_sets:
if not isinstance(tags, dict):
raise TypeError(
"Tag set %r invalid, must be an instance of dict, or"
"bson.son.SON" % (tags,))
return tag_sets
class _ServerMode(object):
"""Base class for all read preferences.
"""
__slots__ = ("__mongos_mode", "__mode", "__tag_sets")
def __init__(self, mode, tag_sets=None):
if mode == _PRIMARY and tag_sets is not None:
raise ConfigurationError("Read preference primary "
"cannot be combined with tags")
self.__mongos_mode = _mongos_modes[mode]
self.__mode = mode
self.__tag_sets = _validate_tag_sets(tag_sets)
@property
def name(self):
"""The name of this read preference.
"""
return self.__class__.__name__
@property
def document(self):
"""Read preference as a document.
"""
if self.__tag_sets in (None, [{}]):
return {'mode': self.__mongos_mode}
return {'mode': self.__mongos_mode, 'tags': self.__tag_sets}
@property
def mode(self):
"""The mode of this read preference instance.
"""
return self.__mode
@property
def tag_sets(self):
"""Set ``tag_sets`` to a list of dictionaries like [{'dc': 'ny'}] to
read only from members whose ``dc`` tag has the value ``"ny"``.
To specify a priority-order for tag sets, provide a list of
tag sets: ``[{'dc': 'ny'}, {'dc': 'la'}, {}]``. A final, empty tag
set, ``{}``, means "read from any member that matches the mode,
ignoring tags." MongoReplicaSetClient tries each set of tags in turn
until it finds a set of tags with at least one matching member.
.. seealso:: `Data-Center Awareness
<http://www.mongodb.org/display/DOCS/Data+Center+Awareness>`_
"""
if self.__tag_sets:
return list(self.__tag_sets)
return [{}]
def __repr__(self):
return "%s(tag_sets=%r)" % (
self.name, self.__tag_sets)
def __eq__(self, other):
if isinstance(other, _ServerMode):
return (self.mode == other.mode and
self.tag_sets == other.tag_sets)
raise NotImplementedError
def __ne__(self, other):
return not self == other
def __getstate__(self):
"""Return value of object for pickling.
Needed explicitly because __slots__() defined.
"""
return {'mode': self.__mode, 'tag_sets': self.__tag_sets}
def __setstate__(self, value):
"""Restore from pickling."""
self.__mode = value['mode']
self.__mongos_mode = _mongos_modes[self.__mode]
self.__tag_sets = _validate_tag_sets(value['tag_sets'])
class Primary(_ServerMode):
"""Primary read preference.
* When directly connected to one mongod queries are allowed if the server
is standalone or a replica set primary.
* When connected to a mongos queries are sent to the primary of a shard.
* When connected to a replica set queries are sent to the primary of
the replica set.
.. versionadded:: 2.9
"""
def __init__(self):
super(Primary, self).__init__(_PRIMARY)
def __repr__(self):
return "Primary()"
def __eq__(self, other):
if isinstance(other, _ServerMode):
return other.mode == _PRIMARY
raise NotImplementedError
class PrimaryPreferred(_ServerMode):
"""PrimaryPreferred read preference.
* When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.
* When connected to a mongos queries are sent to the primary of a shard if
available, otherwise a shard secondary.
* When connected to a replica set queries are sent to the primary if
available, otherwise a secondary.
:Parameters:
- `tag_sets`: The :attr:`~tag_sets` to use if the primary is not
available.
.. versionadded:: 2.9
"""
def __init__(self, tag_sets=None):
super(PrimaryPreferred, self).__init__(_PRIMARY_PREFERRED, tag_sets)
class Secondary(_ServerMode):
"""Secondary read preference.
* When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.
* When connected to a mongos queries are distributed among shard
secondaries. An error is raised if no secondaries are available.
* When connected to a replica set queries are distributed among
secondaries. An error is raised if no secondaries are available.
:Parameters:
- `tag_sets`: The :attr:`~tag_sets` to use with this read_preference
.. versionadded:: 2.9
"""
def __init__(self, tag_sets=None):
super(Secondary, self).__init__(_SECONDARY, tag_sets)
class SecondaryPreferred(_ServerMode):
"""SecondaryPreferred read preference.
* When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.
* When connected to a mongos queries are distributed among shard
secondaries, or the shard primary if no secondary is available.
* When connected to a replica set queries are distributed among
secondaries, or the primary if no secondary is available.
:Parameters:
- `tag_sets`: The :attr:`~tag_sets` to use with this read_preference
.. versionadded:: 2.9
"""
def __init__(self, tag_sets=None):
super(SecondaryPreferred, self).__init__(_SECONDARY_PREFERRED, tag_sets)
class Nearest(_ServerMode):
"""Nearest read preference.
* When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.
* When connected to a mongos queries are distributed among all members of
a shard.
* When connected to a replica set queries are distributed among all
members.
:Parameters:
- `tag_sets`: The :attr:`~tag_sets` to use with this read_preference
.. versionadded:: 2.9
"""
def __init__(self, tag_sets=None):
super(Nearest, self).__init__(_NEAREST, tag_sets)