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client_session.py
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715 lines (590 loc) · 26.3 KB
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# Copyright 2017 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.
"""Logical sessions for ordering sequential operations.
Requires MongoDB 3.6.
.. versionadded:: 3.6
Causally Consistent Reads
=========================
.. code-block:: python
with client.start_session(causal_consistency=True) as session:
collection = client.db.collection
collection.update_one({'_id': 1}, {'$set': {'x': 10}}, session=session)
secondary_c = collection.with_options(
read_preference=ReadPreference.SECONDARY)
# A secondary read waits for replication of the write.
secondary_c.find_one({'_id': 1}, session=session)
If `causal_consistency` is True (the default), read operations that use
the session are causally after previous read and write operations. Using a
causally consistent session, an application can read its own writes and is
guaranteed monotonic reads, even when reading from replica set secondaries.
.. mongodoc:: causal-consistency
.. _transactions-ref:
Transactions
============
MongoDB 4.0 adds support for transactions on replica set primaries. A
transaction is associated with a :class:`ClientSession`. To start a transaction
on a session, use :meth:`ClientSession.start_transaction` in a with-statement.
Then, execute an operation within the transaction by passing the session to the
operation:
.. code-block:: python
orders = client.db.orders
inventory = client.db.inventory
with client.start_session() as session:
with session.start_transaction():
orders.insert_one({"sku": "abc123", "qty": 100}, session=session)
inventory.update_one({"sku": "abc123", "qty": {"$gte": 100}},
{"$inc": {"qty": -100}}, session=session)
Upon normal completion of ``with session.start_transaction()`` block, the
transaction automatically calls :meth:`ClientSession.commit_transaction`.
If the block exits with an exception, the transaction automatically calls
:meth:`ClientSession.abort_transaction`.
For multi-document transactions, you can only specify read/write (CRUD)
operations on existing collections. For example, a multi-document transaction
cannot include a create or drop collection/index operations, including an
insert operation that would result in the creation of a new collection.
A session may only have a single active transaction at a time, multiple
transactions on the same session can be executed in sequence.
.. versionadded:: 3.7
Sharded Transactions
^^^^^^^^^^^^^^^^^^^^
PyMongo 3.9 adds support for transactions on sharded clusters running MongoDB
4.2. Sharded transactions have the same API as replica set transactions.
When running a transaction against a sharded cluster, the session is
pinned to the mongos server selected for the first operation in the
transaction. All subsequent operations that are part of the same transaction
are routed to the same mongos server. When the transaction is completed, by
running either commitTransaction or abortTransaction, the session is unpinned.
.. versionadded:: 3.9
.. mongodoc:: transactions
Classes
=======
"""
import collections
import sys
import uuid
from bson.binary import Binary
from bson.int64 import Int64
from bson.py3compat import abc, reraise_instance
from bson.son import SON
from bson.timestamp import Timestamp
from pymongo import monotonic
from pymongo.errors import (ConfigurationError,
ConnectionFailure,
InvalidOperation,
OperationFailure,
ServerSelectionTimeoutError,
WriteConcernError,
WTimeoutError)
from pymongo.helpers import _RETRYABLE_ERROR_CODES
from pymongo.read_concern import ReadConcern
from pymongo.read_preferences import ReadPreference, _ServerMode
from pymongo.write_concern import WriteConcern
class SessionOptions(object):
"""Options for a new :class:`ClientSession`.
:Parameters:
- `causal_consistency` (optional): If True (the default), read
operations are causally ordered within the session.
- `default_transaction_options` (optional): The default
TransactionOptions to use for transactions started on this session.
"""
def __init__(self,
causal_consistency=True,
default_transaction_options=None):
self._causal_consistency = causal_consistency
if default_transaction_options is not None:
if not isinstance(default_transaction_options, TransactionOptions):
raise TypeError(
"default_transaction_options must be an instance of "
"pymongo.client_session.TransactionOptions, not: %r" %
(default_transaction_options,))
self._default_transaction_options = default_transaction_options
@property
def causal_consistency(self):
"""Whether causal consistency is configured."""
return self._causal_consistency
@property
def default_transaction_options(self):
"""The default TransactionOptions to use for transactions started on
this session.
.. versionadded:: 3.7
"""
return self._default_transaction_options
class TransactionOptions(object):
"""Options for :meth:`ClientSession.start_transaction`.
:Parameters:
- `read_concern`: The :class:`~pymongo.read_concern.ReadConcern` to use
for this transaction.
- `write_concern`: The :class:`~pymongo.write_concern.WriteConcern` to
use for this transaction.
.. versionadded:: 3.7
"""
def __init__(self, read_concern=None, write_concern=None,
read_preference=None):
self._read_concern = read_concern
self._write_concern = write_concern
self._read_preference = read_preference
if read_concern is not None:
if not isinstance(read_concern, ReadConcern):
raise TypeError("read_concern must be an instance of "
"pymongo.read_concern.ReadConcern, not: %r" %
(read_concern,))
if write_concern is not None:
if not isinstance(write_concern, WriteConcern):
raise TypeError("write_concern must be an instance of "
"pymongo.write_concern.WriteConcern, not: %r" %
(write_concern,))
if not write_concern.acknowledged:
raise ConfigurationError(
"transactions do not support unacknowledged write concern"
": %r" % (write_concern,))
if read_preference is not None:
if not isinstance(read_preference, _ServerMode):
raise TypeError("%r is not valid for read_preference. See "
"pymongo.read_preferences for valid "
"options." % (read_preference,))
@property
def read_concern(self):
"""This transaction's :class:`~pymongo.read_concern.ReadConcern`."""
return self._read_concern
@property
def write_concern(self):
"""This transaction's :class:`~pymongo.write_concern.WriteConcern`."""
return self._write_concern
@property
def read_preference(self):
"""This transaction's :class:`~pymongo.read_preferences.ReadPreference`.
"""
return self._read_preference
def _validate_session_write_concern(session, write_concern):
"""Validate that an explicit session is not used with an unack'ed write.
Returns the session to use for the next operation.
"""
if session:
if write_concern is not None and not write_concern.acknowledged:
# For unacknowledged writes without an explicit session,
# drivers SHOULD NOT use an implicit session. If a driver
# creates an implicit session for unacknowledged writes
# without an explicit session, the driver MUST NOT send the
# session ID.
if session._implicit:
return None
else:
raise ConfigurationError(
'Explicit sessions are incompatible with '
'unacknowledged write concern: %r' % (
write_concern,))
return session
class _TransactionContext(object):
"""Internal transaction context manager for start_transaction."""
def __init__(self, session):
self.__session = session
def __enter__(self):
return self
def __exit__(self, exc_type, exc_val, exc_tb):
if self.__session._in_transaction:
if exc_val is None:
self.__session.commit_transaction()
else:
self.__session.abort_transaction()
class _TxnState(object):
NONE = 1
STARTING = 2
IN_PROGRESS = 3
COMMITTED = 4
COMMITTED_EMPTY = 5
ABORTED = 6
class _Transaction(object):
"""Internal class to hold transaction information in a ClientSession."""
def __init__(self, opts):
self.opts = opts
self.state = _TxnState.NONE
self.sharded = False
self.pinned_address = None
self.recovery_token = None
def active(self):
return self.state in (_TxnState.STARTING, _TxnState.IN_PROGRESS)
def reset(self):
self.state = _TxnState.NONE
self.sharded = False
self.pinned_address = None
self.recovery_token = None
def _reraise_with_unknown_commit(exc):
"""Re-raise an exception with the UnknownTransactionCommitResult label."""
exc._add_error_label("UnknownTransactionCommitResult")
reraise_instance(exc, trace=sys.exc_info()[2])
# From the transactions spec, all the retryable writes errors plus
# WriteConcernFailed.
_UNKNOWN_COMMIT_ERROR_CODES = _RETRYABLE_ERROR_CODES | frozenset([
64, # WriteConcernFailed
])
class ClientSession(object):
"""A session for ordering sequential operations."""
def __init__(self, client, server_session, options, authset, implicit):
# A MongoClient, a _ServerSession, a SessionOptions, and a set.
self._client = client
self._server_session = server_session
self._options = options
self._authset = authset
self._cluster_time = None
self._operation_time = None
# Is this an implicitly created session?
self._implicit = implicit
self._transaction = _Transaction(None)
def end_session(self):
"""Finish this session. If a transaction has started, abort it.
It is an error to use the session after the session has ended.
"""
self._end_session(lock=True)
def _end_session(self, lock):
if self._server_session is not None:
try:
if self._in_transaction:
self.abort_transaction()
finally:
self._client._return_server_session(self._server_session, lock)
self._server_session = None
def _check_ended(self):
if self._server_session is None:
raise InvalidOperation("Cannot use ended session")
def __enter__(self):
return self
def __exit__(self, exc_type, exc_val, exc_tb):
self._end_session(lock=True)
@property
def client(self):
"""The :class:`~pymongo.mongo_client.MongoClient` this session was
created from.
"""
return self._client
@property
def options(self):
"""The :class:`SessionOptions` this session was created with."""
return self._options
@property
def session_id(self):
"""A BSON document, the opaque server session identifier."""
self._check_ended()
return self._server_session.session_id
@property
def cluster_time(self):
"""The cluster time returned by the last operation executed
in this session.
"""
return self._cluster_time
@property
def operation_time(self):
"""The operation time returned by the last operation executed
in this session.
"""
return self._operation_time
def _inherit_option(self, name, val):
"""Return the inherited TransactionOption value."""
if val:
return val
txn_opts = self.options.default_transaction_options
val = txn_opts and getattr(txn_opts, name)
if val:
return val
return getattr(self.client, name)
def start_transaction(self, read_concern=None, write_concern=None,
read_preference=None):
"""Start a multi-statement transaction.
Takes the same arguments as :class:`TransactionOptions`.
.. versionadded:: 3.7
"""
self._check_ended()
if self._in_transaction:
raise InvalidOperation("Transaction already in progress")
read_concern = self._inherit_option("read_concern", read_concern)
write_concern = self._inherit_option("write_concern", write_concern)
read_preference = self._inherit_option(
"read_preference", read_preference)
self._transaction.opts = TransactionOptions(
read_concern, write_concern, read_preference)
self._transaction.reset()
self._transaction.state = _TxnState.STARTING
self._start_retryable_write()
return _TransactionContext(self)
def commit_transaction(self):
"""Commit a multi-statement transaction.
.. versionadded:: 3.7
"""
self._check_ended()
retry = False
state = self._transaction.state
if state is _TxnState.NONE:
raise InvalidOperation("No transaction started")
elif state in (_TxnState.STARTING, _TxnState.COMMITTED_EMPTY):
# Server transaction was never started, no need to send a command.
self._transaction.state = _TxnState.COMMITTED_EMPTY
return
elif state is _TxnState.ABORTED:
raise InvalidOperation(
"Cannot call commitTransaction after calling abortTransaction")
elif state is _TxnState.COMMITTED:
# We're explicitly retrying the commit, move the state back to
# "in progress" so that _in_transaction returns true.
self._transaction.state = _TxnState.IN_PROGRESS
retry = True
try:
self._finish_transaction_with_retry("commitTransaction", retry)
except ConnectionFailure as exc:
# We do not know if the commit was successfully applied on the
# server or if it satisfied the provided write concern, set the
# unknown commit error label.
exc._remove_error_label("TransientTransactionError")
_reraise_with_unknown_commit(exc)
except WTimeoutError as exc:
# We do not know if the commit has satisfied the provided write
# concern, add the unknown commit error label.
_reraise_with_unknown_commit(exc)
except OperationFailure as exc:
if exc.code not in _UNKNOWN_COMMIT_ERROR_CODES:
# The server reports errorLabels in the case.
raise
# We do not know if the commit was successfully applied on the
# server or if it satisfied the provided write concern, set the
# unknown commit error label.
_reraise_with_unknown_commit(exc)
finally:
self._transaction.state = _TxnState.COMMITTED
def abort_transaction(self):
"""Abort a multi-statement transaction.
.. versionadded:: 3.7
"""
self._check_ended()
state = self._transaction.state
if state is _TxnState.NONE:
raise InvalidOperation("No transaction started")
elif state is _TxnState.STARTING:
# Server transaction was never started, no need to send a command.
self._transaction.state = _TxnState.ABORTED
return
elif state is _TxnState.ABORTED:
raise InvalidOperation("Cannot call abortTransaction twice")
elif state in (_TxnState.COMMITTED, _TxnState.COMMITTED_EMPTY):
raise InvalidOperation(
"Cannot call abortTransaction after calling commitTransaction")
try:
self._finish_transaction_with_retry("abortTransaction", False)
except (OperationFailure, ConnectionFailure):
# The transactions spec says to ignore abortTransaction errors.
pass
finally:
self._transaction.state = _TxnState.ABORTED
def _finish_transaction_with_retry(self, command_name, explict_retry):
"""Run commit or abort with one retry after any retryable error.
:Parameters:
- `command_name`: Either "commitTransaction" or "abortTransaction".
- `explict_retry`: True when this is an explict commit retry attempt,
ie the application called session.commit_transaction() twice.
"""
# This can be refactored with MongoClient._retry_with_session.
try:
return self._finish_transaction(command_name, explict_retry)
except ServerSelectionTimeoutError:
raise
except ConnectionFailure as exc:
try:
return self._finish_transaction(command_name, True)
except ServerSelectionTimeoutError:
# Raise the original error so the application can infer that
# an attempt was made.
raise exc
except OperationFailure as exc:
if exc.code not in _RETRYABLE_ERROR_CODES:
raise
try:
return self._finish_transaction(command_name, True)
except ServerSelectionTimeoutError:
# Raise the original error so the application can infer that
# an attempt was made.
raise exc
def _finish_transaction(self, command_name, retrying):
# Transaction spec says that after the initial commit attempt,
# subsequent commitTransaction commands should be upgraded to use
# w:"majority" and set a default value of 10 seconds for wtimeout.
wc = self._transaction.opts.write_concern
is_commit = command_name == "commitTransaction"
if retrying and is_commit:
wc_doc = wc.document
wc_doc["w"] = "majority"
wc_doc.setdefault("wtimeout", 10000)
wc = WriteConcern(**wc_doc)
cmd = SON([(command_name, 1)])
if self._transaction.recovery_token and is_commit:
cmd['recoveryToken'] = self._transaction.recovery_token
with self._client._socket_for_writes(self) as sock_info:
return self._client.admin._command(
sock_info,
cmd,
session=self,
write_concern=wc,
parse_write_concern_error=True)
def _advance_cluster_time(self, cluster_time):
"""Internal cluster time helper."""
if self._cluster_time is None:
self._cluster_time = cluster_time
elif cluster_time is not None:
if cluster_time["clusterTime"] > self._cluster_time["clusterTime"]:
self._cluster_time = cluster_time
def advance_cluster_time(self, cluster_time):
"""Update the cluster time for this session.
:Parameters:
- `cluster_time`: The
:data:`~pymongo.client_session.ClientSession.cluster_time` from
another `ClientSession` instance.
"""
if not isinstance(cluster_time, abc.Mapping):
raise TypeError(
"cluster_time must be a subclass of collections.Mapping")
if not isinstance(cluster_time.get("clusterTime"), Timestamp):
raise ValueError("Invalid cluster_time")
self._advance_cluster_time(cluster_time)
def _advance_operation_time(self, operation_time):
"""Internal operation time helper."""
if self._operation_time is None:
self._operation_time = operation_time
elif operation_time is not None:
if operation_time > self._operation_time:
self._operation_time = operation_time
def advance_operation_time(self, operation_time):
"""Update the operation time for this session.
:Parameters:
- `operation_time`: The
:data:`~pymongo.client_session.ClientSession.operation_time` from
another `ClientSession` instance.
"""
if not isinstance(operation_time, Timestamp):
raise TypeError("operation_time must be an instance "
"of bson.timestamp.Timestamp")
self._advance_operation_time(operation_time)
def _process_response(self, reply):
"""Process a response to a command that was run with this session."""
self._advance_cluster_time(reply.get('$clusterTime'))
self._advance_operation_time(reply.get('operationTime'))
if self._in_transaction and self._transaction.sharded:
recovery_token = reply.get('recoveryToken')
if recovery_token:
self._transaction.recovery_token = recovery_token
@property
def has_ended(self):
"""True if this session is finished."""
return self._server_session is None
@property
def _in_transaction(self):
"""True if this session has an active multi-statement transaction."""
return self._transaction.active()
@property
def _pinned_address(self):
"""The mongos address this transaction was created on."""
if self._transaction.active():
return self._transaction.pinned_address
return None
def _pin_mongos(self, server):
"""Pin this session to the given mongos Server."""
self._transaction.sharded = True
self._transaction.pinned_address = server.description.address
def _unpin_mongos(self):
"""Unpin this session from any pinned mongos address."""
self._transaction.pinned_address = None
def _txn_read_preference(self):
"""Return read preference of this transaction or None."""
if self._in_transaction:
return self._transaction.opts.read_preference
return None
def _apply_to(self, command, is_retryable, read_preference):
self._check_ended()
self._server_session.last_use = monotonic.time()
command['lsid'] = self._server_session.session_id
if not self._in_transaction:
self._transaction.reset()
if is_retryable:
command['txnNumber'] = self._server_session.transaction_id
return
if self._in_transaction:
if read_preference != ReadPreference.PRIMARY:
raise InvalidOperation(
'read preference in a transaction must be primary, not: '
'%r' % (read_preference,))
if self._transaction.state == _TxnState.STARTING:
# First command begins a new transaction.
self._transaction.state = _TxnState.IN_PROGRESS
command['startTransaction'] = True
if self._transaction.opts.read_concern:
rc = self._transaction.opts.read_concern.document
else:
rc = {}
if (self.options.causal_consistency
and self.operation_time is not None):
rc['afterClusterTime'] = self.operation_time
if rc:
command['readConcern'] = rc
command['txnNumber'] = self._server_session.transaction_id
command['autocommit'] = False
def _start_retryable_write(self):
self._check_ended()
self._server_session.inc_transaction_id()
class _ServerSession(object):
def __init__(self):
# Ensure id is type 4, regardless of CodecOptions.uuid_representation.
self.session_id = {'id': Binary(uuid.uuid4().bytes, 4)}
self.last_use = monotonic.time()
self._transaction_id = 0
def timed_out(self, session_timeout_minutes):
idle_seconds = monotonic.time() - self.last_use
# Timed out if we have less than a minute to live.
return idle_seconds > (session_timeout_minutes - 1) * 60
@property
def transaction_id(self):
"""Positive 64-bit integer."""
return Int64(self._transaction_id)
def inc_transaction_id(self):
self._transaction_id += 1
class _ServerSessionPool(collections.deque):
"""Pool of _ServerSession objects.
This class is not thread-safe, access it while holding the Topology lock.
"""
def pop_all(self):
ids = []
while self:
ids.append(self.pop().session_id)
return ids
def get_server_session(self, session_timeout_minutes):
# Although the Driver Sessions Spec says we only clear stale sessions
# in return_server_session, PyMongo can't take a lock when returning
# sessions from a __del__ method (like in Cursor.__die), so it can't
# clear stale sessions there. In case many sessions were returned via
# __del__, check for stale sessions here too.
self._clear_stale(session_timeout_minutes)
# The most recently used sessions are on the left.
while self:
s = self.popleft()
if not s.timed_out(session_timeout_minutes):
return s
return _ServerSession()
def return_server_session(self, server_session, session_timeout_minutes):
self._clear_stale(session_timeout_minutes)
if not server_session.timed_out(session_timeout_minutes):
self.appendleft(server_session)
def return_server_session_no_lock(self, server_session):
self.appendleft(server_session)
def _clear_stale(self, session_timeout_minutes):
# Clear stale sessions. The least recently used are on the right.
while self:
if self[-1].timed_out(session_timeout_minutes):
self.pop()
else:
# The remaining sessions also haven't timed out.
break