mongo_client.py

# Copyright 2009-present 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.

"""Tools for connecting to MongoDB.

.. seealso:: :doc:`/examples/high_availability` for examples of connecting
   to replica sets or sets of mongos servers.

To get a :class:`~pymongo.database.Database` instance from a
:class:`MongoClient` use either dictionary-style or attribute-style
access:

.. doctest::

  >>> from pymongo import MongoClient
  >>> c = MongoClient()
  >>> c.test_database
  Database(MongoClient(host=['localhost:27017'], document_class=dict, tz_aware=False, connect=True), u'test_database')
  >>> c['test-database']
  Database(MongoClient(host=['localhost:27017'], document_class=dict, tz_aware=False, connect=True), u'test-database')
"""

import contextlib
import datetime
import threading
import warnings
import weakref

from collections import defaultdict

from bson.codec_options import DEFAULT_CODEC_OPTIONS
from bson.py3compat import (integer_types,
                            string_type)
from bson.son import SON
from pymongo import (common,
                     database,
                     helpers,
                     message,
                     periodic_executor,
                     uri_parser,
                     client_session)
from pymongo.change_stream import ClusterChangeStream
from pymongo.client_options import ClientOptions
from pymongo.command_cursor import CommandCursor
from pymongo.cursor_manager import CursorManager
from pymongo.errors import (AutoReconnect,
                            BulkWriteError,
                            ConfigurationError,
                            ConnectionFailure,
                            InvalidOperation,
                            NetworkTimeout,
                            NotMasterError,
                            OperationFailure,
                            PyMongoError,
                            ServerSelectionTimeoutError)
from pymongo.read_preferences import ReadPreference
from pymongo.server_selectors import (writable_preferred_server_selector,
                                      writable_server_selector)
from pymongo.server_type import SERVER_TYPE
from pymongo.topology import Topology
from pymongo.topology_description import TOPOLOGY_TYPE
from pymongo.settings import TopologySettings
from pymongo.write_concern import DEFAULT_WRITE_CONCERN


class MongoClient(common.BaseObject):
    HOST = "localhost"
    PORT = 27017
    # Define order to retrieve options from ClientOptions for __repr__.
    # No host/port; these are retrieved from TopologySettings.
    _constructor_args = ('document_class', 'tz_aware', 'connect')

    def __init__(
            self,
            host=None,
            port=None,
            document_class=dict,
            tz_aware=None,
            connect=None,
            **kwargs):
        """Client for a MongoDB instance, a replica set, or a set of mongoses.

        The client object is thread-safe and has connection-pooling built in.
        If an operation fails because of a network error,
        :class:`~pymongo.errors.ConnectionFailure` is raised and the client
        reconnects in the background. Application code should handle this
        exception (recognizing that the operation failed) and then continue to
        execute.

        The `host` parameter can be a full `mongodb URI
        <http://dochub.mongodb.org/core/connections>`_, in addition to
        a simple hostname. It can also be a list of hostnames or
        URIs. Any port specified in the host string(s) will override
        the `port` parameter. If multiple mongodb URIs containing
        database or auth information are passed, the last database,
        username, and password present will be used.  For username and
        passwords reserved characters like ':', '/', '+' and '@' must be
        percent encoded following RFC 2396::

            try:
                # Python 3.x
                from urllib.parse import quote_plus
            except ImportError:
                # Python 2.x
                from urllib import quote_plus

            uri = "mongodb://%s:%s@%s" % (
                quote_plus(user), quote_plus(password), host)
            client = MongoClient(uri)

        Unix domain sockets are also supported. The socket path must be percent
        encoded in the URI::

            uri = "mongodb://%s:%s@%s" % (
                quote_plus(user), quote_plus(password), quote_plus(socket_path))
            client = MongoClient(uri)

        But not when passed as a simple hostname::

            client = MongoClient('/tmp/mongodb-27017.sock')

        Starting with version 3.6, PyMongo supports mongodb+srv:// URIs. The
        URI must include one, and only one, hostname. The hostname will be
        resolved to one or more DNS `SRV records
        <https://en.wikipedia.org/wiki/SRV_record>`_ which will be used
        as the seed list for connecting to the MongoDB deployment. When using
        SRV URIs, the `authSource` and `replicaSet` configuration options can
        be specified using `TXT records
        <https://en.wikipedia.org/wiki/TXT_record>`_. See the
        `Initial DNS Seedlist Discovery spec
        <https://github.com/mongodb/specifications/blob/master/source/
        initial-dns-seedlist-discovery/initial-dns-seedlist-discovery.rst>`_
        for more details. Note that the use of SRV URIs implicitly enables
        TLS support. Pass ssl=false in the URI to override.

        .. note:: MongoClient creation will block waiting for answers from
          DNS when mongodb+srv:// URIs are used.

        .. note:: Starting with version 3.0 the :class:`MongoClient`
          constructor no longer blocks while connecting to the server or
          servers, and it no longer raises
          :class:`~pymongo.errors.ConnectionFailure` if they are
          unavailable, nor :class:`~pymongo.errors.ConfigurationError`
          if the user's credentials are wrong. Instead, the constructor
          returns immediately and launches the connection process on
          background threads. You can check if the server is available
          like this::

            from pymongo.errors import ConnectionFailure
            client = MongoClient()
            try:
                # The ismaster command is cheap and does not require auth.
                client.admin.command('ismaster')
            except ConnectionFailure:
                print("Server not available")

        .. warning:: When using PyMongo in a multiprocessing context, please
          read :ref:`multiprocessing` first.

        :Parameters:
          - `host` (optional): hostname or IP address or Unix domain socket
            path of a single mongod or mongos instance to connect to, or a
            mongodb URI, or a list of hostnames / mongodb URIs. If `host` is
            an IPv6 literal it must be enclosed in '[' and ']' characters
            following the RFC2732 URL syntax (e.g. '[::1]' for localhost).
            Multihomed and round robin DNS addresses are **not** supported.
          - `port` (optional): port number on which to connect
          - `document_class` (optional): default class to use for
            documents returned from queries on this client
          - `tz_aware` (optional): if ``True``,
            :class:`~datetime.datetime` instances returned as values
            in a document by this :class:`MongoClient` will be timezone
            aware (otherwise they will be naive)
          - `connect` (optional): if ``True`` (the default), immediately
            begin connecting to MongoDB in the background. Otherwise connect
            on the first operation.

          | **Other optional parameters can be passed as keyword arguments:**

          - `maxPoolSize` (optional): The maximum allowable number of
            concurrent connections to each connected server. Requests to a
            server will block if there are `maxPoolSize` outstanding
            connections to the requested server. Defaults to 100. Cannot be 0.
          - `minPoolSize` (optional): The minimum required number of concurrent
            connections that the pool will maintain to each connected server.
            Default is 0.
          - `maxIdleTimeMS` (optional): The maximum number of milliseconds that
            a connection can remain idle in the pool before being removed and
            replaced. Defaults to `None` (no limit).
          - `socketTimeoutMS`: (integer or None) Controls how long (in
            milliseconds) the driver will wait for a response after sending an
            ordinary (non-monitoring) database operation before concluding that
            a network error has occurred. Defaults to ``None`` (no timeout).
          - `connectTimeoutMS`: (integer or None) Controls how long (in
            milliseconds) the driver will wait during server monitoring when
            connecting a new socket to a server before concluding the server
            is unavailable. Defaults to ``20000`` (20 seconds).
          - `serverSelectionTimeoutMS`: (integer) Controls how long (in
            milliseconds) the driver will wait to find an available,
            appropriate server to carry out a database operation; while it is
            waiting, multiple server monitoring operations may be carried out,
            each controlled by `connectTimeoutMS`. Defaults to ``30000`` (30
            seconds).
          - `waitQueueTimeoutMS`: (integer or None) How long (in milliseconds)
            a thread will wait for a socket from the pool if the pool has no
            free sockets. Defaults to ``None`` (no timeout).
          - `waitQueueMultiple`: (integer or None) Multiplied by maxPoolSize
            to give the number of threads allowed to wait for a socket at one
            time. Defaults to ``None`` (no limit).
          - `heartbeatFrequencyMS`: (optional) The number of milliseconds
            between periodic server checks, or None to accept the default
            frequency of 10 seconds.
          - `appname`: (string or None) The name of the application that
            created this MongoClient instance. MongoDB 3.4 and newer will
            print this value in the server log upon establishing each
            connection. It is also recorded in the slow query log and
            profile collections.
          - `driver`: (pair or None) A driver implemented on top of PyMongo can
            pass a :class:`~pymongo.driver_info.DriverInfo` to add its name,
            version, and platform to the message printed in the server log when
            establishing a connection.
          - `event_listeners`: a list or tuple of event listeners. See
            :mod:`~pymongo.monitoring` for details.
          - `retryWrites`: (boolean) Whether supported write operations
            executed within this MongoClient will be retried once after a
            network error on MongoDB 3.6+. Defaults to ``False``.
            The supported write operations are:

              - :meth:`~pymongo.collection.Collection.bulk_write`, as long as
                :class:`~pymongo.operations.UpdateMany` or
                :class:`~pymongo.operations.DeleteMany` are not included.
              - :meth:`~pymongo.collection.Collection.delete_one`
              - :meth:`~pymongo.collection.Collection.insert_one`
              - :meth:`~pymongo.collection.Collection.insert_many`
              - :meth:`~pymongo.collection.Collection.replace_one`
              - :meth:`~pymongo.collection.Collection.update_one`
              - :meth:`~pymongo.collection.Collection.find_one_and_delete`
              - :meth:`~pymongo.collection.Collection.find_one_and_replace`
              - :meth:`~pymongo.collection.Collection.find_one_and_update`

            Unsupported write operations include, but are not limited to,
            :meth:`~pymongo.collection.Collection.aggregate` using the ``$out``
            pipeline operator and any operation with an unacknowledged write
            concern (e.g. {w: 0})). See
            https://github.com/mongodb/specifications/blob/master/source/retryable-writes/retryable-writes.rst
          - `socketKeepAlive`: (boolean) **DEPRECATED** Whether to send
            periodic keep-alive packets on connected sockets. Defaults to
            ``True``. Disabling it is not recommended, see
            https://docs.mongodb.com/manual/faq/diagnostics/#does-tcp-keepalive-time-affect-mongodb-deployments",
          - `compressors`: Comma separated list of compressors for wire
            protocol compression. The list is used to negotiate a compressor
            with the server. Currently supported options are "snappy" and
            "zlib". Support for snappy requires the
            `python-snappy <https://pypi.org/project/python-snappy/>`_ package.
            zlib support requires the Python standard library zlib module.
            By default no compression is used. Compression support must also be
            enabled on the server. MongoDB 3.4+ supports snappy compression.
            MongoDB 3.6+ supports snappy and zlib.
          - `zlibCompressionLevel`: (int) The zlib compression level to use
            when zlib is used as the wire protocol compressor. Supported values
            are -1 through 9. -1 tells the zlib library to use its default
            compression level (usually 6). 0 means no compression. 1 is best
            speed. 9 is best compression. Defaults to -1.

          | **Write Concern options:**
          | (Only set if passed. No default values.)

          - `w`: (integer or string) If this is a replica set, write operations
            will block until they have been replicated to the specified number
            or tagged set of servers. `w=<int>` always includes the replica set
            primary (e.g. w=3 means write to the primary and wait until
            replicated to **two** secondaries). Passing w=0 **disables write
            acknowledgement** and all other write concern options.
          - `wtimeout`: (integer) Used in conjunction with `w`. Specify a value
            in milliseconds to control how long to wait for write propagation
            to complete. If replication does not complete in the given
            timeframe, a timeout exception is raised.
          - `j`: If ``True`` block until write operations have been committed
            to the journal. Cannot be used in combination with `fsync`. Prior
            to MongoDB 2.6 this option was ignored if the server was running
            without journaling. Starting with MongoDB 2.6 write operations will
            fail with an exception if this option is used when the server is
            running without journaling.
          - `fsync`: If ``True`` and the server is running without journaling,
            blocks until the server has synced all data files to disk. If the
            server is running with journaling, this acts the same as the `j`
            option, blocking until write operations have been committed to the
            journal. Cannot be used in combination with `j`.

          | **Replica set keyword arguments for connecting with a replica set
            - either directly or via a mongos:**

          - `replicaSet`: (string or None) The name of the replica set to
            connect to. The driver will verify that all servers it connects to
            match this name. Implies that the hosts specified are a seed list
            and the driver should attempt to find all members of the set.
            Defaults to ``None``.

          | **Read Preference:**

          - `readPreference`: The replica set read preference for this client.
            One of ``primary``, ``primaryPreferred``, ``secondary``,
            ``secondaryPreferred``, or ``nearest``. Defaults to ``primary``.
          - `readPreferenceTags`: Specifies a tag set as a comma-separated list
            of colon-separated key-value pairs. For example ``dc:ny,rack:1``.
            Defaults to ``None``.
          - `maxStalenessSeconds`: (integer) The maximum estimated
            length of time a replica set secondary can fall behind the primary
            in replication before it will no longer be selected for operations.
            Defaults to ``-1``, meaning no maximum. If maxStalenessSeconds
            is set, it must be a positive integer greater than or equal to
            90 seconds.

          | **Authentication:**

          - `username`: A string.
          - `password`: A string.

            Although username and password must be percent-escaped in a MongoDB
            URI, they must not be percent-escaped when passed as parameters. In
            this example, both the space and slash special characters are passed
            as-is::

              MongoClient(username="user name", password="pass/word")

          - `authSource`: The database to authenticate on. Defaults to the
            database specified in the URI, if provided, or to "admin".
          - `authMechanism`: See :data:`~pymongo.auth.MECHANISMS` for options.
            If no mechanism is specified, PyMongo automatically uses MONGODB-CR
            when connected to a pre-3.0 version of MongoDB, SCRAM-SHA-1 when
            connected to MongoDB 3.0 through 3.6, and negotiates the mechanism
            to use (SCRAM-SHA-1 or SCRAM-SHA-256) when connected to MongoDB
            4.0+.
          - `authMechanismProperties`: Used to specify authentication mechanism
            specific options. To specify the service name for GSSAPI
            authentication pass authMechanismProperties='SERVICE_NAME:<service
            name>'

          .. seealso:: :doc:`/examples/authentication`

          | **SSL configuration:**

          - `ssl`: If ``True``, create the connection to the server using SSL.
            Defaults to ``False``.
          - `ssl_certfile`: The certificate file used to identify the local
            connection against mongod. Implies ``ssl=True``. Defaults to
            ``None``.
          - `ssl_keyfile`: The private keyfile used to identify the local
            connection against mongod.  If included with the ``certfile`` then
            only the ``ssl_certfile`` is needed.  Implies ``ssl=True``.
            Defaults to ``None``.
          - `ssl_pem_passphrase`: The password or passphrase for decrypting
            the private key in ``ssl_certfile`` or ``ssl_keyfile``. Only
            necessary if the private key is encrypted. Only supported by python
            2.7.9+ (pypy 2.5.1+) and 3.3+. Defaults to ``None``.
          - `ssl_cert_reqs`: Specifies whether a certificate is required from
            the other side of the connection, and whether it will be validated
            if provided. It must be one of the three values ``ssl.CERT_NONE``
            (certificates ignored), ``ssl.CERT_REQUIRED`` (certificates
            required and validated), or ``ssl.CERT_OPTIONAL`` (the same as
            CERT_REQUIRED, unless the server was configured to use anonymous
            ciphers). If the value of this parameter is not ``ssl.CERT_NONE``
            and a value is not provided for ``ssl_ca_certs`` PyMongo will
            attempt to load system provided CA certificates. If the python
            version in use does not support loading system CA certificates
            then the ``ssl_ca_certs`` parameter must point to a file of CA
            certificates. Implies ``ssl=True``. Defaults to
            ``ssl.CERT_REQUIRED`` if not provided and ``ssl=True``.
          - `ssl_ca_certs`: The ca_certs file contains a set of concatenated
            "certification authority" certificates, which are used to validate
            certificates passed from the other end of the connection.
            Implies ``ssl=True``. Defaults to ``None``.
          - `ssl_crlfile`: The path to a PEM or DER formatted certificate
            revocation list. Only supported by python 2.7.9+ (pypy 2.5.1+)
            and 3.4+. Defaults to ``None``.
          - `ssl_match_hostname`: If ``True`` (the default), and
            `ssl_cert_reqs` is not ``ssl.CERT_NONE``, enables hostname
            verification using the :func:`~ssl.match_hostname` function from
            python's :mod:`~ssl` module. Think very carefully before setting
            this to ``False`` as that could make your application vulnerable to
            man-in-the-middle attacks.

          | **Read Concern options:**
          | (If not set explicitly, this will use the server default)

          - `readConcernLevel`: (string) The read concern level specifies the
            level of isolation for read operations.  For example, a read
            operation using a read concern level of ``majority`` will only
            return data that has been written to a majority of nodes. If the
            level is left unspecified, the server default will be used.

        .. mongodoc:: connections

        .. versionchanged:: 3.6
           Added support for mongodb+srv:// URIs.
           Added the ``retryWrites`` keyword argument and URI option.

        .. versionchanged:: 3.7
           Added the ``driver`` keyword argument.

        .. versionchanged:: 3.5
           Add ``username`` and ``password`` options. Document the
           ``authSource``, ``authMechanism``, and ``authMechanismProperties ``
           options.
           Deprecated the `socketKeepAlive` keyword argument and URI option.
           `socketKeepAlive` now defaults to ``True``.

        .. versionchanged:: 3.0
           :class:`~pymongo.mongo_client.MongoClient` is now the one and only
           client class for a standalone server, mongos, or replica set.
           It includes the functionality that had been split into
           :class:`~pymongo.mongo_client.MongoReplicaSetClient`: it can connect
           to a replica set, discover all its members, and monitor the set for
           stepdowns, elections, and reconfigs.

           The :class:`~pymongo.mongo_client.MongoClient` constructor no
           longer blocks while connecting to the server or servers, and it no
           longer raises :class:`~pymongo.errors.ConnectionFailure` if they
           are unavailable, nor :class:`~pymongo.errors.ConfigurationError`
           if the user's credentials are wrong. Instead, the constructor
           returns immediately and launches the connection process on
           background threads.

           Therefore the ``alive`` method is removed since it no longer
           provides meaningful information; even if the client is disconnected,
           it may discover a server in time to fulfill the next operation.

           In PyMongo 2.x, :class:`~pymongo.MongoClient` accepted a list of
           standalone MongoDB servers and used the first it could connect to::

               MongoClient(['host1.com:27017', 'host2.com:27017'])

           A list of multiple standalones is no longer supported; if multiple
           servers are listed they must be members of the same replica set, or
           mongoses in the same sharded cluster.

           The behavior for a list of mongoses is changed from "high
           availability" to "load balancing". Before, the client connected to
           the lowest-latency mongos in the list, and used it until a network
           error prompted it to re-evaluate all mongoses' latencies and
           reconnect to one of them. In PyMongo 3, the client monitors its
           network latency to all the mongoses continuously, and distributes
           operations evenly among those with the lowest latency. See
           :ref:`mongos-load-balancing` for more information.

           The ``connect`` option is added.

           The ``start_request``, ``in_request``, and ``end_request`` methods
           are removed, as well as the ``auto_start_request`` option.

           The ``copy_database`` method is removed, see the
           :doc:`copy_database examples </examples/copydb>` for alternatives.

           The :meth:`MongoClient.disconnect` method is removed; it was a
           synonym for :meth:`~pymongo.MongoClient.close`.

           :class:`~pymongo.mongo_client.MongoClient` no longer returns an
           instance of :class:`~pymongo.database.Database` for attribute names
           with leading underscores. You must use dict-style lookups instead::

               client['__my_database__']

           Not::

               client.__my_database__
        """
        if host is None:
            host = self.HOST
        if isinstance(host, string_type):
            host = [host]
        if port is None:
            port = self.PORT
        if not isinstance(port, int):
            raise TypeError("port must be an instance of int")

        seeds = set()
        username = None
        password = None
        dbase = None
        opts = {}
        for entity in host:
            if "://" in entity:
                res = uri_parser.parse_uri(entity, port, warn=True)
                seeds.update(res["nodelist"])
                username = res["username"] or username
                password = res["password"] or password
                dbase = res["database"] or dbase
                opts = res["options"]
            else:
                seeds.update(uri_parser.split_hosts(entity, port))
        if not seeds:
            raise ConfigurationError("need to specify at least one host")

        # _pool_class, _monitor_class, and _condition_class are for deep
        # customization of PyMongo, e.g. Motor.
        pool_class = kwargs.pop('_pool_class', None)
        monitor_class = kwargs.pop('_monitor_class', None)
        condition_class = kwargs.pop('_condition_class', None)

        keyword_opts = kwargs
        keyword_opts['document_class'] = document_class
        if tz_aware is None:
            tz_aware = opts.get('tz_aware', False)
        if connect is None:
            connect = opts.get('connect', True)
        keyword_opts['tz_aware'] = tz_aware
        keyword_opts['connect'] = connect
        # Validate all keyword options.
        keyword_opts = dict(common.validate(k, v)
                            for k, v in keyword_opts.items())
        opts.update(keyword_opts)
        # Username and password passed as kwargs override user info in URI.
        username = opts.get("username", username)
        password = opts.get("password", password)
        if 'socketkeepalive' in opts:
            warnings.warn(
                "The socketKeepAlive option is deprecated. It now"
                "defaults to true and disabling it is not recommended, see "
                "https://docs.mongodb.com/manual/faq/diagnostics/"
                "#does-tcp-keepalive-time-affect-mongodb-deployments",
                DeprecationWarning, stacklevel=2)
        self.__options = options = ClientOptions(
            username, password, dbase, opts)

        self.__default_database_name = dbase
        self.__lock = threading.Lock()
        self.__cursor_manager = None
        self.__kill_cursors_queue = []

        self._event_listeners = options.pool_options.event_listeners

        # Cache of existing indexes used by ensure_index ops.
        self.__index_cache = {}
        self.__index_cache_lock = threading.Lock()

        super(MongoClient, self).__init__(options.codec_options,
                                          options.read_preference,
                                          options.write_concern,
                                          options.read_concern)

        self.__all_credentials = {}
        creds = options.credentials
        if creds:
            self._cache_credentials(creds.source, creds)

        self._topology_settings = TopologySettings(
            seeds=seeds,
            replica_set_name=options.replica_set_name,
            pool_class=pool_class,
            pool_options=options.pool_options,
            monitor_class=monitor_class,
            condition_class=condition_class,
            local_threshold_ms=options.local_threshold_ms,
            server_selection_timeout=options.server_selection_timeout,
            heartbeat_frequency=options.heartbeat_frequency)

        self._topology = Topology(self._topology_settings)
        if connect:
            self._topology.open()

        def target():
            client = self_ref()
            if client is None:
                return False  # Stop the executor.
            MongoClient._process_periodic_tasks(client)
            return True

        executor = periodic_executor.PeriodicExecutor(
            interval=common.KILL_CURSOR_FREQUENCY,
            min_interval=0.5,
            target=target,
            name="pymongo_kill_cursors_thread")

        # We strongly reference the executor and it weakly references us via
        # this closure. When the client is freed, stop the executor soon.
        self_ref = weakref.ref(self, executor.close)
        self._kill_cursors_executor = executor
        executor.open()

    def _cache_credentials(self, source, credentials, connect=False):
        """Save a set of authentication credentials.

        The credentials are used to login a socket whenever one is created.
        If `connect` is True, verify the credentials on the server first.
        """
        # Don't let other threads affect this call's data.
        all_credentials = self.__all_credentials.copy()

        if source in all_credentials:
            # Nothing to do if we already have these credentials.
            if credentials == all_credentials[source]:
                return
            raise OperationFailure('Another user is already authenticated '
                                   'to this database. You must logout first.')

        if connect:
            server = self._get_topology().select_server(
                writable_preferred_server_selector)

            # get_socket() logs out of the database if logged in with old
            # credentials, and logs in with new ones.
            with server.get_socket(all_credentials) as sock_info:
                sock_info.authenticate(credentials)

        # If several threads run _cache_credentials at once, last one wins.
        self.__all_credentials[source] = credentials

    def _purge_credentials(self, source):
        """Purge credentials from the authentication cache."""
        self.__all_credentials.pop(source, None)

    def _cached(self, dbname, coll, index):
        """Test if `index` is cached."""
        cache = self.__index_cache
        now = datetime.datetime.utcnow()
        with self.__index_cache_lock:
            return (dbname in cache and
                    coll in cache[dbname] and
                    index in cache[dbname][coll] and
                    now < cache[dbname][coll][index])

    def _cache_index(self, dbname, collection, index, cache_for):
        """Add an index to the index cache for ensure_index operations."""
        now = datetime.datetime.utcnow()
        expire = datetime.timedelta(seconds=cache_for) + now

        with self.__index_cache_lock:
            if dbname not in self.__index_cache:
                self.__index_cache[dbname] = {}
                self.__index_cache[dbname][collection] = {}
                self.__index_cache[dbname][collection][index] = expire

            elif collection not in self.__index_cache[dbname]:
                self.__index_cache[dbname][collection] = {}
                self.__index_cache[dbname][collection][index] = expire

            else:
                self.__index_cache[dbname][collection][index] = expire

    def _purge_index(self, database_name,
                     collection_name=None, index_name=None):
        """Purge an index from the index cache.

        If `index_name` is None purge an entire collection.

        If `collection_name` is None purge an entire database.
        """
        with self.__index_cache_lock:
            if not database_name in self.__index_cache:
                return

            if collection_name is None:
                del self.__index_cache[database_name]
                return

            if not collection_name in self.__index_cache[database_name]:
                return

            if index_name is None:
                del self.__index_cache[database_name][collection_name]
                return

            if index_name in self.__index_cache[database_name][collection_name]:
                del self.__index_cache[database_name][collection_name][index_name]

    def _server_property(self, attr_name):
        """An attribute of the current server's description.

        If the client is not connected, this will block until a connection is
        established or raise ServerSelectionTimeoutError if no server is
        available.

        Not threadsafe if used multiple times in a single method, since
        the server may change. In such cases, store a local reference to a
        ServerDescription first, then use its properties.
        """
        server = self._topology.select_server(
            writable_server_selector)

        return getattr(server.description, attr_name)

    def watch(self, pipeline=None, full_document='default', resume_after=None,
              max_await_time_ms=None, batch_size=None, collation=None,
              start_at_operation_time=None, session=None):
        """Watch changes on this cluster.

        Performs an aggregation with an implicit initial ``$changeStream``
        stage and returns a
        :class:`~pymongo.change_stream.ClusterChangeStream` cursor which
        iterates over changes on all databases on this cluster.

        Introduced in MongoDB 4.0.

        .. code-block:: python

           with client.watch() as stream:
               for change in stream:
                   print(change)

        The :class:`~pymongo.change_stream.ClusterChangeStream` iterable
        blocks until the next change document is returned or an error is
        raised. If the
        :meth:`~pymongo.change_stream.ClusterChangeStream.next` method
        encounters a network error when retrieving a batch from the server,
        it will automatically attempt to recreate the cursor such that no
        change events are missed. Any error encountered during the resume
        attempt indicates there may be an outage and will be raised.

        .. code-block:: python

            try:
                with client.watch(
                        [{'$match': {'operationType': 'insert'}}]) as stream:
                    for insert_change in stream:
                        print(insert_change)
            except pymongo.errors.PyMongoError:
                # The ChangeStream encountered an unrecoverable error or the
                # resume attempt failed to recreate the cursor.
                logging.error('...')

        For a precise description of the resume process see the
        `change streams specification`_.

        :Parameters:
          - `pipeline` (optional): A list of aggregation pipeline stages to
            append to an initial ``$changeStream`` stage. Not all
            pipeline stages are valid after a ``$changeStream`` stage, see the
            MongoDB documentation on change streams for the supported stages.
          - `full_document` (optional): The fullDocument to pass as an option
            to the ``$changeStream`` stage. Allowed values: 'default',
            'updateLookup'.  Defaults to 'default'.
            When set to 'updateLookup', the change notification for partial
            updates will include both a delta describing the changes to the
            document, as well as a copy of the entire document that was
            changed from some time after the change occurred.
          - `resume_after` (optional): The logical starting point for this
            change stream.
          - `max_await_time_ms` (optional): The maximum time in milliseconds
            for the server to wait for changes before responding to a getMore
            operation.
          - `batch_size` (optional): The maximum number of documents to return
            per batch.
          - `collation` (optional): The :class:`~pymongo.collation.Collation`
            to use for the aggregation.
          - `start_at_operation_time` (optional): If provided, the resulting
            change stream will only return changes that occurred at or after
            the specified :class:`~bson.timestamp.Timestamp`. Requires
            MongoDB >= 4.0.
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        :Returns:
          A :class:`~pymongo.change_stream.ClusterChangeStream` cursor.

        .. versionadded:: 3.7

        .. mongodoc:: changeStreams

        .. _change streams specification:
            https://github.com/mongodb/specifications/blob/master/source/change-streams/change-streams.rst
        """
        return ClusterChangeStream(
            self.admin, pipeline, full_document, resume_after, max_await_time_ms,
            batch_size, collation, start_at_operation_time, session
        )

    @property
    def event_listeners(self):
        """The event listeners registered for this client.

        See :mod:`~pymongo.monitoring` for details.
        """
        return self._event_listeners.event_listeners

    @property
    def address(self):
        """(host, port) of the current standalone, primary, or mongos, or None.

        Accessing :attr:`address` raises :exc:`~.errors.InvalidOperation` if
        the client is load-balancing among mongoses, since there is no single
        address. Use :attr:`nodes` instead.

        If the client is not connected, this will block until a connection is
        established or raise ServerSelectionTimeoutError if no server is
        available.

        .. versionadded:: 3.0
        """
        topology_type = self._topology._description.topology_type
        if topology_type == TOPOLOGY_TYPE.Sharded:
            raise InvalidOperation(
                'Cannot use "address" property when load balancing among'
                ' mongoses, use "nodes" instead.')
        if topology_type not in (TOPOLOGY_TYPE.ReplicaSetWithPrimary,
                                 TOPOLOGY_TYPE.Single):
            return None
        return self._server_property('address')

    @property
    def primary(self):
        """The (host, port) of the current primary of the replica set.

        Returns ``None`` if this client is not connected to a replica set,
        there is no primary, or this client was created without the
        `replicaSet` option.

        .. versionadded:: 3.0
           MongoClient gained this property in version 3.0 when
           MongoReplicaSetClient's functionality was merged in.
        """
        return self._topology.get_primary()

    @property
    def secondaries(self):
        """The secondary members known to this client.

        A sequence of (host, port) pairs. Empty if this client is not
        connected to a replica set, there are no visible secondaries, or this
        client was created without the `replicaSet` option.

        .. versionadded:: 3.0
           MongoClient gained this property in version 3.0 when
           MongoReplicaSetClient's functionality was merged in.
        """
        return self._topology.get_secondaries()

    @property
    def arbiters(self):
        """Arbiters in the replica set.

        A sequence of (host, port) pairs. Empty if this client is not
        connected to a replica set, there are no arbiters, or this client was
        created without the `replicaSet` option.
        """
        return self._topology.get_arbiters()

    @property
    def is_primary(self):
        """If this client is connected to a server that can accept writes.

        True if the current server is a standalone, mongos, or the primary of
        a replica set. If the client is not connected, this will block until a
        connection is established or raise ServerSelectionTimeoutError if no
        server is available.
        """
        return self._server_property('is_writable')

    @property
    def is_mongos(self):
        """If this client is connected to mongos. If the client is not
        connected, this will block until a connection is established or raise
        ServerSelectionTimeoutError if no server is available..
        """
        return self._server_property('server_type') == SERVER_TYPE.Mongos

    @property
    def max_pool_size(self):
        """The maximum allowable number of concurrent connections to each
        connected server. Requests to a server will block if there are
        `maxPoolSize` outstanding connections to the requested server.
        Defaults to 100. Cannot be 0.

        When a server's pool has reached `max_pool_size`, operations for that
        server block waiting for a socket to be returned to the pool. If
        ``waitQueueTimeoutMS`` is set, a blocked operation will raise
        :exc:`~pymongo.errors.ConnectionFailure` after a timeout.
        By default ``waitQueueTimeoutMS`` is not set.
        """
        return self.__options.pool_options.max_pool_size

    @property
    def min_pool_size(self):
        """The minimum required number of concurrent connections that the pool
        will maintain to each connected server. Default is 0.
        """
        return self.__options.pool_options.min_pool_size

    @property
    def max_idle_time_ms(self):
        """The maximum number of milliseconds that a connection can remain
        idle in the pool before being removed and replaced. Defaults to
        `None` (no limit).
        """
        seconds = self.__options.pool_options.max_idle_time_seconds
        if seconds is None:
            return None
        return 1000 * seconds

    @property
    def nodes(self):
        """Set of all currently connected servers.

        .. warning:: When connected to a replica set the value of :attr:`nodes`
          can change over time as :class:`MongoClient`'s view of the replica
          set changes. :attr:`nodes` can also be an empty set when
          :class:`MongoClient` is first instantiated and hasn't yet connected
          to any servers, or a network partition causes it to lose connection
          to all servers.
        """
        description = self._topology.description
        return frozenset(s.address for s in description.known_servers)

    @property
    def max_bson_size(self):
        """The largest BSON object the connected server accepts in bytes.

        If the client is not connected, this will block until a connection is
        established or raise ServerSelectionTimeoutError if no server is
        available.
        """
        return self._server_property('max_bson_size')

    @property
    def max_message_size(self):
        """The largest message the connected server accepts in bytes.

        If the client is not connected, this will block until a connection is
        established or raise ServerSelectionTimeoutError if no server is
        available.
        """
        return self._server_property('max_message_size')

    @property
    def max_write_batch_size(self):
        """The maxWriteBatchSize reported by the server.

        If the client is not connected, this will block until a connection is
        established or raise ServerSelectionTimeoutError if no server is
        available.

        Returns a default value when connected to server versions prior to
        MongoDB 2.6.
        """
        return self._server_property('max_write_batch_size')

    @property
    def local_threshold_ms(self):
        """The local threshold for this instance."""
        return self.__options.local_threshold_ms

    @property
    def server_selection_timeout(self):
        """The server selection timeout for this instance in seconds."""
        return self.__options.server_selection_timeout

    @property
    def retry_writes(self):
        """If this instance should retry supported write operations."""
        return self.__options.retry_writes

    def _is_writable(self):
        """Attempt to connect to a writable server, or return False.
        """
        topology = self._get_topology()  # Starts monitors if necessary.
        try:
            svr = topology.select_server(writable_server_selector)

            # When directly connected to a secondary, arbiter, etc.,
            # select_server returns it, whatever the selector. Check
            # again if the server is writable.
            return svr.description.is_writable
        except ConnectionFailure:
            return False

    def _end_sessions(self, session_ids):
        """Send endSessions command(s) with the given session ids."""
        try:
            # Use SocketInfo.command directly to avoid implicitly creating
            # another session.
            with self._socket_for_reads(
                    ReadPreference.PRIMARY_PREFERRED) as (sock_info, slave_ok):
                if not sock_info.supports_sessions:
                    return

                for i in range(0, len(session_ids), common._MAX_END_SESSIONS):
                    spec = SON([('endSessions',
                                 session_ids[i:i + common._MAX_END_SESSIONS])])
                    sock_info.command(
                        'admin', spec, slave_ok=slave_ok, client=self)
        except PyMongoError:
            # Drivers MUST ignore any errors returned by the endSessions
            # command.
            pass

    def close(self):
        """Cleanup client resources and disconnect from MongoDB.

        On MongoDB >= 3.6, end all server sessions created by this client by
        sending one or more endSessions commands.

        Close all sockets in the connection pools and stop the monitor threads.
        If this instance is used again it will be automatically re-opened and
        the threads restarted.

        .. versionchanged:: 3.6
           End all server sessions created by this client.
        """
        session_ids = self._topology.pop_all_sessions()
        if session_ids:
            self._end_sessions(session_ids)
        # Run _process_periodic_tasks to send pending killCursor requests
        # before closing the topology.
        self._process_periodic_tasks()
        self._topology.close()

    def set_cursor_manager(self, manager_class):
        """DEPRECATED - Set this client's cursor manager.

        Raises :class:`TypeError` if `manager_class` is not a subclass of
        :class:`~pymongo.cursor_manager.CursorManager`. A cursor manager
        handles closing cursors. Different managers can implement different
        policies in terms of when to actually kill a cursor that has
        been closed.

        :Parameters:
          - `manager_class`: cursor manager to use

        .. versionchanged:: 3.3
           Deprecated, for real this time.

        .. versionchanged:: 3.0
           Undeprecated.
        """
        warnings.warn(
            "set_cursor_manager is Deprecated",
            DeprecationWarning,
            stacklevel=2)
        manager = manager_class(self)
        if not isinstance(manager, CursorManager):
            raise TypeError("manager_class must be a subclass of "
                            "CursorManager")

        self.__cursor_manager = manager

    def _get_topology(self):
        """Get the internal :class:`~pymongo.topology.Topology` object.

        If this client was created with "connect=False", calling _get_topology
        launches the connection process in the background.
        """
        self._topology.open()
        return self._topology

    @contextlib.contextmanager
    def _get_socket(self, server):
        try:
            with server.get_socket(self.__all_credentials) as sock_info:
                yield sock_info
        except NetworkTimeout:
            # The socket has been closed. Don't reset the server.
            # Server Discovery And Monitoring Spec: "When an application
            # operation fails because of any network error besides a socket
            # timeout...."
            raise
        except NotMasterError:
            # "When the client sees a "not master" error it MUST replace the
            # server's description with type Unknown. It MUST request an
            # immediate check of the server."
            self._reset_server_and_request_check(server.description.address)
            raise
        except ConnectionFailure:
            # "Client MUST replace the server's description with type Unknown
            # ... MUST NOT request an immediate check of the server."
            self.__reset_server(server.description.address)
            raise
        except OperationFailure as exc:
            if exc.code in helpers._RETRYABLE_ERROR_CODES:
                # Do not request an immediate check since the server is likely
                # shutting down.
                self.__reset_server(server.description.address)
            raise

    def _socket_for_writes(self):
        server = self._get_topology().select_server(writable_server_selector)
        return self._get_socket(server)

    @contextlib.contextmanager
    def _socket_for_reads(self, read_preference):
        assert read_preference is not None, "read_preference must not be None"
        # Get a socket for a server matching the read preference, and yield
        # sock_info, slave_ok. Server Selection Spec: "slaveOK must be sent to
        # mongods with topology type Single. If the server type is Mongos,
        # follow the rules for passing read preference to mongos, even for
        # topology type Single."
        # Thread safe: if the type is single it cannot change.
        topology = self._get_topology()
        single = topology.description.topology_type == TOPOLOGY_TYPE.Single
        server = topology.select_server(read_preference)

        with self._get_socket(server) as sock_info:
            slave_ok = (single and not sock_info.is_mongos) or (
                read_preference != ReadPreference.PRIMARY)
            yield sock_info, slave_ok

    def _send_message_with_response(self, operation, exhaust=False,
                                    address=None):
        """Send a message to MongoDB and return a Response.

        :Parameters:
          - `operation`: a _Query or _GetMore object.
          - `read_preference` (optional): A ReadPreference.
          - `exhaust` (optional): If True, the socket used stays checked out.
            It is returned along with its Pool in the Response.
          - `address` (optional): Optional address when sending a message
            to a specific server, used for getMore.
        """
        with self.__lock:
            # If needed, restart kill-cursors thread after a fork.
            self._kill_cursors_executor.open()

        topology = self._get_topology()
        if address:
            server = topology.select_server_by_address(address)
            if not server:
                raise AutoReconnect('server %s:%d no longer available'
                                    % address)
        else:
            server = topology.select_server(operation.read_preference)

        # If this is a direct connection to a mongod, *always* set the slaveOk
        # bit. See bullet point 2 in server-selection.rst#topology-type-single.
        set_slave_ok = (
            topology.description.topology_type == TOPOLOGY_TYPE.Single
            and server.description.server_type != SERVER_TYPE.Mongos) or (
                operation.read_preference != ReadPreference.PRIMARY)

        return self._reset_on_error(
            server,
            server.send_message_with_response,
            operation,
            set_slave_ok,
            self.__all_credentials,
            self._event_listeners,
            exhaust)

    def _reset_on_error(self, server, func, *args, **kwargs):
        """Execute an operation. Reset the server on network error.

        Returns fn()'s return value on success. On error, clears the server's
        pool and marks the server Unknown.

        Re-raises any exception thrown by fn().
        """
        try:
            return func(*args, **kwargs)
        except NetworkTimeout:
            # The socket has been closed. Don't reset the server.
            raise
        except ConnectionFailure:
            self.__reset_server(server.description.address)
            raise

    def _retry_with_session(self, retryable, func, session, bulk):
        """Execute an operation with at most one consecutive retries

        Returns func()'s return value on success. On error retries the same
        command once.

        Re-raises any exception thrown by func().
        """
        retryable = (retryable and self.retry_writes
                     and session and not session._in_transaction)
        last_error = None
        retrying = False

        def is_retrying():
            return bulk.retrying if bulk else retrying
        while True:
            try:
                server = self._get_topology().select_server(
                    writable_server_selector)
                supports_session = (
                    session is not None and
                    server.description.retryable_writes_supported)
                with self._get_socket(server) as sock_info:
                    if retryable and not supports_session:
                        if is_retrying():
                            # A retry is not possible because this server does
                            # not support sessions raise the last error.
                            raise last_error
                        retryable = False
                    if is_retrying():
                        # Reset the transaction id and retry the operation.
                        session._retry_transaction_id()
                    return func(session, sock_info, retryable)
            except ServerSelectionTimeoutError:
                if is_retrying():
                    # The application may think the write was never attempted
                    # if we raise ServerSelectionTimeoutError on the retry
                    # attempt. Raise the original exception instead.
                    raise last_error
                # A ServerSelectionTimeoutError error indicates that there may
                # be a persistent outage. Attempting to retry in this case will
                # most likely be a waste of time.
                raise
            except ConnectionFailure as exc:
                if not retryable or is_retrying():
                    raise
                if bulk:
                    bulk.retrying = True
                else:
                    retrying = True
                last_error = exc
            except BulkWriteError as exc:
                if not retryable or is_retrying():
                    raise
                # Check the last writeConcernError to determine if this
                # BulkWriteError is retryable.
                wces = exc.details['writeConcernErrors']
                wce = wces[-1] if wces else {}
                if wce.get('code', 0) not in helpers._RETRYABLE_ERROR_CODES:
                    raise
                if bulk:
                    bulk.retrying = True
                else:
                    retrying = True
                last_error = exc
            except OperationFailure as exc:
                if not retryable or is_retrying():
                    raise
                if exc.code not in helpers._RETRYABLE_ERROR_CODES:
                    raise
                if bulk:
                    bulk.retrying = True
                else:
                    retrying = True
                last_error = exc

    def _retryable_write(self, retryable, func, session):
        """Internal retryable write helper."""
        with self._tmp_session(session) as s:
            return self._retry_with_session(retryable, func, s, None)

    def __reset_server(self, address):
        """Clear our connection pool for a server and mark it Unknown."""
        self._topology.reset_server(address)

    def _reset_server_and_request_check(self, address):
        """Clear our pool for a server, mark it Unknown, and check it soon."""
        self._topology.reset_server_and_request_check(address)

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self.address == other.address
        return NotImplemented

    def __ne__(self, other):
        return not self == other

    def _repr_helper(self):
        def option_repr(option, value):
            """Fix options whose __repr__ isn't usable in a constructor."""
            if option == 'document_class':
                if value is dict:
                    return 'document_class=dict'
                else:
                    return 'document_class=%s.%s' % (value.__module__,
                                                     value.__name__)
            if option in common.TIMEOUT_VALIDATORS and value is not None:
                return "%s=%s" % (option, int(value * 1000))

            return '%s=%r' % (option, value)

        # Host first...
        options = ['host=%r' % [
            '%s:%d' % (host, port) if port is not None else host
            for host, port in self._topology_settings.seeds]]
        # ... then everything in self._constructor_args...
        options.extend(
            option_repr(key, self.__options._options[key])
            for key in self._constructor_args)
        # ... then everything else.
        options.extend(
            option_repr(key, self.__options._options[key])
            for key in self.__options._options
            if key not in set(self._constructor_args)
            and key != 'username' and key != 'password')
        return ', '.join(options)

    def __repr__(self):
        return ("MongoClient(%s)" % (self._repr_helper(),))

    def __getattr__(self, name):
        """Get a database by name.

        Raises :class:`~pymongo.errors.InvalidName` if an invalid
        database name is used.

        :Parameters:
          - `name`: the name of the database to get
        """
        if name.startswith('_'):
            raise AttributeError(
                "MongoClient has no attribute %r. To access the %s"
                " database, use client[%r]." % (name, name, name))
        return self.__getitem__(name)

    def __getitem__(self, name):
        """Get a database by name.

        Raises :class:`~pymongo.errors.InvalidName` if an invalid
        database name is used.

        :Parameters:
          - `name`: the name of the database to get
        """
        return database.Database(self, name)

    def close_cursor(self, cursor_id, address=None):
        """DEPRECATED - Send a kill cursors message soon with the given id.

        Raises :class:`TypeError` if `cursor_id` is not an instance of
        ``(int, long)``. What closing the cursor actually means
        depends on this client's cursor manager.

        This method may be called from a :class:`~pymongo.cursor.Cursor`
        destructor during garbage collection, so it isn't safe to take a
        lock or do network I/O. Instead, we schedule the cursor to be closed
        soon on a background thread.

        :Parameters:
          - `cursor_id`: id of cursor to close
          - `address` (optional): (host, port) pair of the cursor's server.
            If it is not provided, the client attempts to close the cursor on
            the primary or standalone, or a mongos server.

        .. versionchanged:: 3.7
           Deprecated.

        .. versionchanged:: 3.0
           Added ``address`` parameter.
        """
        warnings.warn(
            "close_cursor is deprecated.",
            DeprecationWarning,
            stacklevel=2)
        if not isinstance(cursor_id, integer_types):
            raise TypeError("cursor_id must be an instance of (int, long)")

        self._close_cursor(cursor_id, address)

    def _close_cursor(self, cursor_id, address):
        """Send a kill cursors message with the given id.

        What closing the cursor actually means depends on this client's
        cursor manager. If there is none, the cursor is closed asynchronously
        on a background thread.
        """
        if self.__cursor_manager is not None:
            self.__cursor_manager.close(cursor_id, address)
        else:
            self.__kill_cursors_queue.append((address, [cursor_id]))

    def _close_cursor_now(self, cursor_id, address=None, session=None):
        """Send a kill cursors message with the given id.

        What closing the cursor actually means depends on this client's
        cursor manager. If there is none, the cursor is closed synchronously
        on the current thread.
        """
        if not isinstance(cursor_id, integer_types):
            raise TypeError("cursor_id must be an instance of (int, long)")

        if self.__cursor_manager is not None:
            self.__cursor_manager.close(cursor_id, address)
        else:
            try:
                self._kill_cursors(
                    [cursor_id], address, self._get_topology(), session)
            except PyMongoError:
                # Make another attempt to kill the cursor later.
                self.__kill_cursors_queue.append((address, [cursor_id]))

    def kill_cursors(self, cursor_ids, address=None):
        """DEPRECATED - Send a kill cursors message soon with the given ids.

        Raises :class:`TypeError` if `cursor_ids` is not an instance of
        ``list``.

        :Parameters:
          - `cursor_ids`: list of cursor ids to kill
          - `address` (optional): (host, port) pair of the cursor's server.
            If it is not provided, the client attempts to close the cursor on
            the primary or standalone, or a mongos server.

        .. versionchanged:: 3.3
           Deprecated.

        .. versionchanged:: 3.0
           Now accepts an `address` argument. Schedules the cursors to be
           closed on a background thread instead of sending the message
           immediately.
        """
        warnings.warn(
            "kill_cursors is deprecated.",
            DeprecationWarning,
            stacklevel=2)

        if not isinstance(cursor_ids, list):
            raise TypeError("cursor_ids must be a list")

        # "Atomic", needs no lock.
        self.__kill_cursors_queue.append((address, cursor_ids))

    def _kill_cursors(self, cursor_ids, address, topology, session):
        """Send a kill cursors message with the given ids."""
        listeners = self._event_listeners
        publish = listeners.enabled_for_commands
        if address:
            # address could be a tuple or _CursorAddress, but
            # select_server_by_address needs (host, port).
            server = topology.select_server_by_address(tuple(address))
        else:
            # Application called close_cursor() with no address.
            server = topology.select_server(writable_server_selector)

        try:
            namespace = address.namespace
            db, coll = namespace.split('.', 1)
        except AttributeError:
            namespace = None
            db = coll = "OP_KILL_CURSORS"

        spec = SON([('killCursors', coll), ('cursors', cursor_ids)])
        with server.get_socket(self.__all_credentials) as sock_info:
            if sock_info.max_wire_version >= 4 and namespace is not None:
                sock_info.command(db, spec, session=session, client=self)
            else:
                if publish:
                    start = datetime.datetime.now()
                request_id, msg = message.kill_cursors(cursor_ids)
                if publish:
                    duration = datetime.datetime.now() - start
                    # Here and below, address could be a tuple or
                    # _CursorAddress. We always want to publish a
                    # tuple to match the rest of the monitoring
                    # API.
                    listeners.publish_command_start(
                        spec, db, request_id, tuple(address))
                    start = datetime.datetime.now()

                try:
                    sock_info.send_message(msg, 0)
                except Exception as exc:
                    if publish:
                        dur = ((datetime.datetime.now() - start) + duration)
                        listeners.publish_command_failure(
                            dur, message._convert_exception(exc),
                            'killCursors', request_id,
                            tuple(address))
                    raise

                if publish:
                    duration = ((datetime.datetime.now() - start) + duration)
                    # OP_KILL_CURSORS returns no reply, fake one.
                    reply = {'cursorsUnknown': cursor_ids, 'ok': 1}
                    listeners.publish_command_success(
                        duration, reply, 'killCursors', request_id,
                        tuple(address))

    # This method is run periodically by a background thread.
    def _process_periodic_tasks(self):
        """Process any pending kill cursors requests and
        maintain connection pool parameters."""
        address_to_cursor_ids = defaultdict(list)

        # Other threads or the GC may append to the queue concurrently.
        while True:
            try:
                address, cursor_ids = self.__kill_cursors_queue.pop()
            except IndexError:
                break

            address_to_cursor_ids[address].extend(cursor_ids)

        # Don't re-open topology if it's closed and there's no pending cursors.
        if address_to_cursor_ids:
            topology = self._get_topology()
            for address, cursor_ids in address_to_cursor_ids.items():
                try:
                    self._kill_cursors(
                        cursor_ids, address, topology, session=None)
                except Exception:
                    helpers._handle_exception()
        try:
            self._topology.update_pool()
        except Exception:
            helpers._handle_exception()

    def __start_session(self, implicit, **kwargs):
        # Driver Sessions Spec: "If startSession is called when multiple users
        # are authenticated drivers MUST raise an error with the error message
        # 'Cannot call startSession when multiple users are authenticated.'"
        authset = set(self.__all_credentials.values())
        if len(authset) > 1:
            raise InvalidOperation("Cannot call start_session when"
                                   " multiple users are authenticated")

        # Raises ConfigurationError if sessions are not supported.
        server_session = self._get_server_session()
        opts = client_session.SessionOptions(**kwargs)
        return client_session.ClientSession(
            self, server_session, opts, authset, implicit)

    def start_session(self,
                      causal_consistency=True,
                      default_transaction_options=None):
        """Start a logical session.

        This method takes the same parameters as
        :class:`~pymongo.client_session.SessionOptions`. See the
        :mod:`~pymongo.client_session` module for details and examples.

        Requires MongoDB 3.6. It is an error to call :meth:`start_session`
        if this client has been authenticated to multiple databases using the
        deprecated method :meth:`~pymongo.database.Database.authenticate`.

        A :class:`~pymongo.client_session.ClientSession` may only be used with
        the MongoClient that started it.

        :Returns:
          An instance of :class:`~pymongo.client_session.ClientSession`.

        .. versionadded:: 3.6
        """
        return self.__start_session(
            False,
            causal_consistency=causal_consistency,
            default_transaction_options=default_transaction_options)

    def _get_server_session(self):
        """Internal: start or resume a _ServerSession."""
        return self._topology.get_server_session()

    def _return_server_session(self, server_session, lock):
        """Internal: return a _ServerSession to the pool."""
        return self._topology.return_server_session(server_session, lock)

    def _ensure_session(self, session=None):
        """If provided session is None, lend a temporary session."""
        if session:
            return session

        try:
            # Don't make implicit sessions causally consistent. Applications
            # should always opt-in.
            return self.__start_session(True, causal_consistency=False)
        except (ConfigurationError, InvalidOperation):
            # Sessions not supported, or multiple users authenticated.
            return None

    @contextlib.contextmanager
    def _tmp_session(self, session, close=True):
        """If provided session is None, lend a temporary session."""
        if session:
            # Don't call end_session.
            yield session
            return

        s = self._ensure_session(session)
        if s and close:
            with s:
                # Call end_session when we exit this scope.
                yield s
        elif s:
            try:
                # Only call end_session on error.
                yield s
            except Exception:
                s.end_session()
                raise
        else:
            yield None

    def _send_cluster_time(self, command, session):
        topology_time = self._topology.max_cluster_time()
        session_time = session.cluster_time if session else None
        if topology_time and session_time:
            if topology_time['clusterTime'] > session_time['clusterTime']:
                cluster_time = topology_time
            else:
                cluster_time = session_time
        else:
            cluster_time = topology_time or session_time
        if cluster_time:
            command['$clusterTime'] = cluster_time

    def _receive_cluster_time(self, reply, session):
        cluster_time = reply.get('$clusterTime')
        self._topology.receive_cluster_time(cluster_time)
        if session is not None:
            session._advance_cluster_time(cluster_time)
            session._advance_operation_time(reply.get("operationTime"))

    def server_info(self, session=None):
        """Get information about the MongoDB server we're connected to.

        :Parameters:
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        .. versionchanged:: 3.6
           Added ``session`` parameter.
        """
        return self.admin.command("buildinfo",
                                  read_preference=ReadPreference.PRIMARY,
                                  session=session)

    def list_databases(self, session=None, **kwargs):
        """Get a cursor over the databases of the connected server.

        :Parameters:
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.
          - `**kwargs` (optional): Optional parameters of the
            `listDatabases command
            <https://docs.mongodb.com/manual/reference/command/listDatabases/>`_
            can be passed as keyword arguments to this method. The supported
            options differ by server version.

        :Returns:
          An instance of :class:`~pymongo.command_cursor.CommandCursor`.

        .. versionadded:: 3.6
        """
        cmd = SON([("listDatabases", 1)])
        cmd.update(kwargs)
        admin = self._database_default_options("admin")
        res = admin.command(cmd, session=session)
        # listDatabases doesn't return a cursor (yet). Fake one.
        cursor = {
            "id": 0,
            "firstBatch": res["databases"],
            "ns": "admin.$cmd",
        }
        return CommandCursor(admin["$cmd"], cursor, None)

    def list_database_names(self, session=None):
        """Get a list of the names of all databases on the connected server.

        :Parameters:
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        .. versionadded:: 3.6
        """
        return [doc["name"]
                for doc in self.list_databases(session, nameOnly=True)]

    def database_names(self, session=None):
        """**DEPRECATED**: Get a list of the names of all databases on the
        connected server.

        :Parameters:
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        .. versionchanged:: 3.7
           Deprecated. Use :meth:`list_database_names` instead.

        .. versionchanged:: 3.6
           Added ``session`` parameter.
        """
        warnings.warn("database_names is deprecated. Use list_database_names "
                      "instead.", DeprecationWarning, stacklevel=2)
        return self.list_database_names(session)

    def drop_database(self, name_or_database, session=None):
        """Drop a database.

        Raises :class:`TypeError` if `name_or_database` is not an instance of
        :class:`basestring` (:class:`str` in python 3) or
        :class:`~pymongo.database.Database`.

        :Parameters:
          - `name_or_database`: the name of a database to drop, or a
            :class:`~pymongo.database.Database` instance representing the
            database to drop
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        .. versionchanged:: 3.6
           Added ``session`` parameter.

        .. note:: The :attr:`~pymongo.mongo_client.MongoClient.write_concern` of
           this client is automatically applied to this operation when using
           MongoDB >= 3.4.

        .. versionchanged:: 3.4
           Apply this client's write concern automatically to this operation
           when connected to MongoDB >= 3.4.

        """
        name = name_or_database
        if isinstance(name, database.Database):
            name = name.name

        if not isinstance(name, string_type):
            raise TypeError("name_or_database must be an instance "
                            "of %s or a Database" % (string_type.__name__,))

        self._purge_index(name)
        with self._socket_for_writes() as sock_info:
            self[name]._command(
                sock_info,
                "dropDatabase",
                read_preference=ReadPreference.PRIMARY,
                write_concern=self._write_concern_for(session),
                parse_write_concern_error=True,
                session=session)

    def get_default_database(self):
        """DEPRECATED - Get the database named in the MongoDB connection URI.

        >>> uri = 'mongodb://host/my_database'
        >>> client = MongoClient(uri)
        >>> db = client.get_default_database()
        >>> assert db.name == 'my_database'
        >>> db = client.get_database()
        >>> assert db.name == 'my_database'

        Useful in scripts where you want to choose which database to use
        based only on the URI in a configuration file.

        .. versionchanged:: 3.5
           Deprecated, use :meth:`get_database` instead.
        """
        warnings.warn("get_default_database is deprecated. Use get_database "
                      "instead.", DeprecationWarning, stacklevel=2)
        if self.__default_database_name is None:
            raise ConfigurationError('No default database defined')

        return self[self.__default_database_name]

    def get_database(self, name=None, codec_options=None, read_preference=None,
                     write_concern=None, read_concern=None):
        """Get a :class:`~pymongo.database.Database` with the given name and
        options.

        Useful for creating a :class:`~pymongo.database.Database` with
        different codec options, read preference, and/or write concern from
        this :class:`MongoClient`.

          >>> client.read_preference
          Primary()
          >>> db1 = client.test
          >>> db1.read_preference
          Primary()
          >>> from pymongo import ReadPreference
          >>> db2 = client.get_database(
          ...     'test', read_preference=ReadPreference.SECONDARY)
          >>> db2.read_preference
          Secondary(tag_sets=None)

        :Parameters:
          - `name` (optional): The name of the database - a string. If ``None``
            (the default) the database named in the MongoDB connection URI is
            returned.
          - `codec_options` (optional): An instance of
            :class:`~bson.codec_options.CodecOptions`. If ``None`` (the
            default) the :attr:`codec_options` of this :class:`MongoClient` is
            used.
          - `read_preference` (optional): The read preference to use. If
            ``None`` (the default) the :attr:`read_preference` of this
            :class:`MongoClient` is used. See :mod:`~pymongo.read_preferences`
            for options.
          - `write_concern` (optional): An instance of
            :class:`~pymongo.write_concern.WriteConcern`. If ``None`` (the
            default) the :attr:`write_concern` of this :class:`MongoClient` is
            used.
          - `read_concern` (optional): An instance of
            :class:`~pymongo.read_concern.ReadConcern`. If ``None`` (the
            default) the :attr:`read_concern` of this :class:`MongoClient` is
            used.

        .. versionchanged:: 3.5
           The `name` parameter is now optional, defaulting to the database
           named in the MongoDB connection URI.
        """
        if name is None:
            if self.__default_database_name is None:
                raise ConfigurationError('No default database defined')
            name = self.__default_database_name

        return database.Database(
            self, name, codec_options, read_preference,
            write_concern, read_concern)

    def _database_default_options(self, name):
        """Get a Database instance with the default settings."""
        return self.get_database(
            name, codec_options=DEFAULT_CODEC_OPTIONS,
            read_preference=ReadPreference.PRIMARY,
            write_concern=DEFAULT_WRITE_CONCERN)

    @property
    def is_locked(self):
        """Is this server locked? While locked, all write operations
        are blocked, although read operations may still be allowed.
        Use :meth:`unlock` to unlock.
        """
        ops = self._database_default_options('admin').current_op()
        return bool(ops.get('fsyncLock', 0))

    def fsync(self, **kwargs):
        """Flush all pending writes to datafiles.

        Optional parameters can be passed as keyword arguments:
          - `lock`: If True lock the server to disallow writes.
          - `async`: If True don't block while synchronizing.
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        .. note:: Starting with Python 3.7 `async` is a reserved keyword.
          The async option to the fsync command can be passed using a
          dictionary instead::

            options = {'async': True}
            client.fsync(**options)

        .. versionchanged:: 3.6
           Added ``session`` parameter.

        .. warning:: `async` and `lock` can not be used together.

        .. warning:: MongoDB does not support the `async` option
                     on Windows and will raise an exception on that
                     platform.
        """
        self.admin.command("fsync",
                           read_preference=ReadPreference.PRIMARY, **kwargs)

    def unlock(self, session=None):
        """Unlock a previously locked server.

        :Parameters:
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.

        .. versionchanged:: 3.6
           Added ``session`` parameter.
        """
        cmd = SON([("fsyncUnlock", 1)])
        with self._socket_for_writes() as sock_info:
            if sock_info.max_wire_version >= 4:
                try:
                    with self._tmp_session(session) as s:
                        sock_info.command(
                            "admin", cmd, session=s, client=self)
                except OperationFailure as exc:
                    # Ignore "DB not locked" to replicate old behavior
                    if exc.code != 125:
                        raise
            else:
                message._first_batch(sock_info, "admin", "$cmd.sys.unlock",
                                     {}, -1, True, self.codec_options,
                                     ReadPreference.PRIMARY, cmd,
                                     self._event_listeners)

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.close()

    def __iter__(self):
        return self

    def __next__(self):
        raise TypeError("'MongoClient' object is not iterable")

    next = __next__