# Source code for cassandra.policies

    # Copyright DataStax, 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.

    from collections import namedtuple
    from functools import lru_cache
    from itertools import islice, cycle, groupby, repeat
    import logging
    from random import randint, shuffle
    from threading import Lock
    import socket
    import warnings

    log = logging.getLogger(__name__)

    from cassandra import WriteType as WT

    # This is done this way because WriteType was originally
    # defined here and in order not to break the API.
    # It may removed in the next mayor.
    WriteType = WT

    from cassandra import ConsistencyLevel, OperationTimedOut

    class HostDistance(object):
        """
        A measure of how "distant" a node is from the client, which
        may influence how the load balancer distributes requests
        and how many connections are opened to the node.
        """

        IGNORED = -1
        """
        A node with this distance should never be queried or have
        connections opened to it.
        """

        LOCAL = 0
        """
        Nodes with ``LOCAL`` distance will be preferred for operations
        under some load balancing policies (such as :class:`.DCAwareRoundRobinPolicy`)
        and will have a greater number of connections opened against
        them by default.

        This distance is typically used for nodes within the same
        datacenter as the client.
        """

        REMOTE = 1
        """
        Nodes with ``REMOTE`` distance will be treated as a last resort
        by some load balancing policies (such as :class:`.DCAwareRoundRobinPolicy`)
        and will have a smaller number of connections opened against
        them by default.

        This distance is typically used for nodes outside of the
        datacenter that the client is running in.
        """

    class HostStateListener(object):

        def on_up(self, host):
            """ Called when a node is marked up. """
            raise NotImplementedError()

        def on_down(self, host):
            """ Called when a node is marked down. """
            raise NotImplementedError()

        def on_add(self, host):
            """
            Called when a node is added to the cluster.  The newly added node
            should be considered up.
            """
            raise NotImplementedError()

        def on_remove(self, host):
            """ Called when a node is removed from the cluster. """
            raise NotImplementedError()

    class LoadBalancingPolicy(HostStateListener):
        """
        Load balancing policies are used to decide how to distribute
        requests among all possible coordinator nodes in the cluster.

        In particular, they may focus on querying "near" nodes (those
        in a local datacenter) or on querying nodes who happen to
        be replicas for the requested data.

        You may also use subclasses of :class:`.LoadBalancingPolicy` for
        custom behavior.
        """

        _hosts_lock = None

        def __init__(self):
            self._hosts_lock = Lock()

        def distance(self, host):
            """
            Returns a measure of how remote a :class:`~.pool.Host` is in
            terms of the :class:`.HostDistance` enums.
            """
            raise NotImplementedError()

        def populate(self, cluster, hosts):
            """
            This method is called to initialize the load balancing
            policy with a set of :class:`.Host` instances before its
            first use.  The `cluster` parameter is an instance of
            :class:`.Cluster`.
            """
            raise NotImplementedError()

        def make_query_plan(self, working_keyspace=None, query=None):
            """
            Given a :class:`~.query.Statement` instance, return a iterable
            of :class:`.Host` instances which should be queried in that
            order.  A generator may work well for custom implementations
            of this method.

            Note that the `query` argument may be :const:`None` when preparing
            statements.

            `working_keyspace` should be the string name of the current keyspace,
            as set through :meth:`.Session.set_keyspace()` or with a ``USE``
            statement.
            """
            raise NotImplementedError()

        def check_supported(self):
            """
            This will be called after the cluster Metadata has been initialized.
            If the load balancing policy implementation cannot be supported for
            some reason (such as a missing C extension), this is the point at
            which it should raise an exception.
            """
            pass

    class RoundRobinPolicy(LoadBalancingPolicy):
        """
        A subclass of :class:`.LoadBalancingPolicy` which evenly
        distributes queries across all nodes in the cluster,
        regardless of what datacenter the nodes may be in.
        """
        _live_hosts = frozenset(())
        _position = 0

        def populate(self, cluster, hosts):
            self._live_hosts = frozenset(hosts)
            if len(hosts) > 1:
                self._position = randint(0, len(hosts) - 1)

        def distance(self, host):
            return HostDistance.LOCAL

        def make_query_plan(self, working_keyspace=None, query=None):
            # not thread-safe, but we don't care much about lost increments
            # for the purposes of load balancing
            pos = self._position
            self._position += 1

            hosts = self._live_hosts
            length = len(hosts)
            if length:
                pos %= length
                return islice(cycle(hosts), pos, pos + length)
            else:
                return []

        def on_up(self, host):
            with self._hosts_lock:
                self._live_hosts = self._live_hosts.union((host, ))

        def on_down(self, host):
            with self._hosts_lock:
                self._live_hosts = self._live_hosts.difference((host, ))

        def on_add(self, host):
            with self._hosts_lock:
                self._live_hosts = self._live_hosts.union((host, ))

        def on_remove(self, host):
            with self._hosts_lock:
                self._live_hosts = self._live_hosts.difference((host, ))

    class DCAwareRoundRobinPolicy(LoadBalancingPolicy):
        """
        Similar to :class:`.RoundRobinPolicy`, but prefers hosts
        in the local datacenter and only uses nodes in remote
        datacenters as a last resort.
        """

        local_dc = None
        used_hosts_per_remote_dc = 0

        def __init__(self, local_dc='', used_hosts_per_remote_dc=0):
            """
            The `local_dc` parameter should be the name of the datacenter
            (such as is reported by ``nodetool ring``) that should
            be considered local. If not specified, the driver will choose
            a local_dc based on the first host among :attr:`.Cluster.contact_points`
            having a valid DC. If relying on this mechanism, all specified
            contact points should be nodes in a single, local DC.

            `used_hosts_per_remote_dc` controls how many nodes in
            each remote datacenter will have connections opened
            against them. In other words, `used_hosts_per_remote_dc` hosts
            will be considered :attr:`~.HostDistance.REMOTE` and the
            rest will be considered :attr:`~.HostDistance.IGNORED`.
            By default, all remote hosts are ignored.
            """
            self.local_dc = local_dc
            self.used_hosts_per_remote_dc = used_hosts_per_remote_dc
            self._dc_live_hosts = {}
            self._position = 0
            self._endpoints = []
            LoadBalancingPolicy.__init__(self)

        def _dc(self, host):
            return host.datacenter or self.local_dc

        def populate(self, cluster, hosts):
            for dc, dc_hosts in groupby(hosts, lambda h: self._dc(h)):
                self._dc_live_hosts[dc] = tuple(set(dc_hosts))

            if not self.local_dc:
                self._endpoints = [
                    endpoint
                    for endpoint in cluster.endpoints_resolved]

            self._position = randint(0, len(hosts) - 1) if hosts else 0

        def distance(self, host):
            dc = self._dc(host)
            if dc == self.local_dc:
                return HostDistance.LOCAL

            if not self.used_hosts_per_remote_dc:
                return HostDistance.IGNORED
            else:
                dc_hosts = self._dc_live_hosts.get(dc)
                if not dc_hosts:
                    return HostDistance.IGNORED

                if host in list(dc_hosts)[:self.used_hosts_per_remote_dc]:
                    return HostDistance.REMOTE
                else:
                    return HostDistance.IGNORED

        def make_query_plan(self, working_keyspace=None, query=None):
            # not thread-safe, but we don't care much about lost increments
            # for the purposes of load balancing
            pos = self._position
            self._position += 1

            local_live = self._dc_live_hosts.get(self.local_dc, ())
            pos = (pos % len(local_live)) if local_live else 0
            for host in islice(cycle(local_live), pos, pos + len(local_live)):
                yield host

            # the dict can change, so get candidate DCs iterating over keys of a copy
            other_dcs = [dc for dc in self._dc_live_hosts.copy().keys() if dc != self.local_dc]
            for dc in other_dcs:
                remote_live = self._dc_live_hosts.get(dc, ())
                for host in remote_live[:self.used_hosts_per_remote_dc]:
                    yield host

        def on_up(self, host):
            # not worrying about threads because this will happen during
            # control connection startup/refresh
            if not self.local_dc and host.datacenter:
                if host.endpoint in self._endpoints:
                    self.local_dc = host.datacenter
                    log.info("Using datacenter '%s' for DCAwareRoundRobinPolicy (via host '%s'); "
                             "if incorrect, please specify a local_dc to the constructor, "
                             "or limit contact points to local cluster nodes" %
                             (self.local_dc, host.endpoint))
                    del self._endpoints

            dc = self._dc(host)
            with self._hosts_lock:
                current_hosts = self._dc_live_hosts.get(dc, ())
                if host not in current_hosts:
                    self._dc_live_hosts[dc] = current_hosts + (host, )

        def on_down(self, host):
            dc = self._dc(host)
            with self._hosts_lock:
                current_hosts = self._dc_live_hosts.get(dc, ())
                if host in current_hosts:
                    hosts = tuple(h for h in current_hosts if h != host)
                    if hosts:
                        self._dc_live_hosts[dc] = hosts
                    else:
                        del self._dc_live_hosts[dc]

        def on_add(self, host):
            self.on_up(host)

        def on_remove(self, host):
            self.on_down(host)

    class TokenAwarePolicy(LoadBalancingPolicy):
        """
        A :class:`.LoadBalancingPolicy` wrapper that adds token awareness to
        a child policy.

        This alters the child policy's behavior so that it first attempts to
        send queries to :attr:`~.HostDistance.LOCAL` replicas (as determined
        by the child policy) based on the :class:`.Statement`'s
        :attr:`~.Statement.routing_key`. If :attr:`.shuffle_replicas` is
        truthy, these replicas will be yielded in a random order. Once those
        hosts are exhausted, the remaining hosts in the child policy's query
        plan will be used in the order provided by the child policy.

        If no :attr:`~.Statement.routing_key` is set on the query, the child
        policy's query plan will be used as is.
        """

        _child_policy = None
        _cluster_metadata = None
        shuffle_replicas = False
        """
        Yield local replicas in a random order.
        """

        def __init__(self, child_policy, shuffle_replicas=False):
            self._child_policy = child_policy
            self.shuffle_replicas = shuffle_replicas

        def populate(self, cluster, hosts):
            self._cluster_metadata = cluster.metadata
            self._child_policy.populate(cluster, hosts)

        def check_supported(self):
            if not self._cluster_metadata.can_support_partitioner():
                raise RuntimeError(
                    '%s cannot be used with the cluster partitioner (%s) because '
                    'the relevant C extension for this driver was not compiled. '
                    'See the installation instructions for details on building '
                    'and installing the C extensions.' %
                    (self.__class__.__name__, self._cluster_metadata.partitioner))

        def distance(self, *args, **kwargs):
            return self._child_policy.distance(*args, **kwargs)

        def make_query_plan(self, working_keyspace=None, query=None):
            if query and query.keyspace:
                keyspace = query.keyspace
            else:
                keyspace = working_keyspace

            child = self._child_policy
            if query is None:
                for host in child.make_query_plan(keyspace, query):
                    yield host
            else:
                routing_key = query.routing_key
                if routing_key is None or keyspace is None:
                    for host in child.make_query_plan(keyspace, query):
                        yield host
                else:
                    replicas = self._cluster_metadata.get_replicas(keyspace, routing_key)
                    if self.shuffle_replicas:
                        shuffle(replicas)
                    for replica in replicas:
                        if replica.is_up and \
                                child.distance(replica) == HostDistance.LOCAL:
                            yield replica

                    for host in child.make_query_plan(keyspace, query):
                        # skip if we've already listed this host
                        if host not in replicas or \
                                child.distance(host) == HostDistance.REMOTE:
                            yield host

        def on_up(self, *args, **kwargs):
            return self._child_policy.on_up(*args, **kwargs)

        def on_down(self, *args, **kwargs):
            return self._child_policy.on_down(*args, **kwargs)

        def on_add(self, *args, **kwargs):
            return self._child_policy.on_add(*args, **kwargs)

        def on_remove(self, *args, **kwargs):
            return self._child_policy.on_remove(*args, **kwargs)

    class WhiteListRoundRobinPolicy(RoundRobinPolicy):
        """
        A subclass of :class:`.RoundRobinPolicy` which evenly
        distributes queries across all nodes in the cluster,
        regardless of what datacenter the nodes may be in, but
        only if that node exists in the list of allowed nodes

        This policy is addresses the issue described in
        https://datastax-oss.atlassian.net/browse/JAVA-145
        Where connection errors occur when connection
        attempts are made to private IP addresses remotely
        """

        def __init__(self, hosts):
            """
            The `hosts` parameter should be a sequence of hosts to permit
            connections to.
            """
            self._allowed_hosts = tuple(hosts)
            self._allowed_hosts_resolved = [endpoint[4][0] for a in self._allowed_hosts
                                            for endpoint in socket.getaddrinfo(a, None, socket.AF_UNSPEC, socket.SOCK_STREAM)]

            RoundRobinPolicy.__init__(self)

        def populate(self, cluster, hosts):
            self._live_hosts = frozenset(h for h in hosts if h.address in self._allowed_hosts_resolved)

            if len(hosts) <= 1:
                self._position = 0
            else:
                self._position = randint(0, len(hosts) - 1)

        def distance(self, host):
            if host.address in self._allowed_hosts_resolved:
                return HostDistance.LOCAL
            else:
                return HostDistance.IGNORED

        def on_up(self, host):
            if host.address in self._allowed_hosts_resolved:
                RoundRobinPolicy.on_up(self, host)

        def on_add(self, host):
            if host.address in self._allowed_hosts_resolved:
                RoundRobinPolicy.on_add(self, host)

    class HostFilterPolicy(LoadBalancingPolicy):
        """
        A :class:`.LoadBalancingPolicy` subclass configured with a child policy,
        and a single-argument predicate. This policy defers to the child policy for
        hosts where ``predicate(host)`` is truthy. Hosts for which
        ``predicate(host)`` is falsy will be considered :attr:`.IGNORED`, and will
        not be used in a query plan.

        This can be used in the cases where you need a whitelist or blacklist
        policy, e.g. to prepare for decommissioning nodes or for testing:

        .. code-block:: python

            def address_is_ignored(host):
                return host.address in [ignored_address0, ignored_address1]

            blacklist_filter_policy = HostFilterPolicy(
                child_policy=RoundRobinPolicy(),
                predicate=address_is_ignored
            )

            cluster = Cluster(
                primary_host,
                load_balancing_policy=blacklist_filter_policy,
            )

        See the note in the :meth:`.make_query_plan` documentation for a caveat on
        how wrapping ordering polices (e.g. :class:`.RoundRobinPolicy`) may break
        desirable properties of the wrapped policy.

        Please note that whitelist and blacklist policies are not recommended for
        general, day-to-day use. You probably want something like
        :class:`.DCAwareRoundRobinPolicy`, which prefers a local DC but has
        fallbacks, over a brute-force method like whitelisting or blacklisting.
        """

        def __init__(self, child_policy, predicate):
            """
            :param child_policy: an instantiated :class:`.LoadBalancingPolicy`
                                 that this one will defer to.
            :param predicate: a one-parameter function that takes a :class:`.Host`.
                              If it returns a falsy value, the :class:`.Host` will
                              be :attr:`.IGNORED` and not returned in query plans.
            """
            super(HostFilterPolicy, self).__init__()
            self._child_policy = child_policy
            self._predicate = predicate

        def on_up(self, host, *args, **kwargs):
            return self._child_policy.on_up(host, *args, **kwargs)

        def on_down(self, host, *args, **kwargs):
            return self._child_policy.on_down(host, *args, **kwargs)

        def on_add(self, host, *args, **kwargs):
            return self._child_policy.on_add(host, *args, **kwargs)

        def on_remove(self, host, *args, **kwargs):
            return self._child_policy.on_remove(host, *args, **kwargs)

        @property
        def predicate(self):
            """
            A predicate, set on object initialization, that takes a :class:`.Host`
            and returns a value. If the value is falsy, the :class:`.Host` is
            :class:`~HostDistance.IGNORED`. If the value is truthy,
            :class:`.HostFilterPolicy` defers to the child policy to determine the
            host's distance.

            This is a read-only value set in ``__init__``, implemented as a
            ``property``.
            """
            return self._predicate

        def distance(self, host):
            """
            Checks if ``predicate(host)``, then returns
            :attr:`~HostDistance.IGNORED` if falsy, and defers to the child policy
            otherwise.
            """
            if self.predicate(host):
                return self._child_policy.distance(host)
            else:
                return HostDistance.IGNORED

        def populate(self, cluster, hosts):
            self._child_policy.populate(cluster=cluster, hosts=hosts)

        def make_query_plan(self, working_keyspace=None, query=None):
            """
            Defers to the child policy's
            :meth:`.LoadBalancingPolicy.make_query_plan` and filters the results.

            Note that this filtering may break desirable properties of the wrapped
            policy in some cases. For instance, imagine if you configure this
            policy to filter out ``host2``, and to wrap a round-robin policy that
            rotates through three hosts in the order ``host1, host2, host3``,
            ``host2, host3, host1``, ``host3, host1, host2``, repeating. This
            policy will yield ``host1, host3``, ``host3, host1``, ``host3, host1``,
            disproportionately favoring ``host3``.
            """
            child_qp = self._child_policy.make_query_plan(
                working_keyspace=working_keyspace, query=query
            )
            for host in child_qp:
                if self.predicate(host):
                    yield host

        def check_supported(self):
            return self._child_policy.check_supported()

    class ConvictionPolicy(object):
        """
        A policy which decides when hosts should be considered down
        based on the types of failures and the number of failures.

        If custom behavior is needed, this class may be subclassed.
        """

        def __init__(self, host):
            """
            `host` is an instance of :class:`.Host`.
            """
            self.host = host

        def add_failure(self, connection_exc):
            """
            Implementations should return :const:`True` if the host should be
            convicted, :const:`False` otherwise.
            """
            raise NotImplementedError()

        def reset(self):
            """
            Implementations should clear out any convictions or state regarding
            the host.
            """
            raise NotImplementedError()

    class SimpleConvictionPolicy(ConvictionPolicy):
        """
        The default implementation of :class:`ConvictionPolicy`,
        which simply marks a host as down after the first failure
        of any kind.
        """

        def add_failure(self, connection_exc):
            return not isinstance(connection_exc, OperationTimedOut)

        def reset(self):
            pass

    class ReconnectionPolicy(object):
        """
        This class and its subclasses govern how frequently an attempt is made
        to reconnect to nodes that are marked as dead.

        If custom behavior is needed, this class may be subclassed.
        """

        def new_schedule(self):
            """
            This should return a finite or infinite iterable of delays (each as a
            floating point number of seconds) in-between each failed reconnection
            attempt.  Note that if the iterable is finite, reconnection attempts
            will cease once the iterable is exhausted.
            """
            raise NotImplementedError()

    class ConstantReconnectionPolicy(ReconnectionPolicy):
        """
        A :class:`.ReconnectionPolicy` subclass which sleeps for a fixed delay
        in-between each reconnection attempt.
        """

        def __init__(self, delay, max_attempts=64):
            """
            `delay` should be a floating point number of seconds to wait in-between
            each attempt.

            `max_attempts` should be a total number of attempts to be made before
            giving up, or :const:`None` to continue reconnection attempts forever.
            The default is 64.
            """
            if delay < 0:
                raise ValueError("delay must not be negative")
            if max_attempts is not None and max_attempts < 0:
                raise ValueError("max_attempts must not be negative")

            self.delay = delay
            self.max_attempts = max_attempts

        def new_schedule(self):
            if self.max_attempts:
                return repeat(self.delay, self.max_attempts)
            return repeat(self.delay)

    class ExponentialReconnectionPolicy(ReconnectionPolicy):
        """
        A :class:`.ReconnectionPolicy` subclass which exponentially increases
        the length of the delay in-between each reconnection attempt up to
        a set maximum delay.

        A random amount of jitter (+/- 15%) will be added to the pure exponential
        delay value to avoid the situations where many reconnection handlers are
        trying to reconnect at exactly the same time.
        """

        # TODO: max_attempts is 64 to preserve legacy default behavior
        # consider changing to None in major release to prevent the policy
        # giving up forever
        def __init__(self, base_delay, max_delay, max_attempts=64):
            """
            `base_delay` and `max_delay` should be in floating point units of
            seconds.

            `max_attempts` should be a total number of attempts to be made before
            giving up, or :const:`None` to continue reconnection attempts forever.
            The default is 64.
            """
            if base_delay < 0 or max_delay < 0:
                raise ValueError("Delays may not be negative")

            if max_delay < base_delay:
                raise ValueError("Max delay must be greater than base delay")

            if max_attempts is not None and max_attempts < 0:
                raise ValueError("max_attempts must not be negative")

            self.base_delay = base_delay
            self.max_delay = max_delay
            self.max_attempts = max_attempts

        def new_schedule(self):
            i, overflowed = 0, False
            while self.max_attempts is None or i < self.max_attempts:
                if overflowed:
                    yield self.max_delay
                else:
                    try:
                        yield self._add_jitter(min(self.base_delay * (2 ** i), self.max_delay))
                    except OverflowError:
                        overflowed = True
                        yield self.max_delay

                i += 1

        # Adds -+ 15% to the delay provided
        def _add_jitter(self, value):
            jitter = randint(85, 115)
            delay = (jitter * value) / 100
            return min(max(self.base_delay, delay), self.max_delay)

    class RetryPolicy(object):
        """
        A policy that describes whether to retry, rethrow, or ignore coordinator
        timeout and unavailable failures. These are failures reported from the
        server side. Timeouts are configured by
        `settings in cassandra.yaml <https://github.com/apache/cassandra/blob/cassandra-2.1.4/conf/cassandra.yaml#L568-L584>`_.
        Unavailable failures occur when the coordinator cannot achieve the consistency
        level for a request. For further information see the method descriptions
        below.

        To specify a default retry policy, set the
        :attr:`.Cluster.default_retry_policy` attribute to an instance of this
        class or one of its subclasses.

        To specify a retry policy per query, set the :attr:`.Statement.retry_policy`
        attribute to an instance of this class or one of its subclasses.

        If custom behavior is needed for retrying certain operations,
        this class may be subclassed.
        """

        RETRY = 0
        """
        This should be returned from the below methods if the operation
        should be retried on the same connection.
        """

        RETHROW = 1
        """
        This should be returned from the below methods if the failure
        should be propagated and no more retries attempted.
        """

        IGNORE = 2
        """
        This should be returned from the below methods if the failure
        should be ignored but no more retries should be attempted.
        """

        RETRY_NEXT_HOST = 3
        """
        This should be returned from the below methods if the operation
        should be retried on another connection.
        """

        def on_read_timeout(self, query, consistency, required_responses,
                            received_responses, data_retrieved, retry_num):
            """
            This is called when a read operation times out from the coordinator's
            perspective (i.e. a replica did not respond to the coordinator in time).
            It should return a tuple with two items: one of the class enums (such
            as :attr:`.RETRY`) and a :class:`.ConsistencyLevel` to retry the
            operation at or :const:`None` to keep the same consistency level.

            `query` is the :class:`.Statement` that timed out.

            `consistency` is the :class:`.ConsistencyLevel` that the operation was
            attempted at.

            The `required_responses` and `received_responses` parameters describe
            how many replicas needed to respond to meet the requested consistency
            level and how many actually did respond before the coordinator timed
            out the request. `data_retrieved` is a boolean indicating whether
            any of those responses contained data (as opposed to just a digest).

            `retry_num` counts how many times the operation has been retried, so
            the first time this method is called, `retry_num` will be 0.

            By default, operations will be retried at most once, and only if
            a sufficient number of replicas responded (with data digests).
            """
            if retry_num != 0:
                return self.RETHROW, None
            elif received_responses >= required_responses and not data_retrieved:
                return self.RETRY, consistency
            else:
                return self.RETHROW, None

        def on_write_timeout(self, query, consistency, write_type,
                             required_responses, received_responses, retry_num):
            """
            This is called when a write operation times out from the coordinator's
            perspective (i.e. a replica did not respond to the coordinator in time).

            `query` is the :class:`.Statement` that timed out.

            `consistency` is the :class:`.ConsistencyLevel` that the operation was
            attempted at.

            `write_type` is one of the :class:`.WriteType` enums describing the
            type of write operation.

            The `required_responses` and `received_responses` parameters describe
            how many replicas needed to acknowledge the write to meet the requested
            consistency level and how many replicas actually did acknowledge the
            write before the coordinator timed out the request.

            `retry_num` counts how many times the operation has been retried, so
            the first time this method is called, `retry_num` will be 0.

            By default, failed write operations will retried at most once, and
            they will only be retried if the `write_type` was
            :attr:`~.WriteType.BATCH_LOG`.
            """
            if retry_num != 0:
                return self.RETHROW, None
            elif write_type == WriteType.BATCH_LOG:
                return self.RETRY, consistency
            else:
                return self.RETHROW, None

        def on_unavailable(self, query, consistency, required_replicas, alive_replicas, retry_num):
            """
            This is called when the coordinator node determines that a read or
            write operation cannot be successful because the number of live
            replicas are too low to meet the requested :class:`.ConsistencyLevel`.
            This means that the read or write operation was never forwarded to
            any replicas.

            `query` is the :class:`.Statement` that failed.

            `consistency` is the :class:`.ConsistencyLevel` that the operation was
            attempted at.

            `required_replicas` is the number of replicas that would have needed to
            acknowledge the operation to meet the requested consistency level.
            `alive_replicas` is the number of replicas that the coordinator
            considered alive at the time of the request.

            `retry_num` counts how many times the operation has been retried, so
            the first time this method is called, `retry_num` will be 0.

            By default, if this is the first retry, it triggers a retry on the next
            host in the query plan with the same consistency level. If this is not the
            first retry, no retries will be attempted and the error will be re-raised.
            """
            return (self.RETRY_NEXT_HOST, None) if retry_num == 0 else (self.RETHROW, None)

        def on_request_error(self, query, consistency, error, retry_num):
            """
            This is called when an unexpected error happens. This can be in the
            following situations:

            * On a connection error
            * On server errors: overloaded, isBootstrapping, serverError, etc.

            `query` is the :class:`.Statement` that timed out.

            `consistency` is the :class:`.ConsistencyLevel` that the operation was
            attempted at.

            `error` the instance of the exception.

            `retry_num` counts how many times the operation has been retried, so
            the first time this method is called, `retry_num` will be 0.

            By default, it triggers a retry on the next host in the query plan
            with the same consistency level.
            """
            # TODO revisit this for the next major
            # To preserve the same behavior than before, we don't take retry_num into account
            return self.RETRY_NEXT_HOST, None

    class FallthroughRetryPolicy(RetryPolicy):
        """
        A retry policy that never retries and always propagates failures to
        the application.
        """

        def on_read_timeout(self, *args, **kwargs):
            return self.RETHROW, None

        def on_write_timeout(self, *args, **kwargs):
            return self.RETHROW, None

        def on_unavailable(self, *args, **kwargs):
            return self.RETHROW, None

        def on_request_error(self, *args, **kwargs):
            return self.RETHROW, None

    class DowngradingConsistencyRetryPolicy(RetryPolicy):
        """
        *Deprecated:* This retry policy will be removed in the next major release.

        A retry policy that sometimes retries with a lower consistency level than
        the one initially requested.

        **BEWARE**: This policy may retry queries using a lower consistency
        level than the one initially requested. By doing so, it may break
        consistency guarantees. In other words, if you use this retry policy,
        there are cases (documented below) where a read at :attr:`~.QUORUM`
        *may not* see a preceding write at :attr:`~.QUORUM`. Do not use this
        policy unless you have understood the cases where this can happen and
        are ok with that. It is also recommended to subclass this class so
        that queries that required a consistency level downgrade can be
        recorded (so that repairs can be made later, etc).

        This policy implements the same retries as :class:`.RetryPolicy`,
        but on top of that, it also retries in the following cases:

        * On a read timeout: if the number of replicas that responded is
          greater than one but lower than is required by the requested
          consistency level, the operation is retried at a lower consistency
          level.
        * On a write timeout: if the operation is an :attr:`~.UNLOGGED_BATCH`
          and at least one replica acknowledged the write, the operation is
          retried at a lower consistency level.  Furthermore, for other
          write types, if at least one replica acknowledged the write, the
          timeout is ignored.
        * On an unavailable exception: if at least one replica is alive, the
          operation is retried at a lower consistency level.

        The reasoning behind this retry policy is as follows: if, based
        on the information the Cassandra coordinator node returns, retrying the
        operation with the initially requested consistency has a chance to
        succeed, do it. Otherwise, if based on that information we know the
        initially requested consistency level cannot be achieved currently, then:

        * For writes, ignore the exception (thus silently failing the
          consistency requirement) if we know the write has been persisted on at
          least one replica.
        * For reads, try reading at a lower consistency level (thus silently
          failing the consistency requirement).

        In other words, this policy implements the idea that if the requested
        consistency level cannot be achieved, the next best thing for writes is
        to make sure the data is persisted, and that reading something is better
        than reading nothing, even if there is a risk of reading stale data.
        """
        def __init__(self, *args, **kwargs):
            super(DowngradingConsistencyRetryPolicy, self).__init__(*args, **kwargs)
            warnings.warn('DowngradingConsistencyRetryPolicy is deprecated '
                          'and will be removed in the next major release.',
                          DeprecationWarning)

        def _pick_consistency(self, num_responses):
            if num_responses >= 3:
                return self.RETRY, ConsistencyLevel.THREE
            elif num_responses >= 2:
                return self.RETRY, ConsistencyLevel.TWO
            elif num_responses >= 1:
                return self.RETRY, ConsistencyLevel.ONE
            else:
                return self.RETHROW, None

        def on_read_timeout(self, query, consistency, required_responses,
                            received_responses, data_retrieved, retry_num):
            if retry_num != 0:
                return self.RETHROW, None
            elif ConsistencyLevel.is_serial(consistency):
                # Downgrading does not make sense for a CAS read query
                return self.RETHROW, None
            elif received_responses < required_responses:
                return self._pick_consistency(received_responses)
            elif not data_retrieved:
                return self.RETRY, consistency
            else:
                return self.RETHROW, None

        def on_write_timeout(self, query, consistency, write_type,
                             required_responses, received_responses, retry_num):
            if retry_num != 0:
                return self.RETHROW, None

            if write_type in (WriteType.SIMPLE, WriteType.BATCH, WriteType.COUNTER):
                if received_responses > 0:
                    # persisted on at least one replica
                    return self.IGNORE, None
                else:
                    return self.RETHROW, None
            elif write_type == WriteType.UNLOGGED_BATCH:
                return self._pick_consistency(received_responses)
            elif write_type == WriteType.BATCH_LOG:
                return self.RETRY, consistency

            return self.RETHROW, None

        def on_unavailable(self, query, consistency, required_replicas, alive_replicas, retry_num):
            if retry_num != 0:
                return self.RETHROW, None
            elif ConsistencyLevel.is_serial(consistency):
                # failed at the paxos phase of a LWT, retry on the next host
                return self.RETRY_NEXT_HOST, None
            else:
                return self._pick_consistency(alive_replicas)

    class AddressTranslator(object):
        """
        Interface for translating cluster-defined endpoints.

        The driver discovers nodes using server metadata and topology change events. Normally,
        the endpoint defined by the server is the right way to connect to a node. In some environments,
        these addresses may not be reachable, or not preferred (public vs. private IPs in cloud environments,
        suboptimal routing, etc). This interface allows for translating from server defined endpoints to
        preferred addresses for driver connections.

        *Note:* :attr:`~Cluster.contact_points` provided while creating the :class:`~.Cluster` instance are not
        translated using this mechanism -- only addresses received from Cassandra nodes are.
        """

        def translate(self, addr):
            """
            Accepts the node ip address, and returns a translated address to be used connecting to this node.
            """
            raise NotImplementedError()

    class IdentityTranslator(AddressTranslator):
        """
        Returns the endpoint with no translation
        """

        def translate(self, addr):
            return addr

    class EC2MultiRegionTranslator(AddressTranslator):
        """
        Resolves private ips of the hosts in the same datacenter as the client, and public ips of hosts in other datacenters.
        """

        def translate(self, addr):
            """
            Reverse DNS the public broadcast_address, then lookup that hostname to get the AWS-resolved IP, which
            will point to the private IP address within the same datacenter.
            """
            # get family of this address so we translate to the same
            family = socket.getaddrinfo(addr, 0, socket.AF_UNSPEC, socket.SOCK_STREAM)[0][0]
            host = socket.getfqdn(addr)
            for a in socket.getaddrinfo(host, 0, family, socket.SOCK_STREAM):
                try:
                    return a[4][0]
                except Exception:
                    pass
            return addr

    class SpeculativeExecutionPolicy(object):
        """
        Interface for specifying speculative execution plans
        """

        def new_plan(self, keyspace, statement):
            """
            Returns

            :param keyspace:
            :param statement:
            :return:
            """
            raise NotImplementedError()

    class SpeculativeExecutionPlan(object):
        def next_execution(self, host):
            raise NotImplementedError()

    class NoSpeculativeExecutionPlan(SpeculativeExecutionPlan):
        def next_execution(self, host):
            return -1

    class NoSpeculativeExecutionPolicy(SpeculativeExecutionPolicy):

        def new_plan(self, keyspace, statement):
            return NoSpeculativeExecutionPlan()

    class ConstantSpeculativeExecutionPolicy(SpeculativeExecutionPolicy):
        """
        A speculative execution policy that sends a new query every X seconds (**delay**) for a maximum of Y attempts (**max_attempts**).
        """

        def __init__(self, delay, max_attempts):
            self.delay = delay
            self.max_attempts = max_attempts

        class ConstantSpeculativeExecutionPlan(SpeculativeExecutionPlan):
            def __init__(self, delay, max_attempts):
                self.delay = delay
                self.remaining = max_attempts

            def next_execution(self, host):
                if self.remaining > 0:
                    self.remaining -= 1
                    return self.delay
                else:
                    return -1

        def new_plan(self, keyspace, statement):
            return self.ConstantSpeculativeExecutionPlan(self.delay, self.max_attempts)

    class WrapperPolicy(LoadBalancingPolicy):

        def __init__(self, child_policy):
            self._child_policy = child_policy

        def distance(self, *args, **kwargs):
            return self._child_policy.distance(*args, **kwargs)

        def populate(self, cluster, hosts):
            self._child_policy.populate(cluster, hosts)

        def on_up(self, *args, **kwargs):
            return self._child_policy.on_up(*args, **kwargs)

        def on_down(self, *args, **kwargs):
            return self._child_policy.on_down(*args, **kwargs)

        def on_add(self, *args, **kwargs):
            return self._child_policy.on_add(*args, **kwargs)

        def on_remove(self, *args, **kwargs):
            return self._child_policy.on_remove(*args, **kwargs)

    class DefaultLoadBalancingPolicy(WrapperPolicy):
        """
        A :class:`.LoadBalancingPolicy` wrapper that adds the ability to target a specific host first.

        If no host is set on the query, the child policy's query plan will be used as is.
        """

        _cluster_metadata = None

        def populate(self, cluster, hosts):
            self._cluster_metadata = cluster.metadata
            self._child_policy.populate(cluster, hosts)

        def make_query_plan(self, working_keyspace=None, query=None):
            if query and query.keyspace:
                keyspace = query.keyspace
            else:
                keyspace = working_keyspace

            # TODO remove next major since execute(..., host=XXX) is now available
            addr = getattr(query, 'target_host', None) if query else None
            target_host = self._cluster_metadata.get_host(addr)

            child = self._child_policy
            if target_host and target_host.is_up:
                yield target_host
                for h in child.make_query_plan(keyspace, query):
                    if h != target_host:
                        yield h
            else:
                for h in child.make_query_plan(keyspace, query):
                    yield h

    # TODO for backward compatibility, remove in next major

    class DSELoadBalancingPolicy(DefaultLoadBalancingPolicy):
        """
        *Deprecated:* This will be removed in the next major release,
        consider using :class:`.DefaultLoadBalancingPolicy`.
        """
        def __init__(self, *args, **kwargs):
            super(DSELoadBalancingPolicy, self).__init__(*args, **kwargs)
            warnings.warn("DSELoadBalancingPolicy will be removed in 4.0. Consider using "
                          "DefaultLoadBalancingPolicy.", DeprecationWarning)

    class NeverRetryPolicy(RetryPolicy):
        def _rethrow(self, *args, **kwargs):
            return self.RETHROW, None

        on_read_timeout = _rethrow
        on_write_timeout = _rethrow
        on_unavailable = _rethrow

    ColDesc = namedtuple('ColDesc', ['ks', 'table', 'col'])

    class ColumnEncryptionPolicy(object):
        """
        A policy enabling (mostly) transparent encryption and decryption of data before it is
        sent to the cluster.

        Key materials and other configurations are specified on a per-column basis.  This policy can
        then be used by driver structures which are aware of the underlying columns involved in their
        work.  In practice this includes the following cases:

        * Prepared statements - data for columns specified by the cluster's policy will be transparently
          encrypted before they are sent
        * Rows returned from any query - data for columns specified by the cluster's policy will be
          transparently decrypted before they are returned to the user

        To enable this functionality, create an instance of this class (or more likely a subclass)
        before creating a cluster.  This policy should then be configured and supplied to the Cluster
        at creation time via the :attr:`.Cluster.column_encryption_policy` attribute.
        """

        def encrypt(self, coldesc, obj_bytes):
            """
            Encrypt the specified bytes using the cryptography materials for the specified column.
            Largely used internally, although this could also be used to encrypt values supplied
            to non-prepared statements in a way that is consistent with this policy.
            """
            raise NotImplementedError()

        def decrypt(self, coldesc, encrypted_bytes):
            """
            Decrypt the specified (encrypted) bytes using the cryptography materials for the
            specified column.  Used internally; could be used externally as well but there's
            not currently an obvious use case.
            """
            raise NotImplementedError()

        def add_column(self, coldesc, key):
            """
            Provide cryptography materials to be used when encrypted and/or decrypting data
            for the specified column.
            """
            raise NotImplementedError()

        def contains_column(self, coldesc):
            """
            Predicate to determine if a specific column is supported by this policy.
            Currently only used internally.
            """
            raise NotImplementedError()

        def encode_and_encrypt(self, coldesc, obj):
            """
            Helper function to enable use of this policy on simple (i.e. non-prepared)
            statements.
            """
            raise NotImplementedError()