Class TokenAwarePolicy

cassandra/policies.py:321–405

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

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):