Class RoundRobinPolicy

cassandra/policies.py:155–200

A subclass of :class:`.LoadBalancingPolicy` which evenly distributes queries across all nodes in the cluster, regardless of what datacenter the nodes may be in.

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):