policies.py 文件源码

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