'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 forwared 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, no retries will be attempted and the error will be re-raised.'

def on_unavailable(self, query, consistency, required_replicas, alive_replicas, retry_num):

    return ((self.RETRY_NEXT_HOST, consistency) if (retry_num == 0) else (self.RETHROW, None))
