code: def on_unavailable(self, query, consistency, required_replicas, alive_replicas, retry_num):
          
          return (self.RETRY_NEXT_HOST, None) if retry_num == 0 else (self.RETHROW, None)
docs: 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.