public function start($data, array $metadata = [], array $options = [])
    {
        $message_array = ['message' => $this->_serializeMessage($data)];
        if (array_key_exists('flags', $options)) {
            $message_array['flags'] = $options['flags'];
        }
        $this->call->startBatch([
            OP_SEND_INITIAL_METADATA => $metadata,
            OP_SEND_MESSAGE => $message_array,
            OP_SEND_CLOSE_FROM_CLIENT => true,
        ]);
    }