public function addServer(string $dsn) : bool
    {
        try {
            $context = new \ZMQContext;
            $this->socket = $context->getSocket(\ZMQ::SOCKET_REQ);
            $this->socket->connect($dsn);
            $this->dsn = $dsn;
            return true;
        } catch (\ZMQException $e) {
            throw new ClientException($e->getMessage());
        }
    }