protected function _slave()
    {
        if (empty($this->_config_slaves)) {
            return $this->_master();
        }
        if (null === $this->_slave) {
            $count = count($this->_config_slaves);
            $index = mt_rand(0, $count-1);
            $config = $this->_config_slaves[$index];
            $this->_slave = $this->_connection($config);
        }
        return $this->_slave;
    }