protected function _checkRequiredColumns()
    {
        if ($this->_modifiedColumn === null) {
            /**
             * @see Zend_Session_SaveHandler_Exception
             */
            // require_once 'Zend/Session/SaveHandler/Exception.php';

            throw new Zend_Session_SaveHandler_Exception(
                "Configuration must define '" . self::MODIFIED_COLUMN . "' which names the "
              . "session table last modification time column.");
        } else if ($this->_lifetimeColumn === null) {
            /**
             * @see Zend_Session_SaveHandler_Exception
             */
            // require_once 'Zend/Session/SaveHandler/Exception.php';

            throw new Zend_Session_SaveHandler_Exception(
                "Configuration must define '" . self::LIFETIME_COLUMN . "' which names the "
              . "session table lifetime column.");
        } else if ($this->_dataColumn === null) {
            /**
             * @see Zend_Session_SaveHandler_Exception
             */
            // require_once 'Zend/Session/SaveHandler/Exception.php';

            throw new Zend_Session_SaveHandler_Exception(
                "Configuration must define '" . self::DATA_COLUMN . "' which names the "
              . "session table data column.");
        }
    }