public static function alias($alias, $source)
    {
        if (empty(static::$_config[$source]) && empty(static::$_config[$alias])) {
            throw new MissingDatasourceConfigException(
                sprintf('Cannot create alias of "%s" as it does not exist.', $alias)
            );
        }
        static::$_aliasMap[$source] = $alias;
    }