private function buildRequestRate($array)
    {
        $result = [];
        foreach ($array as $pair) {
            $string = self::DIRECTIVE_REQUEST_RATE . ':' . $pair['ratio'];
            if (isset($pair['from']) &&
                isset($pair['to'])
            ) {
                $string .= ' ' . $pair['from'] . '-' . $pair['to'];
            }
            $result[] = $string;
        }
        return $result;
    }