protected function getInterval(array $config)
    {
        if (isset($config['interval'])) {
            if (!is_int($config['interval']) || $config['interval'] <= 0) {
                throw new \DomainException(
                    'interval must reference a positive integer value',
                    self::ERR_INTERVAL_INVALID
                );
            }
            return $config['interval'];
        }
        return 300; // default to 5 minutes
    }