protected function getCache($key) {
        $this->logger->debug('LRUcache: get key' . $key);
        if ( ! isset($this->cache[$key])) {
            $this->logger->debug('LRUcache: key' . $key . ' Not Found' );
            return null;
        }
        // Put the value gotten to last.
        $tmpValue = $this->cache[$key];
        unset($this->cache[$key]);
        $this->cache[$key] = $tmpValue;
        $this->logger->debug('LRUcache: key' . $key . ' Found' );
        return $tmpValue;
    }