protected function getDefaultCache()
    {
        if (extension_loaded('apcu')) {
            return new ApcuCachePool();
        }

        if (class_exists('\Cache\Adapter\Filesystem\FilesystemCachePool')) {
            $filesystemAdapter = new Local($this->cacheDir);
            $filesystem        = new Filesystem($filesystemAdapter);
            return new FilesystemCachePool($filesystem);
        }

        return null;
    }