protected function readMetadata($id)
    {
        $filename = $this->getFilename($id, true);
        $this->assertFile($filename);
        if (is_file($filename)) {
            if (false === ($data = file_get_contents($filename))) {
                throw new RawFileCacheException(sprintf('Cannot read file : %s', $filename), RawFileCacheException::CANNOT_READ_FILE);
            }
            return (array)unserialize($data);
        }
        return [];
    }