paceVersion();

        return sprintf('%s[%s][%s]', $this->namespace, $id, $namespaceVersion);
    }

    /**
     * Returns the namespace cache key.
     */
    private function getNamespaceCacheKey(): string
    {
        return sprintf(self::DOCTRINE_NAMESPACE_CACHEKEY, $this->namespace);
    }

    /**
     * Returns the namespace version.
     */
    private function getNamespaceVersion(): int
    {
        if ($this->namespaceVersion !== null) {
            return $this->namespaceVersion;
        }

        $namespaceCacheKey      = $this->getNamespaceCacheKey();
        $this->namespaceVersion = (int) $this->doFetch($namespaceCacheKey) ?: 1;

        return $this->namespaceVersion;
    }

    /**
     * Default implementation of doFetchMultiple. Each driver that supports multi-get should owerwrite it.
     *
     * @param string[] $keys Array of keys to retrieve from cache
     *
     * @return mixed[] Array of values retrieved for the given keys.
     */
    protected function doFetchMultiple(array $keys)
    {
        $returnValues = [];

        foreach ($keys as $key) {
            $item = $this->doFetch($key);
            if ($item === false && ! $this->doContains($key)) {
                continue;
            }

            $returnValues[$key] = $item;
        }

        return $returnValues;
    }

    /**
     * Fetches an entry from the cache.
     *
     * @param string $id The id of the cache entry to fetch.
     *
     * @return mixed|false The cached data or FALSE, if no cache entry exists for the given id.
     */
    abstract protected function doFetch($id);

    /**
     * Tests if an entry exists in the cache.
     *
     * @param string $id The cache id of the entry to check for.
     *
     * @return bool TRUE if a cache entry exists for the given cache id, FALSE otherwise.
     */
    abstract protected function doContains($id);

    /**
     * Default implementation of doSaveMultiple. Each driver that supports multi-put should override it.
     *
     * @param mixed[] $keysAndValues Array of keys and values to save in cache
     * @param int     $lifetime      The lifetime. If != 0, sets a specific lifetime for these
     *                               cache entries (0 => infinite lifeTime).
     *
     * @return bool TRUE if the operation was successful, FALSE if it wasn't.
     */
    protected function doSaveMultiple(array $keysAndValues, $lifetime = 0)
    {
        $success = true;

        foreach ($keysAndValues as $key => $value) {
            if ($this->doSave($key, $value, $lifetime)) {
                continue;
            }

            $success = false;
        }

        return $success;
    }

    /**
     * Puts data into the cache.
     *
     * @param string $id       The cache id.
     * @param string $data     The cache entry/data.
     * @param int    $lifeTime The lifetime. If != 0, sets a specific lifetime for this
     *                           cache entry (0 => infinite lifeTime).
     *
     * @return bool TRUE if the entry was successfully stored in the cache, FALSE otherwise.
     */
    abstract protected function doSave($id, $data, $lifeTime = 0);

    /**
     * Default implementation of doDeleteMultiple. Each driver that supports multi-delete should override it.
     *
     * @param string[] $keys Array of keys to delete from cache
     *
     * @return bool TRUE if the operation was successful, FALSE if it wasn't
     */
    protected function doDeleteMultiple(array $keys)
    {
        $success = true;

        foreach ($keys as $key) {
            if ($this->doDelete($key)) {
                continue;
            }

            $success = false;
        }

        return $success;
    }

    /**
     * Deletes a cache entry.
     *
     * @param string $id The cache id.
     *
     * @return bool TRUE if the cache entry was successfully deleted, FALSE otherwise.
     */
    abstract protected function doDelete($id);

    /**
     * Flushes all cache entries.
     *
     * @return bool TRUE if the cache entries were successfully flushed, FALSE otherwise.
     */
    abstract protected function doFlush();

    /**
     * Retrieves cached information from the data store.
     *
     * @return mixed[]|null An associative array with server's statistics if available, NULL otherwise.
     */
    abstract protected function doGetStats();
}