public function compute($group_name, $identifier, RendererInterface $renderer = null)
    {
        if (false === $this->isValidGroup($group_name)) {
            throw new InvalidArgumentException("$group_name is not a valid cache identifier appender group.");
        }

        foreach ($this->appenders[$group_name] as $appender) {
            $identifier = $appender->computeIdentifier($identifier, $renderer);
        }

        return $identifier;
    }