/**
     * Polls a URL queue.
     *
     * @param sessionId The session ID.
     * @return The URL queue.
     */
    QUEUE poll(String sessionId);

    /**
     * Saves the session.
     *
     * @param sessionId The session ID.
     */
    void saveSession(String sessionId);

    /**
     * Checks if a URL has been visited.
     *
     * @param urlQueue The URL queue.