/**
     * 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.

                SortBuilders.fieldSort(OpenSearchUrlQueue.WEIGHT).order(SortOrder.DESC),
                SortBuilders.fieldSort(CREATE_TIME).order(SortOrder.ASC));
    }

    /**
     * Saves the session state (currently not implemented).
     *
     * @param sessionId The session ID.
     */
    @Override
    public void saveSession(final String sessionId) {
        // TODO use cache
    }