* @return true if the URL exists in the queue, false otherwise.
     */
    @Override
    protected boolean exists(final String sessionId, final String url) {
        final boolean ret = super.exists(sessionId, url);
        if (!ret) {
            final QueueHolder queueHolder = getQueueHolder(sessionId);
            final Queue<OpenSearchUrlQueue> waitingQueue = queueHolder.waitingQueue;