* @return The next URL queue entry, or null if none available.
     */
    @Override
    public OpenSearchUrlQueue poll(final String sessionId) {
        final QueueHolder queueHolder = getQueueHolder(sessionId);
        final Queue<OpenSearchUrlQueue> waitingQueue = queueHolder.waitingQueue;
        final Queue<OpenSearchUrlQueue> crawlingQueue = queueHolder.crawlingQueue;