final Queue<OpenSearchUrlQueue> crawlingQueue = queueHolder.crawlingQueue;

        OpenSearchUrlQueue urlQueue = waitingQueue.poll();
        if (urlQueue == null) {
            synchronized (queueHolder) {
                urlQueue = waitingQueue.poll();
                if (urlQueue == null) {
                    final List<OpenSearchUrlQueue> urlQueueList = fetchUrlQueueList(sessionId);

  @CanIgnoreReturnValue
  @Override
  public @Nullable E poll() {
    Monitor monitor = this.monitor;
    if (monitor.enterIf(notEmpty)) {
      try {
        return extract();
      } finally {
        monitor.leave();
      }
    } else {
      return null;
    }
  }

  @CanIgnoreReturnValue
  @Override
  public @Nullable E poll(long timeout, TimeUnit unit) throws InterruptedException {

    //                                                         Poll Loop Safety
    //                                                         ==================

    @Test
    public void test_poll_callsHeartbeatEventsCleanup() {
        // Verify poll calls sendHeartbeat, fetchNewEvents+dispatch, cleanupExpiredDocuments

   */
  protected void startUp() throws Exception {}

  /**
   * Run the service. This method is invoked on the execution thread. Implementations must respond
   * to stop requests. You could poll for lifecycle changes in a work loop:
   *
   * <pre>
   *   public void run() {
   *     while ({@link #isRunning()}) {
   *       // perform a unit of work
   *     }
   *   }
   * </pre>
   *

        finalizer != null
            ? new ReflectiveStarter(getStartFinalizer(finalizer))
            : new DirectStarter();
  }

  /** The actual reference queue that our background thread will poll. */
  final ReferenceQueue<Object> queue;

  final PhantomReference<Object> frqRef;

  /** Whether or not the background thread started successfully. */
  final boolean threadStarted;

     * notification nearly simultaneously, we sometimes receive only an interrupt. Thus, when we're
     * interrupted, we can't just poll whether it's time to end the wait because our "end the wait"
     * notification has been lost. (This is in contrast to how we can poll with, say, a
     * CountDownLatch.) In order to avoid hiding the requested notification from the caller, we need

        try {
            JobProcess jobProcess = processHelper.startProcess(sessionId, cmdList, pbCall, 1024, outputCallback);
            assertNotNull(jobProcess);

            // Poll for process to complete
            for (int i = 0; i < 50; i++) {
                if (!jobProcess.getProcess().isAlive()) {
                    break;
                }
                Thread.sleep(100);

        finalizer != null
            ? new ReflectiveStarter(getStartFinalizer(finalizer))
            : new DirectStarter();
  }

  /** The actual reference queue that our background thread will poll. */
  final ReferenceQueue<Object> queue;

  final PhantomReference<Object> frqRef;

  /** Whether or not the background thread started successfully. */
  final boolean threadStarted;

        indexUpdater.maxErrorCount = 0;

        // Make dataService throw runtime exception
        ((TestDataService) dataService).setThrowRuntimeException(true);

        indexUpdater.start();

        // Poll for thread to process exceptions
        for (int i = 0; i < 20; i++) {
            if (!indexUpdater.isAlive() || indexUpdater.getState() == Thread.State.WAITING

 * to perform its tasks.
 * </p>
 *
 * <p>
 * The crawling process involves the following steps:
 * </p>
 * <ol>
 *   <li>Fetching a URL from the queue using {@link UrlQueueService#poll(String)}.</li>
 *   <li>Checking if the URL is valid using {@link #isValid(UrlQueue)}.</li>
 *   <li>Accessing the content using a {@link CrawlerClient} obtained from {@link CrawlerClientFactory}.</li>