public Crawler(OkHttpClient client, int queueLimit, int hostLimit) {
    this.client = client;
    this.queue = new LinkedBlockingQueue<>(queueLimit);
    this.hostLimit = hostLimit;
  }

  private void parallelDrainQueue(int threadCount) {
    ExecutorService executor = Executors.newFixedThreadPool(threadCount);
    for (int i = 0; i < threadCount; i++) {
      executor.execute(() -> {
        try {
          drainQueue();