return true;
    }

    /**
     * Stores child URLs from the given set into the crawling queue for future processing.
     * This method filters out blank URLs and increments the depth for child URLs.
     *
     * @param urlQueue the parent URL queue item
     * @param childUrlSet the set of child URLs to be queued for crawling
     */

  private void weaklyReferenceQueue() {
    frq = new FinalizableReferenceQueue();
    queueReference = new WeakReference<>(frq.queue);

    /*
     * Queue and clear a reference for good measure. We test later on that
     * the finalizer thread stopped, but we should test that it actually
     * started first.
     */
    reference =

    assertFalse(segment.clearValueForTesting(key, hash, valueRef));
    segment.setWeakValueReferenceForTesting(entry, valueRef);
    assertTrue(segment.clearValueForTesting(key, hash, valueRef));
  }

  // reference queues

  public void testDrainKeyReferenceQueueOnWrite() {
    for (MapMaker maker : allWeakKeyStrengthMakers()) {
      MapMakerInternalMap<Object, Object, ?, ?> map = makeMap(maker.concurrencyLevel(1));

    assertFalse(segment.clearValueForTesting(key, hash, valueRef));
    segment.setWeakValueReferenceForTesting(entry, valueRef);
    assertTrue(segment.clearValueForTesting(key, hash, valueRef));
  }

  // reference queues

  public void testDrainKeyReferenceQueueOnWrite() {
    for (MapMaker maker : allWeakKeyStrengthMakers()) {
      MapMakerInternalMap<Object, Object, ?, ?> map = makeMap(maker.concurrencyLevel(1));

# Number of primary shards for crawler queue index.
index.document.crawler.queue.number_of_shards=10
# Number of primary shards for crawler data index.
index.document.crawler.data.number_of_shards=10
# Number of primary shards for crawler filter index.
index.document.crawler.filter.number_of_shards=10
# Number of replicas for crawler queue index.
index.document.crawler.queue.number_of_replicas=1

import java.util.List;
import java.util.ListIterator;
import java.util.Locale;
import java.util.Map;
import java.util.NavigableMap;
import java.util.NavigableSet;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Random;
import java.util.Set;
import java.util.SortedMap;
import java.util.SortedSet;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.UUID;

import java.util.NavigableSet;
import java.util.Optional;
import java.util.OptionalDouble;
import java.util.OptionalInt;
import java.util.OptionalLong;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Random;
import java.util.Set;
import java.util.SortedMap;
import java.util.SortedSet;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.UUID;

    }

    @Override
    public @Nullable ReferenceEntry<K, V> getEntry() {
      return null;
    }

    @Override
    public ValueReference<K, V> copyFor(
        ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
      return this;
    }

    public void setLoading(boolean loading) {
      this.loading = loading;
    }

    @Override

    }

    @Override
    public @Nullable ReferenceEntry<K, V> getEntry() {
      return null;
    }

    @Override
    public ValueReference<K, V> copyFor(
        ReferenceQueue<V> queue, V value, ReferenceEntry<K, V> entry) {
      return this;
    }

    public void setLoading(boolean loading) {
      this.loading = loading;
    }

    @Override

They tend to require more complex configurations, a message/job queue manager, like RabbitMQ or Redis, but they allow you to run background tasks in multiple processes, and especially, in multiple servers.