sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Reshuffle.java

/*
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 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.beam.sdk.transforms;

import java.util.concurrent.ThreadLocalRandom;
import org.apache.beam.sdk.annotations.Experimental;
import org.apache.beam.sdk.annotations.Internal;
import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
import org.apache.beam.sdk.transforms.windowing.ReshuffleTrigger;
import org.apache.beam.sdk.transforms.windowing.TimestampCombiner;
import org.apache.beam.sdk.transforms.windowing.Window;
import org.apache.beam.sdk.util.IdentityWindowFn;
import org.apache.beam.sdk.values.KV;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.sdk.values.TimestampedValue;
import org.apache.beam.sdk.values.WindowingStrategy;
import org.joda.time.Duration;

/**
 * <b>For internal use only; no backwards compatibility guarantees.</b>
 *
 * <p>A {@link PTransform} that returns a {@link PCollection} equivalent to its input but
 * operationally provides some of the side effects of a {@link GroupByKey}, in particular preventing
 * fusion of the surrounding transforms, checkpointing and deduplication by id.
 *
 * <p>Performs a {@link GroupByKey} so that the data is key-partitioned. Configures the {@link
 * WindowingStrategy} so that no data is dropped, but doesn't affect the need for the user to
 * specify allowed lateness and accumulation mode before a user-inserted GroupByKey.
 *
 * @param <K> The type of key being reshuffled on.
 * @param <V> The type of value being reshuffled.
 * @deprecated this transform's intended side effects are not portable; it will likely be removed
 */
@Internal
@Deprecated
public class Reshuffle<K, V> extends PTransform<PCollection<KV<K, V>>, PCollection<KV<K, V>>> {

  private Reshuffle() {}

  public static <K, V> Reshuffle<K, V> of() {
    return new Reshuffle<>();
  }

  /**
   * Encapsulates the sequence "pair input with unique key, apply {@link Reshuffle#of}, drop the
   * key" commonly used to break fusion.
   */
  @Experimental
  public static <T> ViaRandomKey<T> viaRandomKey() {
    return new ViaRandomKey<>();
  }

  @Override
  public PCollection<KV<K, V>> expand(PCollection<KV<K, V>> input) {
    WindowingStrategy<?, ?> originalStrategy = input.getWindowingStrategy();
    // If the input has already had its windows merged, then the GBK that performed the merge
    // will have set originalStrategy.getWindowFn() to InvalidWindows, causing the GBK contained
    // here to fail. Instead, we install a valid WindowFn that leaves all windows unchanged.
    // The TimestampCombiner is set to ensure the GroupByKey does not shift elements forwards in
    // time.
    // Because this outputs as fast as possible, this should not hold the watermark.
    Window<KV<K, V>> rewindow =
        Window.<KV<K, V>>into(new IdentityWindowFn<>(originalStrategy.getWindowFn().windowCoder()))
            .triggering(new ReshuffleTrigger<>())
            .discardingFiredPanes()
            .withTimestampCombiner(TimestampCombiner.EARLIEST)
            .withAllowedLateness(Duration.millis(BoundedWindow.TIMESTAMP_MAX_VALUE.getMillis()));

    return input
        .apply(rewindow)
        .apply("ReifyOriginalTimestamps", Reify.timestampsInValue())
        .apply(GroupByKey.create())
        // Set the windowing strategy directly, so that it doesn't get counted as the user having
        // set allowed lateness.
        .setWindowingStrategyInternal(originalStrategy)
        .apply(
            "ExpandIterable",
            ParDo.of(
                new DoFn<KV<K, Iterable<TimestampedValue<V>>>, KV<K, TimestampedValue<V>>>() {
                  @ProcessElement
                  public void processElement(
                      @Element KV<K, Iterable<TimestampedValue<V>>> element,
                      OutputReceiver<KV<K, TimestampedValue<V>>> r) {
                    K key = element.getKey();
                    for (TimestampedValue<V> value : element.getValue()) {
                      r.output(KV.of(key, value));
                    }
                  }
                }))
        .apply("RestoreOriginalTimestamps", ReifyTimestamps.extractFromValues());
  }

  /** Implementation of {@link #viaRandomKey()}. */
  public static class ViaRandomKey<T> extends PTransform<PCollection<T>, PCollection<T>> {
    private ViaRandomKey() {}

    @Override
    public PCollection<T> expand(PCollection<T> input) {
      return input
          .apply("Pair with random key", ParDo.of(new AssignShardFn<>()))
          .apply(Reshuffle.of())
          .apply(Values.create());
    }

    private static class AssignShardFn<T> extends DoFn<T, KV<Integer, T>> {
      private int shard;

      @Setup
      public void setup() {
        shard = ThreadLocalRandom.current().nextInt();
      }

      @ProcessElement
      public void processElement(@Element T element, OutputReceiver<KV<Integer, T>> r) {
        ++shard;
        // Smear the shard into something more random-looking, to avoid issues
        // with runners that don't properly hash the key being shuffled, but rely
        // on it being random-looking. E.g. Spark takes the Java hashCode() of keys,
        // which for Integer is a no-op and it is an issue:
        // http://hydronitrogen.com/poor-hash-partitioning-of-timestamps-integers-and-longs-in-
        // spark.html
        // This hashing strategy is copied from
        // org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Hashing.smear().
        int hashOfShard = 0x1b873593 * Integer.rotateLeft(shard * 0xcc9e2d51, 15);
        r.output(KV.of(hashOfShard, element));
      }
    }
  }
}