BatchEventTimerExample.java

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 * 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
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package example;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.apache.beam.sdk.Pipeline;
import org.apache.beam.sdk.coders.Coder;
import org.apache.beam.sdk.coders.ListCoder;
import org.apache.beam.sdk.coders.VarLongCoder;
import org.apache.beam.sdk.options.PipelineOptions;
import org.apache.beam.sdk.options.PipelineOptionsFactory;
import org.apache.beam.sdk.state.StateSpec;
import org.apache.beam.sdk.state.StateSpecs;
import org.apache.beam.sdk.state.TimeDomain;
import org.apache.beam.sdk.state.Timer;
import org.apache.beam.sdk.state.TimerSpec;
import org.apache.beam.sdk.state.TimerSpecs;
import org.apache.beam.sdk.state.ValueState;
import org.apache.beam.sdk.transforms.Create;
import org.apache.beam.sdk.transforms.DoFn;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.transforms.WithKeys;
import org.apache.beam.sdk.values.KV;
import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.MoreObjects;
import org.joda.time.Duration;
import org.joda.time.Instant;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class BatchEventTimerExample {

  public static void main(String[] args) {
    PipelineOptions options = PipelineOptionsFactory.fromArgs(args).withValidation().as(PipelineOptions.class);
    Pipeline pipeline = Pipeline.create(options);
    pipeline
        .apply(Create.of(Stream.iterate(0L, i -> i + 1).limit(1000).collect(Collectors.toList())))
        .apply(WithKeys.of(0L))
        .apply(useEventTimeAsTimer(VarLongCoder.of(), Duration.standardSeconds(1L)))
        .apply(Log.of());

    pipeline.run();
  }

  private static <T> ParDo.SingleOutput<KV<Long, T>, T> useEventTimeAsTimer(
      Coder<T> coder, Duration interval) {
    return ParDo.of(new UseEventTimeAsTimerFn<>(coder, interval));
  }

  private static class UseEventTimeAsTimerFn<T> extends DoFn<KV<Long, T>, T> {
    private static final String TIMER_ID = "timerId";
    private static final String QUEUE_ID = "queue";
    private static final String RECORDED_TIMER_TIMESTAMP_ID = "recordedTimerTimestamp";

    @TimerId(TIMER_ID)
    private final TimerSpec timerSpec = TimerSpecs.timer(TimeDomain.EVENT_TIME);

    @StateId(QUEUE_ID)
    private final StateSpec<ValueState<List<T>>> queueStateSpec;

    @StateId(RECORDED_TIMER_TIMESTAMP_ID)
    private final StateSpec<ValueState<Long>> recordedTimerTimestampStateSpec = StateSpecs.value();

    private final Duration interval;

    private UseEventTimeAsTimerFn(Coder<T> coder, Duration interval) {
      queueStateSpec = StateSpecs.value(ListCoder.of(coder));
      this.interval = interval;
    }

    @ProcessElement
    public void process(
        @Element KV<Long, T> element,
        @Timestamp Instant elementTimestamp,
        @StateId(QUEUE_ID) ValueState<List<T>> queueState,
        @TimerId(TIMER_ID) Timer timer,
        @StateId(RECORDED_TIMER_TIMESTAMP_ID) ValueState<Long> recordedTimerTimestampState) {
      List<T> queue = MoreObjects.firstNonNull(queueState.read(), new ArrayList<>());
      queue.add(element.getValue());
      queueState.write(queue);
      if (recordedTimerTimestampState.read() == null) {
        timer.offset(interval).setRelative();
        recordedTimerTimestampState.write(timer.getCurrentRelativeTime().getMillis());
      }
    }

    @OnTimer(TIMER_ID)
    public void onTimer(
        @TimerId(TIMER_ID) Timer timer,
        @StateId(QUEUE_ID) ValueState<List<T>> queueState,
        @StateId(RECORDED_TIMER_TIMESTAMP_ID) ValueState<Long> recordedTimerTimestampState,
        OutputReceiver<T> receiver) {
      List<T> queue = MoreObjects.firstNonNull(queueState.read(), new ArrayList<>());
      if (queue.isEmpty()) {
        timer.clear();
        return;
      }

      receiver.output(queue.remove(0));
      queueState.write(queue);

      if (queue.isEmpty()) {
        return;
      }

      timer.offset(interval).setRelative();
      recordedTimerTimestampState.write(timer.getCurrentRelativeTime().getMillis());
    }
  }

  private static class Log {
    private static final Logger LOG = LoggerFactory.getLogger(Log.class);
    private static final Instant START = Instant.now();

    static <T> ParDo.SingleOutput<T, T> of() {
      return ParDo.of(new Log.Fn<>());
    }

    private static class Fn<T> extends DoFn<T, T> {
      @ProcessElement
      public void process(@Element T t, OutputReceiver<T> receiver) {
        Duration elapsed = Duration.millis(Instant.now().getMillis() - START.getMillis());
        LOG.info("{}: {}", elapsed, t);
        receiver.output(t);
      }
    }
  }
}

Running this example using local runner or Dataflow outputs to log as shown below (not all the output shown). The PTn.nnnS are the elapsed durations from the start of the pipeline. Notice that despite the 1s interval suggesting we might see PT1.nnn, PT2.nnn, and so on. However, we see PT1.120S, PT1.143S, PT1.144S. This is due to the fact that an EVENT_TIME based timer was used and fired with respect to element timestamps and not the processing time.

INFO example.BatchEventTimerExample$Log - PT1.120S: 234
INFO example.BatchEventTimerExample$Log - PT1.143S: 235
INFO example.BatchEventTimerExample$Log - PT1.144S: 236