StreamEventTimerExample.java

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 * to you under the Apache License, Version 2.0 (the
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package example;

import static org.apache.beam.sdk.util.Preconditions.checkStateNotNull;
import static org.apache.beam.sdk.values.TypeDescriptors.longs;

import java.util.ArrayList;
import java.util.List;
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.DoFn;
import org.apache.beam.sdk.transforms.MapElements;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.transforms.PeriodicImpulse;
import org.apache.beam.sdk.transforms.WithKeys;
import org.apache.beam.sdk.transforms.WithTimestamps;
import org.apache.beam.sdk.transforms.windowing.GlobalWindow;
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 StreamEventTimerExample {

  public static void main(String[] args) {
    Instant start = Instant.now();
    PipelineOptions options = PipelineOptionsFactory.fromArgs(args).withValidation().as(PipelineOptions.class);
    Pipeline pipeline = Pipeline.create(options);
    pipeline
        .apply(PeriodicImpulse.create().withInterval(Duration.millis(25L)))
        .apply(
            MapElements.into(longs())
                .via(instant -> checkStateNotNull(instant).getMillis() - start.getMillis()))
        .apply(WithTimestamps.of(ignored -> GlobalWindow.INSTANCE.maxTimestamp()))
        .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 using local or Dataflow runner outputs the following (not all shown). The PTn.nnnS are the elapsed durations from the start of the pipeline. As expected, these elapsed durations are not as consistent as ProcessTime domain timers.

PT926.843S: 12465
PT929.045S: 12490