How to handle lack of messages from Pub/Sub in Google Dataflow

Question

The below is just random example code:

import argparse
import apache_beam as beam
from apache_beam.transforms import window
from apache_beam.transforms.trigger import AfterWatermark, AccumulationMode

class FormatDoFn(beam.DoFn):
    def process(self, viewing, window=beam.DoFn.WindowParam):
        print(viewing)

def main(argv=None):

    parser = argparse.ArgumentParser()
    known_args, pipeline_args = parser.parse_known_args(argv)

    with beam.Pipeline(argv=pipeline_args) as p:

        # Read from PubSub messages.
        input = p | beam.io.ReadFromPubSub("projects/example-project/topics/example")

        transformed = (
            input
            | beam.WindowInto(
                                window.FixedWindows(5),
                                trigger = AfterWatermark(),
                                accumulation_mode = AccumulationMode.DISCARDING
                            )

            | 'Format' >> beam.ParDo(FormatDoFn())
        )

if __name__ == '__main__':
    main()

I have a working Dataflow job that processes Pub/Sub messages per fixed window interval and inserts the aggregated results to a table. However, it only makes inserts when messages are received.

How can I make the dataflow job insert a Zero Value row once a window has passed without any messages received.

Answer 1

The solution turned out to be very simple for my use case. I just generated dummy KVs and combined it with the regular Pubsub message transformed KVs.

PCollection<Long> input = pipeline
                            .apply("unbounded longs",
                                    GenerateSequence
                                            .from(0)
                                            .withRate(2, Duration.standardSeconds(1))
                            );

Answer 2

As mentioned by @CaptainNabla and @Bruno Volpato, you can usetimers which will allow delayed processing. You can keep the count of every half minute in a state and use a timer to emit the results periodically and drop the unnecessary counts. The timer can be set or unset. For more information, you can use Timely and Stateful processing. For more information, you can check this link .