google cloud dataflow - Early results from GroupByKey transform

Question

How can I get GroupByKey to trigger early results, rather than wait for all the data to arrive (which in my case is a pretty long time).I tried to split my input PCollection into windows with an early trigger, but it just doesn`t work. It still waits for all the data to arrive before giving out the results.

PCollection<List<String>> input = ...
PCollection<KV<Integer,List<String>>> keyedInput = input.apply(ParDo.of(new AddArbitraryKey()))
keyedInput.apply(Window<KV<Integer,List<String>>>into(
          FixedWindows.of(Duration.standardSeconds(1)))
         .triggering(Repeatedly.forever(AfterWatermark.pastEndOfWindow()))
         .withAllowedLateness(Duration.ZERO).discardingFiredPanes())
 .apply(GroupByKey.<Integer,List<String>>create())
       .apply(ParDo.of(new RemoveArbitraryKey()))
       .apply(ParDo.of(new FurtherProcessing())

I am doing this to prevent fusing . The AddArbitraryKey transform outputs its elements with Timestamp. However, GroupByKey holds up everything until all the data arrives (for all the windows) . Could someone please tell me how i can get it to trigger early. Thank You .

Answer 1

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