Gist for article: https://medium.com/evenbit/aggregate-thousands-of-inputs-per-second-with-firebase-76111212b850

beam.java

pipeline
  .apply("Receive PubSub", PubsubIO.readMessagesWithAttributes().fromTopic(fromTopic))
  .apply("Transform to KeyValue", ParDo.of(new TransformPubSubMessage()))
  .apply("Apply window of time", Window.into(FixedWindows.of(WINDOW_SIZE)))
  .apply("Group by contestant", GroupByKey.create())
  .apply("Count votes", Combine.groupedValues(new CountFn()))
  .apply("Transform to PubSub Message", ParDo.of(new MakePubSubPayload()))
  .apply("Send PubSub", PubsubIO.writeMessages().to(toTopic));