I'm working on a streaming Java Apache Beam (2.13.0) pipeline that is running in Google Cloud Dataflow. I have a long running PTransform (for a single input, it does a lot of work, outputs multiple outputs and can take >10 minutes).
I want to return early results from the processing to the user. I have a Window and Combine step afterwards. Early triggers do not seem to work with a long running PTransform. The Combine step outputs elements after the PTransform finishes processing the element (rather than returning early results).
I've tried many different early Window functions. E.g. I've tried doing forever element count triggers and it does not work. Ditto for forever processing time-based triggers (e.g. every 10 processing seconds). I've tried GlobalWindows, Fixed Windows, Session Windows, etc.
Here is rough pseudo code for what I'm doing.
p.apply(PubsubIO.readStrings().fromSubscription(options.getInput()));
.apply(FlatMapElements.via(new LongRunningCalculation()))
.apply(<I've tried a variety of window functions>)
.apply(Combine.perKey(new SumMetrics()))
.apply(DatastoreIO.v1().write().withProjectId(options.getProject()));
For the Window functions, I've tried many different Window functions to see if I can get anything to return early. I can't get it to return early.
Here is a basic one.
Window.into(new GlobalWindows())
.triggering(Repeatedly.forever(AfterPane.elementCountAtLeast(10)))
.withAllowedLateness(Duration.ZERO)
.discardingFiredPanes());
Even for this one, Even if the Window has added >>10 elements, the GroupBy in the Combine step does not output any rows.
Expected: If I have a long running PTransform, I'd still expect early triggers to still fire.
Actual: I can't seem to get early triggers to work.
Any advice?