a question on window trigger and delta output

[kant kodali <ka...@gmail.com>]

Hi All,

I set a transformation like this and my events in the stream have a
sequential timestamp like 1,2,3,.... and I set the watermark to event time.

   myStream
       .keyBy(0)
       .timeWindow(Time.of(1000, TimeUnit.MILLISECONDS))
       .aggregate(new myAggregateFunction())
       .print()

This stream prints only when there are events that are multiples of 1000.
This is not quite what I want because I want to see some output every
second. so I tried this

myStream
.map(new PartitionMapper<>()).returns(typeInfo)
.keyBy(0)
.timeWindow(Time.of(1000, TimeUnit.MILLISECONDS))
.trigger(ContinuousProcessingTimeTrigger.of(Time.seconds(1)))

This looked much better however it prints the entire state every second,
which makes sense because thats what ContinuousProcessingTimeTrigger is
supposed to do. But what I am looking for is combination of both like
Trigger only if there are some events in the window with a max timeout of 1
second. if there are no events in the window then don't trigger because I
don't want to see the same output every second.

What would be the right trigger here (something that doesn't wait until the
watermark event or doesn't print the same output every second)?

Also. I don't want to output the whole state in the transformation above
instead I only want to output the delta from previous trigger to current
trigger. I looked into DeltaTriggers and Delta evictors and confused by
what the threshold parameter will do and also looking for some explanation
on what they would do?

Other words, I want to output something similar to retractStream() when an
outer join between two tables is executed since it outputs only delta and
only when there are some events (doesn't wait until the watermark event or
doesn't print the same output every second)

Thanks!