Timer.withOutputTimestamp().offset().setRelative seems unusable with event time

[Jan Lukavský]

Hi,

I have come across a bug with timer output timestamp - when using event 
time and relative timers, setting the timer can arbitrarily throw 
IllegalArgumentException if the firing timestamp (input watermark) is 
ahead of the output timestamp (like .java.lang.IllegalArgumentException: 
Attempted to set an event-time timer with an output timestamp of 
2021-04-29T07:16:19.369Z that is after the timer firing timestamp 
-290308-12-21T19:59:05.225Z). But there is no way to access the firing 
timestamp from user code. This means that the use has to either catch 
the IllegalArgumentException, or not use this construct at all.

Catching the exception should probably not be part of a contract, so we 
should do one of the following:

 a) either throw the exception right away and disable using relative 
timers with output timestamp completely, or

 b) support it correctly

What is the actual reason not to support output timestamps, that are 
ahead of firing timestamp? From my understanding, that should not be an 
issue, because there is TimestampCombiner.EARLIEST on the 
watermarkholdstate that corresponds to the output timestamp. If that is 
correct can we simply remove the check?

 Jan