What runner are you using? Are you trying this out in the DirectRunner? As
> far as I know, no runner supports stateful processing in session windows
> yet. It is probably a bug that your pipeline was accepted by the runner
> when it includes features that the runner cannot execute. It would be a
> real mistake to have missed this validation for the DirectRunner.
Oh, ok, got it. :-(
I was actually trying this on the DirectRunner and it didn't fire errors,
so I guess no validation is performed in that sense.
Support for stateful processing in merging windows is definitely important.
> There's only a Jira filed for Dataflow  as far as I can tell. I just
> cloned it for the DirectRunner  since that is how you would test your
> pipeline. If you want to follow the same feature for a different runner, I
> can route a new ticket to the right person.
>  https://issues.apache.org/jira/browse/BEAM-2507
>  https://issues.apache.org/jira/browse/BEAM-3686
Awesome, I'm glad there is interest in adding this feature, it really fits
our use case. (and for the moment we have only interests for the two target
runners mentioned by you)