Event time matching of side input

[Jan Lukavský]

Hi,

I have a question about matching side inputs to main input. First a 
recap, to make sure I understand the current state correctly:

 a) elements from main input are pushed back (stored in state) until a 
first side input pane arrives (that might be on time, or early)

 b) after that, elements from the main input are matched to the current 
side input view - the view is updated as new data arrives, but is 
matched to the main input elements in processing time

If this is the current state, my question is, would it be possible to 
add a new mode of matching of side inputs? Something like

 ParDo.of(new MyDoFn()).withSideInput("name", myView, 
TimeDomain.EVENT_TIME)

the semantics would be that the elements from the main PCollection would 
be stored into state as pairs with the value of the current main input 
watermark and on update of side-input watermark only main input elements 
with associated input watermark less than that of the side input would 
be matched with the side input and sent downstream. Although this 
approach is necessarily more expensive and introducing additional 
latency than processing time matching, there are situations when 
processing time matching is inapropriate for correctness reasons.

WDYT?

 Jan