Main input element accesses expired side input windows

[Shen Li]

Hi,

When a main input element tries to access an expired side input window
(violating maximumLookback), should ParDo discard the element or treat it
as an error?

Besides, what should ParDo do in the following situation:
1. The side input window W is not expired but unready when the main input
element X arrives. So the ParDo pushes back the main input element.
2. Later the side input window W expires before X is processed.
In this case, should ParDo throw away X?

Thanks,
Shen