Hi Stephan,
I faced the similar issue, the way implemented this(though a workaround) is
by making the input to fold function i.e. the initial value to fold
symmetric to what goes into the window function.
I made the initial value to fold function a tuple with all non
required/available index values in that initial tuple as 'null'.
The idea was to have a consistent tuple pass onto both functions and let
individual functions operate/update the index of their choice in the tuple.
The last tuple i.e. returned tuple after both these operations would have
all the index values set up.
val DEFAULT_ACCUMULATOR_VALUE = (null, List[String]())
.apply(DEFAULT_ACCUMULATOR_VALUE,
new MyFoldFunction(), // This operates/folds
values in index 1 i.e. the list
new MyWindowFunction()) // This simply puts in the key
at index 0
In the end I achieve the aggregation task through fold function + element
processing through window function.
Hope this helps!
Regards,
Anchit
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