I need to create a sliding window of 4 events in Flink streaming application.
The window should slide by one event per every new arriving event. Furthermore,
per each new arriving event I need to calculate the aggregate sum of a
particular field in the 4 events I have in the window. I have referred 
which describes how this can be implemented based on time. But my use case if
little different, hence I am wondering how the window can be slided when a new
event gets added to the window.