GroupApply

Splits a stream by key, runs the same query on each group, and merges the results.

GroupApply(keySelector, g => query, resultSelector) runs query once per key and gives the result selector the group key, so each output can say which group it came from. This is how per-key windowed statistics are written.

In the diagram, TumblingWindowLifetime(5) puts the events at 1, 2 and 4 in [5,10) and those at 7 and 9 in [10,15). Per key: a averages 1 and 3 to 2 in [5,10); b has only 2 there; in [10,15) b is 4 and a is 5.

See also: Count (tumbling window), Sum (hopping window), TumblingWindowLifetime, SelectByKey, Map, Reduce, Pivot