Count (tumbling window)

Counts the events alive at each moment; after a tumbling window, that is a count per window.

Trill aggregates are snapshot aggregates: the result at each moment is computed over the events alive at that moment, and a new output event starts whenever that set changes. Windowing operators set lifetimes so that “alive at the same moment” means “in the same window”.

In the diagram, TumblingWindowLifetime(4) puts the events at 1, 2 and 4 in [4,8), so the count is 3 there; the window [8,12) holds one event and [12,16) one.

See also: TumblingWindowLifetime, Sum (hopping window), GroupApply