Early and late firings (panes)

Early and late firings (panes) - emitting one window's result more than once as data keeps landing: in the early and late firings panes model, every emission is a pane, a snapshot of the window at that moment. An early pane fires before the watermark, on speculation ("here is the count so far"). The on-time pane fires when the watermark passes the window's end. Late panes fire again for each straggler that arrives within the allowed lateness.

The correctness bill lands downstream. A window that emits three times has three answers in flight, so every consumer must know which pane wins, which is the accumulation mode problem, and a sink that naively appends panes double-counts.

Do you need it? This is the deep end of the Dataflow model, built for pipelines where a partial answer minutes early is worth real money. To be fair, that describes very few jobs. Ubik does not do panes: a window emits once, when the watermark passes its end, --watermark-delay widens the grace period if you need one, and anything later is counted in late_dropped rather than refiring the window. One window, one answer.