@Experimental(value=TRIGGER) public class AfterWatermark extends java.lang.Object
AfterWatermark triggers fire based on progress of the system watermark. This time is a
lower-bound, sometimes heuristically established, on event times that have been fully processed
by the pipeline.
For sources that provide non-heuristic watermarks (e.g. PubsubIO when using arrival times as
event times), the watermark is a strict guarantee that no data with an event time earlier than
that watermark will ever be observed in the pipeline. In this case, it's safe to assume that any
pane triggered by an AfterWatermark trigger with a reference point at or beyond the end
of the window will be the last pane ever for that window.
For sources that provide heuristic watermarks (e.g. PubsubIO when using user-supplied event
times), the watermark itself becomes an estimate that no data with an event time earlier
than that watermark (i.e. "late data") will ever be observed in the pipeline. These heuristics
can often be quite accurate, but the chance of seeing late data for any given window is non-zero.
Thus, if absolute correctness over time is important to your use case, you may want to consider
using a trigger that accounts for late data. The default trigger, Repeatedly.forever(AfterWatermark.pastEndOfWindow()), which fires once when the watermark passes
the end of the window and then immediately therafter when any late data arrives, is one such
example.
The watermark is the clock that defines TimeDomain.EVENT_TIME.
Additionaly firings before or after the watermark can be requested by calling AfterWatermark.pastEndOfWindow.withEarlyFirings(OnceTrigger) or AfterWatermark.pastEndOfWindow.withLateFirings(OnceTrigger).
| Modifier and Type | Class and Description |
|---|---|
static class |
AfterWatermark.AfterWatermarkEarlyAndLate |
static class |
AfterWatermark.FromEndOfWindow
A watermark trigger targeted relative to the end of the window.
|
| Modifier and Type | Method and Description |
|---|---|
static AfterWatermark.FromEndOfWindow |
pastEndOfWindow()
Creates a trigger that fires when the watermark passes the end of the window.
|
public static AfterWatermark.FromEndOfWindow pastEndOfWindow()