damccorm opened a new issue, #19654:
URL: https://github.com/apache/beam/issues/19654
This is kind of hard to reproduce, but I've seen it happen a few times in
the wild now.
We have a DoFn that sets an event-time timer at window.maxTimestamp, the
timer callback does something like:
```
def onWindowClose(
@StateId(...) key: ValueState[K],
@StateId(...) values: CombiningState[V],
out: OutputReceiver[O],
...
) {
val k = key.read()
val values = values.read()
out.output(KV.of(k,
values)
key.clear()
values.clear()
}
```
Essentially, keep track of the key, accumulate values seen in a window, and
emit them at the end of the window.
ProcessElement is pretty simple as well:
```
def processElement(
ctx: ProcessContext,
@StateId(...) key: ValueState[K],
@StateId(...)
values: CombiningState[V],
...
) {
key.write(ctx.element().getKey())
value.add(ctx.element().getValue())
timer.set(window.maxTimestamp())
}
```
However, *ONLY* when running on streaming engine (this doesn't happen
otherwise), I'll see cases where the onWindowClose timer fires with a null key,
and empty values.
This can only happen if the timer fired twice, since it wouldn't have been
set if no elements had arrived, and if late data had arrived, it would have set
the key (and added to the combining state). Also, we never have late date in
our pipeline.
An interesting other thing I noticed is that these "phantom firings" seem to
happen ~10-15 minutes _AFTER_ the window closes.
Again, its pretty rate, we'll have millions of keys in a window, and I'll
only see the error happen every few hours (with hourly windows).
Let me know if I can clarify anything else!
Imported from Jira
[BEAM-7614](https://issues.apache.org/jira/browse/BEAM-7614). Original Jira may
contain additional context.
Reported by: SteveNiemitz.
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