fhueske commented on code in PR #27935:
URL: https://github.com/apache/flink/pull/27935#discussion_r3109096730
##########
docs/content.zh/docs/dev/table/functions/ptfs.md:
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@@ -1021,6 +1021,18 @@ class TimerFunction extends ProcessTableFunction<String>
{
{{< /tab >}}
{{< /tabs >}}
+### Handling of Late Records
+
+A late record is a record with a time attribute value that is less than or
equal to the current
+watermark. PTFs handle late records just like non-late records by calling the
`eval()` method. If
+the `on_time` argument is specified, the late timestamp is preserved in the
output. This behavior is
+the same for PTFs with row and set semantics.
+
+Registering a timer for a time that is less than or equal to the current
watermark is allowed.
+If registered from within `eval()`, the timer fires on the next watermark
advance. If registered
+from within `onTimer()`, the timer fires immediately after the current timer
finishes. Note that
+unconditionally re-registering a past-time timer from within `onTimer()`
causes an infinite loop.
Review Comment:
This is the same behavior as for other functions with access to timers such
as KeyedProcessFunction.
Before this change, we ignored the registrations of timers with a timestamp
< current_watermark, which is inconsistent with Flink's other functions and
surprising for Flink users.
We might be able to introduce guards against such behavior, but I don't
think it is necessary. I don't think this is a common problem and adding logic
to detect and prevent such logic would cause more problems than it solves.
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