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commit a72d91bed91692077577b176e97a5cb753a9c109
Author: Piotr Nowojski <piotr.nowoj...@gmail.com>
AuthorDate: Tue May 29 15:18:16 2018 +0200
[hotfix][docs] Document event time behaviour for idle sources
This closes #6076.
---
docs/dev/event_time.md | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/docs/dev/event_time.md b/docs/dev/event_time.md
index e3090ad..1d747aa 100644
--- a/docs/dev/event_time.md
+++ b/docs/dev/event_time.md
@@ -215,6 +215,19 @@ arrive after the system's event time clock (as signaled by
the watermarks) has a
timestamp. See [Allowed Lateness]({{ site.baseurl
}}/dev/stream/operators/windows.html#allowed-lateness) for more information on
how to work
with late elements in event time windows.
+## Idling sources
+
+Currently, with pure event time watermarks generators, watermarks can not
progress if there are no elements
+to be processed. That means in case of gap in the incoming data, event time
will not progress and for
+example the window operator will not be triggered and thus existing windows
will not be able to produce any
+output data.
+
+To circumvent this one can use periodic watermark assigners that don't only
assign based on
+element timestamps. An example solution could be an assigner that switches to
using current processing time
+as the time basis after not observing new events for a while.
+
+Sources can be marked as idle using
`SourceFunction.SourceContext#markAsTemporarilyIdle`. For details please refer
to the Javadoc of
+this method as well as `StreamStatus`.
## Debugging Watermarks
