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commit 78254b3eee73e8d002dfd7e9afcc92e81e79d83e
Author: Piotr Nowojski <piotr.nowoj...@gmail.com>
AuthorDate: Fri May 25 09:55:04 2018 +0200
[hotfix][docs] Specify operators behaviour on processing watermarks
---
docs/dev/event_time.md | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/docs/dev/event_time.md b/docs/dev/event_time.md
index 886bf22..e3090ad 100644
--- a/docs/dev/event_time.md
+++ b/docs/dev/event_time.md
@@ -180,6 +180,8 @@ Once a watermark reaches an operator, the operator can
advance its internal *eve
<img src="{{ site.baseurl }}/fig/stream_watermark_out_of_order.svg" alt="A
data stream with events (out of order) and watermarks" class="center"
width="65%" />
+Note that event time is inherited by a freshly created stream element (or
elements) from either the event that produced them or
+from watermark that triggered creation of those elements.
## Watermarks in Parallel Streams
@@ -219,4 +221,17 @@ with late elements in event time windows.
Please refer to the [Debugging Windows & Event Time]({{ site.baseurl
}}/monitoring/debugging_event_time.html) section for debugging
watermarks at runtime.
+## How operators are processing watermarks
+
+As a general rule, operators are required to completely process a given
watermark before forwarding it downstream. For example,
+`WindowOperator` will first evaluate which windows should be fired, and only
after producing all of the output triggered by
+the watermark will the watermark itself be sent downstream. In other words,
all elements produced due to occurrence of a watermark
+will be emitted before the watermark.
+
+The same rule applies to `TwoInputStreamOperator`. However, in this case the
current watermark of the operator is defined as
+the minimum of both of its inputs.
+
+The details of this behavior are defined by the implementations of the
`OneInputStreamOperator#processWatermark`,
+`TwoInputStreamOperator#processWatermark1` and
`TwoInputStreamOperator#processWatermark2` methods.
+
{% top %}
