Github user aljoscha commented on a diff in the pull request:
https://github.com/apache/flink/pull/3191#discussion_r97826641
--- Diff: docs/dev/windows.md ---
@@ -204,72 +221,120 @@ input
{% highlight scala %}
val input: DataStream[T] = ...
-// tumbling event-time windows
-input
- .keyBy(<key selector>)
- .window(TumblingEventTimeWindows.of(Time.seconds(5)))
- .<windowed transformation>(<window function>)
-
// sliding event-time windows
input
.keyBy(<key selector>)
.window(SlidingEventTimeWindows.of(Time.seconds(10), Time.seconds(5)))
.<windowed transformation>(<window function>)
-// event-time session windows
+// sliding processing-time windows
input
.keyBy(<key selector>)
- .window(EventTimeSessionWindows.withGap(Time.minutes(10)))
+ .window(SlidingProcessingTimeWindows.of(Time.seconds(10),
Time.seconds(5)))
.<windowed transformation>(<window function>)
-// tumbling processing-time windows
+// sliding processing-time windows offset by -8 hours
input
.keyBy(<key selector>)
- .window(TumblingProcessingTimeWindows.of(Time.seconds(5)))
+ .window(SlidingProcessingTimeWindows.of(Time.hours(12), Time.hours(1),
Time.hours(-8)))
.<windowed transformation>(<window function>)
+{% endhighlight %}
+</div>
+</div>
-// sliding processing-time windows
+Time intervals can be specified by using one of `Time.milliseconds(x)`,
`Time.seconds(x)`,
+`Time.minutes(x)`, and so on.
+
+As shown in the last example, sliding window assigners also take an
optional `offset` parameter
+that can be used to change the alignment of windows. For example, without
offsets hourly windows
+sliding by 30 minutes are aligned with epoch, that is you will get windows
such as
+`1:00:00.000 - 1:59:59.999`, `1:30:00.000 - 2:29:59.999` and so on. If you
want to change that
+you can give an offset. With an offset of 15 minutes you would, for
example, get
+`1:15:00.000 - 2:14:59.999`, `1:45:00.000 - 2:44:59.999` etc.
+An important use case for offsets is to adjust windows to timezones other
than UTC-0.
+For example, in China you would have to specify an offset of
`Time.hours(-8)`.
+
+### Session Windows
+
+The *session windows* assigner groups elements by sessions of activity.
Session windows do not overlap and
+do not have a fixed start and end time in contrast to *tumbling windows*
and *sliding windows*. Instead a
+session window assigner closes a window when it does not receive elements
for a certain period
+of time, i.e., when a gap of inactivity occurred. A session window
assigner is configured with the *session gap* which
--- End diff --
This sentence looks a bit problematic since it is not the assigner that
decides when to close a window and when to open a new one. I see that it makes
sense from an explanation point of view but it misrepresents how it actually
works.
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