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From aljoscha <...@git.apache.org>
Subject [GitHub] flink pull request #3191: [FLINK-5529] [FLINK-4752] [docs] Improve / extends...
Date Wed, 25 Jan 2017 17:32:13 GMT
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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