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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-3899) Document window processing with Reduce/FoldFunction + WindowFunction
Date Wed, 24 Aug 2016 07:45:21 GMT

    [ https://issues.apache.org/jira/browse/FLINK-3899?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15434409#comment-15434409

ASF GitHub Bot commented on FLINK-3899:

Github user fhueske commented on a diff in the pull request:

    --- Diff: docs/apis/streaming/windows.md ---
    @@ -459,42 +459,106 @@ ready for processing. This allows to get the benefit of incremental
window compu
     the additional meta information that writing a `WindowFunction` provides.
     This is an example that shows how incremental aggregation functions can be combined with
    -a `WindowFunction`.
    +a `WindowFunction`.  The `FoldFunction`/`WindowFunction` example shows how to extract
    +ending event-time of a window of sensor readings that contain a timestamp, 
    +and the `ReduceFunction`/`WindowFunctions` example shows how to do eager window
    +aggregation (only a single element is kept in the window).
     <div class="codetabs" markdown="1">
     <div data-lang="java" markdown="1">
     {% highlight java %}
    -DataStream<Tuple2<String, Long>> input = ...;
    +DataStream<SensorReading> input = ...;
     // for folding incremental computation
         .keyBy(<key selector>)
         .window(<window assigner>)
    -    .apply(<initial value>, new MyFoldFunction(), new MyWindowFunction());
    +    .apply(Long.MIN_VALUE, new MyFoldFunction(), new MyWindowFunction());
    +/* ... */
    +private static  class myFoldFunction implements FoldFunction<SensorReading, Long>
    +    public Long fold(Long acc, SensorReading s) {
    +        return Math.max(acc, s.timestamp());
    +    }
    +private static class MyWindowFunction implements WindowFunction<Long, Long, String,
TimeWindow> {
    +    public void apply(String key, TimeWindow window, Iterable<Long> timestamps,
Collector<Long> out) {
    +            out.collect(timestamps.iterator().next());
    --- End diff --
    The example looks good, thanks! Two minor suggestions: 1) I think we can omit setting
key and time in the `FoldFunction`, 2) the `WindowFunction` could fetch the count in a separate
variable. This would make the `out.collect` line a bit shorter.
    Regarding the type restriction: You discovered a bug that we would like to fix but can't
until Flink 2.0.0 because we promoted the interface to be `@Public` and the API is stable
in Flink 1.0 releases. IMO it makes sense to point out this accidental restriction in the

> Document window processing with Reduce/FoldFunction + WindowFunction
> --------------------------------------------------------------------
>                 Key: FLINK-3899
>                 URL: https://issues.apache.org/jira/browse/FLINK-3899
>             Project: Flink
>          Issue Type: Improvement
>          Components: Documentation, Streaming
>    Affects Versions: 1.1.0
>            Reporter: Fabian Hueske
> The streaming documentation does not describe how windows can be processed with FoldFunction
or ReduceFunction and a subsequent WindowFunction. This combination allows for eager window
aggregation (only a single element is kept in the window) and access of the Window object,
e.g., to have access to the window's start and end time.

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