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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: Fold vs Reduce in DataStream API
Date Wed, 30 Dec 2015 17:14:13 GMT
Yes, this is correct right now. It should not be too hard to add the pre-aggregation behavior
for fold, however.
> On 30 Dec 2015, at 17:31, Brian Chhun <brian.chhun@getbraintree.com> wrote:

> 
> Hi All,
> 
> Are certain considerations when using these functions on windowed streams?
> 
> From reading the code, it looks using reduce (or another aggregation function) on a windowed
stream will pre-aggregate the result value as elements are added to the window, keeping the
size of window constant. On the other hand, the fold function will accumulate elements into
the window and wait until the window is fired before computing the aggregation. Does this
sound correct?
> 
> On Thu, Nov 19, 2015 at 1:27 PM, Stephan Ewen <sewen@apache.org> wrote:
> Hi Ron!
> 
> Yes, we had to change a few things in the API between 0.9 and 0.10. The API in 0.9 had
quite a few problems. This one now looks good, we are confident that it will stay.
> 
> Greetings,
> Stephan
> 
> 
> On Thu, Nov 19, 2015 at 8:15 PM, Ron Crocker <rcrocker@newrelic.com> wrote:
> Thanks Stephan, that helps quite a bit. Looks like another one of those API changes that
I'll be struggling with for a little bit.
> 
> On Thu, Nov 19, 2015 at 10:40 AM, Stephan Ewen <sewen@apache.org> wrote:
> Hi Ron!
> 
> You are right, there is a copy/paste error in the docs, it should be a FoldFunction that
is passed to fold(), not a ReduceFunction.
> 
> In Flink-0.10, the FoldFunction is only available on
> 
>   - KeyedStream (https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/streaming/api/datastream/KeyedStream.html#fold(R,%20org.apache.flink.api.common.functions.FoldFunction))
> 
>   - WindowedStream (https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/streaming/api/datastream/WindowedStream.html#fold(R,%20org.apache.flink.api.common.functions.FoldFunction,%20org.apache.flink.api.common.typeinfo.TypeInformation))
> 
> In most cases, you probably want the variant on the WindowedStream, if you aggregate
values over time.
> 
> --------------------------------------------------------
> 
> To the difference between fold() and reduce(): It is very subtle. The fold function can
also convert to another type whenever it integrates a new element.
> 
> Here is an example (with lists, not streams, but same principle).
> 
> --------------------------------------------------------
> 
> ReduceFunction<Integer> {
> 
>   public Integer reduce(Integer a, Integer b) { return a + b; }
> }
> 
> [1, 2, 3, 4, 5] -> reduce()  means: ((((1 + 2) + 3) + 4) + 5) = 15
> 
> --------------------------------------------------------
> 
> FoldFunction<String, Integer> {
> 
>   public String fold(String current, Integer i) { return current + String.valueOf(i);
}
> }
> 
> [1, 2, 3, 4, 5] -> fold("start-")  means: ((((("start-" + 1) + 2) + 3) + 4) + 5) =
"start-12345" (as a String)
> 
> 
> I hope that example illustrates the difference.
> 
> 
> Greetings,
> Stephan
> 
> 
> On Thu, Nov 19, 2015 at 7:00 PM, Ron Crocker <rcrocker@newrelic.com> wrote:
> Hi Fabian -
> 
> Thanks Fabian, that is a helpful description. 
> 
> That document WAS my source of information and it seems to also be the source of my confusion.
Further, it appears to be wrong - there is a FoldFunction<O,T> (https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/api/common/functions/FoldFunction.html)
that should be passed into fold()?  
> 
> Separate note: fold() doesn't appear in the javadocs for 0.10.0 DataStream (see https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/streaming/api/datastream/DataStream.html).
So this made me look in the freshly-downloaded flink-streaming-java:0.10.0 and fold() does
not appear in org.apache.flink.streaming.api.datastream.DataStream either. Am I looking in
the wrong place for it? In 0.9.1, it's located in that same class with this signature: fold(R
initialValue, FoldFunction<OUT, R> folder). 
> 
> Ron 
> 
> On Wed, Nov 18, 2015 at 9:39 AM, Fabian Hueske <fhueske@gmail.com> wrote:
> Hi Ron,
> 
> Have you checked: https://ci.apache.org/projects/flink/flink-docs-release-0.10/apis/streaming_guide.html#transformations?
> 
> Fold is like reduce, except that you define a start element (of a different type than
the input type) and the result type is the type of the initial value. In reduce, the result
type must be identical to the input type.
> 
> Best, Fabian
> 
> 2015-11-18 18:32 GMT+01:00 Ron Crocker <rcrocker@newrelic.com>:
> Is there a succinct description of the distinction between these transforms?
> 
> -- 
> Ron Crocker
> Principal Software Engineer
> ( ( •)) New Relic
> rcrocker@newrelic.com
> M: +1 630 363 8835
> 
> 
> 
> 
> -- 
> Ron Crocker
> Principal Software Engineer
> ( ( •)) New Relic
> rcrocker@newrelic.com
> M: +1 630 363 8835
> 
> 


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