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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: Fold vs Reduce in DataStream API
Date Thu, 31 Dec 2015 08:50:01 GMT
No, the internal behavior is not documented right now. But I think it will change before the
1.0 release.

> On 30 Dec 2015, at 18:44, Brian Chhun <brian.chhun@getbraintree.com> wrote:
> 
> Thanks for the clarification. Is there a resource besides the code that has these kinds
of things documented? Understandable if there isn't much out there yet and that these things
are still in flux.
> 
> On Wed, Dec 30, 2015 at 11:14 AM, Aljoscha Krettek <aljoscha@apache.org> wrote:
> 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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