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.ch...@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 <se...@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 <rcroc...@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 <se...@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 <rcroc...@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 <fhue...@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 <rcroc...@newrelic.com>: > Is there a succinct description of the distinction between these transforms? > > -- > Ron Crocker > Principal Software Engineer > ( ( •)) New Relic > rcroc...@newrelic.com > M: +1 630 363 8835 > > > > > -- > Ron Crocker > Principal Software Engineer > ( ( •)) New Relic > rcroc...@newrelic.com > M: +1 630 363 8835 > >
