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 >
