A happy +1. This simplifies the code base, and if we find a compelling use,
it shouldn't be too bad to add it back in.
On Fri, Apr 21, 2017 at 10:24 AM, Kenneth Knowles <k...@google.com.invalid>
wrote:
> Hi all,
>
> I propose that we remove KeyedCombineFn before the first stable release.
>
> I don't think it adds enough value for the complexity it adds to e.g.
> CombineWithContext [1] and state [2, 3], and it doesn't seem to me that
> users really use it when we might expect. I am happy to be demonstrated
> wrong.
>
> It is very likely that you have never written [4, 5] or thought about
> KeyedCombineFn. So for context, here are excepts from signatures just to
> show the difference from CombineFn:
>
> CombineFn<InputT, AccumT, OutputT> {
> AccumT createAccumulator();
> AccumT addInput(AccumT accum, InputT input);
> AccumT mergeAccumulators(Iterable<AccumT> accums);
> OutputT extractOutput(AccumT accum);
> }
>
> KeyedCombineFn<K, InputT, AccumT, OutputT> {
> AccumT createAccumulator(K key);
> AccumT addInput(K key, AccumT accum, InputT input);
> AccumT mergeAccumulators(K key, Iterable<AccumT> accums);
> OutputT extractOutput(K key, AccumT accum);
> }
>
> So what are the particular reasons for this, versus a CombineFn that has
> KVs as its input and accumulator types?
>
> - There are some performance improvements potentially from not passing
> keys around, based on the assumption they are always available.
>
> - There is also a spec difference because it only has to be associative
> and commutative per key, cannot be applied in a global combine, and
> addInput is automatically key preserving.
>
> But in fact, in all of my code crawling the class is almost never used
> (even over the course of its history at Google) and even the few uses I
> found were often mistakes where the key is totally ignored, probably
> because a user thinks "I am doing a keyed combine so I need a keyed combine
> function". So the number of users actually affected is about zero.
>
> I would be curious if anyone has a compelling case for keeping
> KeyedCombineFn.
>
> Kenn
>
> [1]
> https://github.com/yafengguo/Apache-beam/blob/master/sdks/
> java/core/src/main/java/org/apache/beam/sdk/transforms/
> CombineWithContext.java
> [2] https://issues.apache.org/jira/browse/BEAM-1336
> [3] https://github.com/apache/beam/pull/2627
> [4]
> https://github.com/search?l=Java&q=KeyedCombineFn&ref=
> advsearch&type=Code&utf8=%E2%9C%93
> [5] https://www.google.com/search?q=KeyedCombineFn
>
