Another perspective, ignoring runners custom implementations and non-Java SDKs could be that the semantics are perfectly well defined: it is a composite and its semantics are defined by its implementation in terms of primitives. It is just that this expansion is not what we want so we should not use it (and also we shouldn't use "whatever the implementation does" as a spec for anything we care about).
On Thu, Oct 5, 2023 at 11:56 AM Kenneth Knowles <k...@apache.org> wrote: > I totally agree. I am motivated right now by the fact that it is already > used all over the place but with no consistent semantics. Maybe it is > simpler to focus on just making the minimal change, which would basically > be to update the expansion of the Reshuffle in the Java SDK. > > Kenn > > On Thu, Oct 5, 2023 at 11:39 AM John Casey <theotherj...@google.com> > wrote: > >> Given that this is a hint, I'm not sure redistribute should be a >> PTransform as opposed to some other way to hint to a runner. >> >> I'm not sure of what the syntax of that would be, but a semantic no-op >> transform that the runner may or may not do anything with is odd. >> >> On Thu, Oct 5, 2023 at 11:30 AM Kenneth Knowles <k...@apache.org> wrote: >> >>> So a high level suggestion from Robert that I want to highlight as a >>> top-post: >>> >>> Instead of focusing on just fixing the SDKs and runners Reshuffle, this >>> could be an opportunity to introduce Redistribute which was proposed in the >>> long-ago thread. The semantics are identical but it is more clear that it >>> *only* is a hint about redistributing data and there is no expectation >>> of a checkpoint. >>> >>> This new name may also be an opportunity to maintain update >>> compatibility (though this may actually be leaving unsafe code in user's >>> hands) and/or separate @RequiresStableInput/checkpointing uses of Reshuffle >>> from redistribution-only uses of Reshuffle. >>> >>> Any other thoughts on this one high level bit? >>> >>> Kenn >>> >>> On Thu, Oct 5, 2023 at 11:15 AM Kenneth Knowles <k...@apache.org> wrote: >>> >>>> >>>> On Wed, Oct 4, 2023 at 7:45 PM Robert Burke <lostl...@apache.org> >>>> wrote: >>>> >>>>> LGTM. >>>>> >>>>> It looks the Go SDK already adheres to these semantics as well for the >>>>> reference impl(well, reshuffle/redistribute_randomly, _by_key isn't >>>>> implemented in the Go SDK, and only uses the existing unqualified >>>>> reshuffle >>>>> URN [0]. >>>>> >>>>> The original strategy, and then for every element, the original >>>>> Window, TS, and Pane are all serialized, shuffled, and then deserialized >>>>> downstream. >>>>> >>>>> >>>>> https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/exec/reshuffle.go#L65 >>>>> >>>>> >>>>> https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/exec/reshuffle.go#L145 >>>>> >>>>> Prism at the moment vaccuously implements reshuffle by omitting the >>>>> node, and rewriting the inputs and outputs [1], as it's a local runner >>>>> with >>>>> single transform per bundle execution, but I was intending to make it a >>>>> fusion break regardless. Ultimately prism's "test" variant will default >>>>> to >>>>> executing the SDKs dictated reference implementation for the composite(s), >>>>> and any "fast" or "prod" variant would simply do the current >>>>> implementation. >>>>> >>>> >>>> Very nice! >>>> >>>> And of course I should have linked out to the existing reshuffle URN in >>>> the proto. >>>> >>>> Kenn >>>> >>>> >>>> >>>>> >>>>> Robert Burke >>>>> Beam Go Busybody >>>>> >>>>> [0]: >>>>> https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L46C3-L46C50 >>>>> [1]: >>>>> https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/runners/prism/internal/handlerunner.go#L82 >>>>> >>>>> >>>>> >>>>> On 2023/09/26 15:43:53 Kenneth Knowles wrote: >>>>> > Hi everyone, >>>>> > >>>>> > Recently there was a bug [1] caused by discrepancies between two of >>>>> > Dataflow's reshuffle implementations. I think the reference >>>>> implementation >>>>> > in the Java SDK [2] also does not match. This all led to discussion >>>>> on the >>>>> > bug and the pull request [3] about what the actual semantics should >>>>> be. I >>>>> > got it wrong, maybe multiple times. So I wrote up a very short >>>>> document to >>>>> > finish the discussion: >>>>> > >>>>> > https://s.apache.org/beam-reshuffle >>>>> > >>>>> > This is also probably among the simplest imaginable use of >>>>> > http://s.apache.org/ptransform-design-doc in case you want to see >>>>> kind of >>>>> > how I intended it to be used. >>>>> > >>>>> > Kenn >>>>> > >>>>> > [1] https://github.com/apache/beam/issues/28219 >>>>> > [2] >>>>> > >>>>> https://github.com/apache/beam/blob/d52b077ad505c8b50f10ec6a4eb83d385cdaf96a/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Reshuffle.java#L84 >>>>> > [3] https://github.com/apache/beam/pull/28272 >>>>> > >>>>> >>>>
