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Re: Evolving a Coder for an added field


For some extra context this change touches more than FileIO, in
reality this will affect updates in any file-based pipelines because
the metadata on each file will have now an extra field for the
lastModifiedDate.

The PR looks perfect, only issue is the backwards compatibility Coder
question. Knowing that probably Dataflow is the only one affected, I
would like to know what can we do?

[1] Should we merge and the Coder updatability be tied to SDK versions
(which makes sense and is probably more aligned with the LTS
discussion)?
[2] Should we have a MetadataCoderV2? (does this imply a repeated
Matadata object) ? In this case where is the right place to identify
and decide what coder to use?

Other ideas... ?

Last thing, the link that Luke shared does not seem to work (looks
like a googley-friendly URL, here it is the full URL for those
interested in the drain/update proposal:

[2] https://docs.google.com/document/d/1UWhnYPgui0gUYOsuGcCjLuoOUlGA4QaY91n8p3wz9MY/edit#
On Fri, Nov 2, 2018 at 10:11 PM Lukasz Cwik <lcwik@xxxxxxxxxx> wrote:
>
> I think the idea is that you would use one coder for paths where you don't need this information and would have FileIO provide a separate path that uses your updated coder.
> Existing users would not be impacted and users of the new FileIO that depend on this information would not be able to have updated their pipeline in the first place.
>
> If the feature in FileIO is experimental, we could choose to break it for existing users though since I don't know how feasible my suggestion above is.
>
>
>
> On Fri, Nov 2, 2018 at 12:56 PM Jeff Klukas <jklukas@xxxxxxxxxxx> wrote:
>>
>> Lukasz - Thanks for those links. That's very helpful context.
>>
>> It sounds like there's no explicit user contract about evolving Coder classes in the Java SDK and users might reasonably assume Coders to be stable between SDK versions. Thus, users of the Dataflow or Flink runners might reasonably expect that they can update the Java SDK version used in their pipeline when performing an update.
>>
>> Based in that understanding, evolving a class like Metadata might not be possible except in a major version bump where it's obvious to users to expect breaking changes and not to expect an "update" operation to work.
>>
>> It's not clear to me what changing the "name" of a coder would look like or whether that's a tenable solution here. Would that change be able to happen within the SDK itself, or is it something users would need to specify?