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https://issues.apache.org/jira/browse/BEAM-3204?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16257739#comment-16257739
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Kenneth Knowles commented on BEAM-3204:
---------------------------------------

In light of the idea to fork {{FunctionSpec}} into e.g. {{UdfSpec}} and 
{{PTransformSpec}} I think we could also just inline the fields into {{Coder}}.

> Coders only should have a FunctionSpec, not an SdkFunctionSpec
> --------------------------------------------------------------
>
>                 Key: BEAM-3204
>                 URL: https://issues.apache.org/jira/browse/BEAM-3204
>             Project: Beam
>          Issue Type: Sub-task
>          Components: beam-model
>            Reporter: Kenneth Knowles
>            Assignee: Henning Rohde
>              Labels: portability
>
> We added environments to coders to account for "custom" coders where it is 
> only really possible for one SDK to understand them, like this:
> {code}
> Coder {
>   spec: SdkFunctionSpec {
>     environment: "java_sdk_docker_container",
>     spec: FunctionSpec {
>       urn: "beam:coder:java_custom_coder",
>       payload: <serialized java bytes>
>     }
>   }
> }
> {code}
> But a coder must be understood by both the producer of a PCollection and its 
> consumers. A coder is not the same as other UDF, though these are 
> user-defined.
> A pipeline where either the producer or consumer cannot handle the coder is 
> invalid, and we will have to build our cross-language APIs to prevent 
> construction of such a pipeline. So we can drop the environment.
> I think there are some folks who want to reserve the ability to add an 
> environment later, perhaps, to not pain ourselves into a corner. In this 
> case, we can just add a field to Coder.



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