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https://issues.apache.org/jira/browse/BEAM-3305?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17546771#comment-17546771
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Kenneth Knowles commented on BEAM-3305:
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This issue has been migrated to https://github.com/apache/beam/issues/18602
> Consider: Go ViewFn support
> ---------------------------
>
> Key: BEAM-3305
> URL: https://issues.apache.org/jira/browse/BEAM-3305
> Project: Beam
> Issue Type: Improvement
> Components: sdk-go
> Reporter: Henning Rohde
> Priority: P3
>
> We can add it as an optional field to beam.SideInput (see:
> pkg/beam/option.go). The execution side needs to support it as well. However,
> with the various side input forms, it's not clear how valuable such a feature
> would be.
> It is strongly recommended to add Map Side Inputs
> https://issues.apache.org/jira/browse/BEAM-3293 before implementing this
> suggestion, and required to have caching implemented
> https://issues.apache.org/jira/browse/BEAM-11097. Otherwise very little
> benefit is acheived.
> See https://issues.apache.org/jira/browse/BEAM-3293 for where code might need
> to be changed.
> Make providing a ViewFn part of the beam.SideInput struct, either as a method
> to validate & update unexported fields, or adding an optional field.
> The ViewFn would be a function that takes in the iterator/multimap version of
> the Side Input and returns whatever the pre-processed type is. That value
> would be what is cached to reduce work per common window.
> The DoFn should then be able to use that type in the position of the
> ProcessElement method for that side input. This avoids issues that only work
> in of the Global Window like trying to process the side input in StartBundle.
> Something would need to be changed about handling the DoFn method signature
> analysis to support additional alternative side input types.
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