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https://issues.apache.org/jira/browse/BEAM-9959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17548590#comment-17548590
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Danny McCormick commented on BEAM-9959:
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This issue has been migrated to https://github.com/apache/beam/issues/20214
> Mistakes Computing Composite Inputs and Outputs
> -----------------------------------------------
>
> Key: BEAM-9959
> URL: https://issues.apache.org/jira/browse/BEAM-9959
> Project: Beam
> Issue Type: Bug
> Components: sdk-go
> Reporter: Robert Burke
> Priority: P3
>
> The Go SDK uses a Scope object to manage beam Composites.
> A bug was discovered when consuming a PCollection in both the composite that
> created it, and in a separate composite.
> Further, the Go SDK should verify that the root hypergraph structure is a DAG
> and provides a reasonable error. In particular, the leaf nodes of the graph
> could form a DAG, but due to how the beam.Scope object is used, might cause
> the hypergraph to not be a DAG.
> Eg. It's possible to write the following in the Go SDK.
> PTransforms A, B, C and PCollections colA, colB, and Composites a, b.
> A and C are in a, and B are in b.
> A generates colA
> B consumes colA, and generates colB.
> C consumes colA and colB.
> ```
> a := s.Scope(a)
> b := s.Scope(b)
> colA := beam.Impulse(*a*)
> colB := beam.ParDo(*b*, <doFn>, colA)
> beam.ParDo0(*a*, <doFn>, colA, beam.SideInput{colB})
> ```
> If it doesn't already, the Go SDK must emit a clear error, and fail pipeline
> construction.
> If the affected composites are roots in the graph, the cycle prevents being
> able to topologically sort the root ptransforms for the pipeline graph, which
> can adversely affect runners.
> The recommendation is always to wrap uses of scope in functions or other
> scopes to prevent such incorrect constructions.
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