Robert Burke created BEAM-9959:
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Summary: 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
Assignee: Robert Burke
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 colB.
```
a := s.Scope(a)
b := s.Scope(b)
colA := beam.Impulse(*a*)
colB := beam.ParDo(*b*, <doFn>, colA)
beam.ParDo0(*a*, <doFn>, colA)
```
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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