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https://issues.apache.org/jira/browse/BEAM-11355?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kenneth Knowles updated BEAM-11355:
-----------------------------------
Status: Open (was: Triage Needed)
> pipeline_from_stages after sort_stages can return non-topologically ordered
> pipelines
> -------------------------------------------------------------------------------------
>
> Key: BEAM-11355
> URL: https://issues.apache.org/jira/browse/BEAM-11355
> Project: Beam
> Issue Type: Bug
> Components: sdk-py-core
> Reporter: Yifan Mai
> Assignee: Yifan Mai
> Priority: P2
>
> translations.sort_stages() sorts stages in topological order. However,
> calling translations.pipeline_from_stages() on sorted stages can result in a
> pipeline that is not topologically ordered. This is because of how it
> constructs the tree of parent to subtransforms.
> Example pipeline:
> * Leaf transforms are A, B and C.
> * Composite transform D has subtransforms B and C.
> * Root transform E has subtransforms A and D.
> * A produces an output that is an input to C, and B produces an output that
> is an input to C.
> * After optimizations and sort stages, the order of leaf stages is B, A, C
> (this is a valid ordering)
> Under the current implementation of translations.pipeline_from_stages():
> # B is added to the pipeline first, which also adds its parent D and its
> grandparent E to the pipeline. D is added as the first subtransform of E and
> B is added as the first subtransform of D.
> # A is added to the pipeline second. A is added as the second subtransform
> of E.
> # C is added to the pipeline third. C is added as the second subtransform of
> D.
> The order is now E(D(B, C), A) which is invalid because C must follow A. A
> valid order would be E(A, D(B, C)).
> The easiest fix is to change translations.pipeline_from_stages() such that
> whenever a leaf transform is added to the pipeline, all its ancestors are
> moved to the last position of the subtransforms of their respective parent.
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