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https://issues.apache.org/jira/browse/FLINK-2398?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14699891#comment-14699891
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Aljoscha Krettek commented on FLINK-2398:
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It works because the output type is not needed because the generic operator is
the very last operator that is executed.
> Decouple StreamGraph Building from the API
> ------------------------------------------
>
> Key: FLINK-2398
> URL: https://issues.apache.org/jira/browse/FLINK-2398
> Project: Flink
> Issue Type: Improvement
> Components: Streaming
> Reporter: Aljoscha Krettek
> Assignee: Aljoscha Krettek
>
> Currently, the building of the StreamGraph is very intertwined with the API
> methods. DataStream knows about the StreamGraph and keeps track of splitting,
> selected names, unions and so on. This leads to the problem that is is very
> hard to understand how the StreamGraph is built because the code that does it
> is all over the place. This also makes it hard to extend/change parts of the
> Streaming system.
> I propose to introduce "Transformations". A transformation hold information
> about one operation: The input streams, types, names, operator and so on. An
> API method creates a transformation instead of fiddling with the StreamGraph
> directly. A new component, the StreamGraphGenerator creates a StreamGraph
> from the tree of transformations that result from program specification using
> the API methods. This would relieve DataStream from knowing about the
> StreamGraph and makes unions, splitting, selection visible transformations
> instead of being scattered across the different API classes as fields.
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