[jira] [Commented] (BEAM-1314) DoFn per-key lifecycle (Setup/Teardown)

["Luke Cwik (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-1314?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16048581#comment-16048581 ] 

Luke Cwik commented on BEAM-1314:
---------------------------------

I believe this is more of an implementation detail of how the Java SDK works. Nothing in the model says that a DoFn which uses user state can't be re-used across different keys within the same bundle. I believe the flaws are within the usage of a KeyedWorkItem and how the current key propagates through all the context objects within the Java SDK runners core.

> DoFn per-key lifecycle (Setup/Teardown)
> ---------------------------------------
>
>                 Key: BEAM-1314
>                 URL: https://issues.apache.org/jira/browse/BEAM-1314
>             Project: Beam
>          Issue Type: Wish
>          Components: beam-model
>            Reporter: Eugene Kirpichov
>
> DoFn's that use state and timers are implicitly per-key. Setup/Teardown methods are usually used to establish expensive resources - long-standing connections and such.
> For per-key DoFn's, we'd often want to use these per-key, so it'd be good to have ability in the model to ask that there be 1 instance of the DoFn per key, reused between e.g. different timer or trigger firings for this key, but not used for other keys.
> E.g. this would be particularly useful for Splittable DoFn - for its ability to reuse expensive resources between checkpoints.



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