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https://issues.apache.org/jira/browse/SPARK-4423?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14313030#comment-14313030
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Patrick Wendell commented on SPARK-4423:
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[~joshrosen] Is this specific to foreach? Isn't this true of map() or other
operators as well?
> Improve foreach() documentation to avoid confusion between local- and
> cluster-mode behavior
> -------------------------------------------------------------------------------------------
>
> Key: SPARK-4423
> URL: https://issues.apache.org/jira/browse/SPARK-4423
> Project: Spark
> Issue Type: Improvement
> Components: Documentation
> Reporter: Josh Rosen
> Assignee: Ilya Ganelin
>
> {{foreach}} seems to be a common source of confusion for new users: in
> {{local}} mode, {{foreach}} can be used to update local variables on the
> driver, but programs that do this will not work properly when executed on
> clusters, since the {{foreach}} will update per-executor variables (note that
> this _will_ work correctly for accumulators, but not for other types of
> mutable objects).
> Similarly, I've seen users become confused when {{.foreach(println)}} doesn't
> print to the driver's standard output.
> At a minimum, we should improve the documentation to warn users against
> unsafe uses of {{foreach}} that won't work properly when transitioning from
> local mode to a real cluster.
> We might also consider changes to local mode so that its behavior more
> closely matches the cluster modes; this will require some discussion, though,
> since any change of behavior here would technically be a user-visible
> backwards-incompatible change (I don't think that we made any explicit
> guarantees about the current local-mode behavior, but someone might be
> relying on the current implicit behavior).
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