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From "Sean Owen (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (SPARK-4423) Improve foreach() documentation to avoid confusion between local- and cluster-mode behavior
Date Wed, 11 Mar 2015 13:20:38 GMT

     [ https://issues.apache.org/jira/browse/SPARK-4423?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Sean Owen resolved SPARK-4423.
------------------------------
       Resolution: Fixed
    Fix Version/s: 1.4.0

Issue resolved by pull request 4696
[https://github.com/apache/spark/pull/4696]

> 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
>             Fix For: 1.4.0
>
>
> {{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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