[jira] [Created] (SPARK-4423) Improve foreach() documentation to avoid confusion between local- and cluster-mode behavior

["Josh Rosen (JIRA)" <ji...@apache.org>]

Josh Rosen created SPARK-4423:
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             Summary: 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


{{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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