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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