[GitHub] spark pull request: SPARK-3642. Document the nuances of shared var...

[JoshRosen <gi...@git.apache.org>]

Github user JoshRosen commented on a diff in the pull request:

    https://github.com/apache/spark/pull/2490#discussion_r22244391
  
    --- Diff: docs/programming-guide.md ---
    @@ -1183,6 +1189,10 @@ running on the cluster can then add to it using the `add` method or the `+=` ope
     However, they cannot read its value.
     Only the driver program can read the accumulator's value, using its `value` method.
     
    +An operation referencing an accumulator may run multiple times, either when parts of its output
    +data become lost or when multiple actions make use of the RDD it produces. In these cases, only the
    +additions reported by the first successful execution contribute to the accumulator's value.
    --- End diff --
    
    I was going to suggest that this might not be true, but it looks like this is partially-true now that #2524 has been merged.  That PR added a paragraph to the programming guide which clarifies that we guard against duplicate updates only for updates performed inside of actions and _not_ for ones performed in transformations: https://github.com/apache/spark/commit/66cc2431462a5354bb50c196a59da0ffc258c466?diff=unified#diff-3
    
    In light of this, do we still need this paragraph?


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