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https://issues.apache.org/jira/browse/SPARK-733?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14275608#comment-14275608
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Apache Spark commented on SPARK-733:
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User 'ilganeli' has created a pull request for this issue:
https://github.com/apache/spark/pull/4022
> Add documentation on use of accumulators in lazy transformation
> ---------------------------------------------------------------
>
> Key: SPARK-733
> URL: https://issues.apache.org/jira/browse/SPARK-733
> Project: Spark
> Issue Type: Bug
> Components: Documentation
> Reporter: Josh Rosen
>
> Accumulators updates are side-effects of RDD computations. Unlike RDDs,
> accumulators do not carry lineage that would allow them to be computed when
> their values are accessed on the master.
> This can lead to confusion when accumulators are used in lazy transformations
> like `map`:
> {code}
> val acc = sc.accumulator(0)
> data.map(x => acc += x; f(x))
> // Here, acc is 0 because no actions have cause the `map` to be computed.
> {code}
> As far as I can tell, our documentation only includes examples of using
> accumulators in `foreach`, for which this problem does not occur.
> This pattern of using accumulators in map() occurs in Bagel and other Spark
> code found in the wild.
> It might be nice to document this behavior in the accumulators section of the
> Spark programming guide.
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