[jira] [Resolved] (SPARK-3105) Calling cache() after RDDs are pipelined has no effect in PySpark

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

     [ https://issues.apache.org/jira/browse/SPARK-3105?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Josh Rosen resolved SPARK-3105.
-------------------------------
    Resolution: Won't Fix

Closing as "Won't Fix", since I don't think we have any plans to change this behavior in the near term, nobody has complained about this, and it's a somewhat invasive tricky change.

> Calling cache() after RDDs are pipelined has no effect in PySpark
> -----------------------------------------------------------------
>
>                 Key: SPARK-3105
>                 URL: https://issues.apache.org/jira/browse/SPARK-3105
>             Project: Spark
>          Issue Type: Bug
>          Components: PySpark
>    Affects Versions: 1.0.0, 1.1.0
>            Reporter: Josh Rosen
>            Assignee: Josh Rosen
>
> PySpark's PipelinedRDD decides whether to pipeline transformations by checking whether those transformations are pipelinable _at the time that the PipelinedRDD objects are created_ rather than at the time that we invoke actions.  This might lead to problems if we call {{cache()}} on an RDD after it's already been used in a pipeline:
> {code}
> rdd1 = sc.parallelize(range(100)).map(lambda x: x)
> rdd2 = rdd1.map(lambda x: 2 * x)
> rdd1.cache()
> rdd2.collect()
> {code}
> When I run this code, I'd expect {cache()}} to break the pipeline and cache intermediate results, but instead the two transformations are pipelined together in Python, effectively ignoring the {{cache()}}.
> Note that {{cache()}} works properly if we call it before performing any other transformations on the RDD:
> {code}
> rdd1 = sc.parallelize(range(100)).map(lambda x: x).cache()
> rdd2 = rdd1.map(lambda x: 2 * x)
> rdd2.collect()
> {code}
> This works as expected and caches {{rdd1}}.
> To fix this, I think we dynamically decide whether to pipeline when we actually perform actions, rather than statically deciding when we create the RDDs.
> We should also add tests for this.
> (Thanks to [~tdas] for pointing out this issue.)



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