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From markhamstra <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-4417] New API: sample RDD to fixed numb...
Date Sun, 28 Dec 2014 19:28:24 GMT
Github user markhamstra commented on the pull request:

    https://github.com/apache/spark/pull/3723#issuecomment-68216759
  
    My biggest problem with this is that, while the existing `sample` is an action, `sampleByCount`
is another one of those unholy beasts that is neither an action nor a transformation -- meaning
that, while it transforms an RDD into another RDD, it isn't lazy while doing so, but rather
embeds several actions (`count`) and makes use of another unholy beast (`zipWithIndex`), all
of which means that invoking `sampleByCount` eagerly launches several jobs in order to create
the new RDD.
    
    This is by no means the only eager transformation (or whatever we end up calling these
unholy beasts), since there is a handful of others that already exist in Spark; but I am really
hesitant to add another.  What we need is a larger strategy and re-organization to properly
handle, name and document eager transformations, but that is well beyond the scope of this
single PR.  In the meantime, eager transformations are just conveniences (inconveniences if
you are trying to launch jobs asynchronously) that packages up one or more actions.  They
can always be broken up into multiple explicit and ordinary transformations and actions (as
Sean was effectively suggesting earlier), so none of them are strictly necessary to achieve
their functionality.
    
    I'm really hesitant to add `sampleByCount` to the Spark API and thereby to the list of
eager transformations that we need to somehow fix in the future.  Perhaps a better way to
handle such convenience packaging of transformations and actions on RDDs is to include them
in [Spark Packages](http://spark-packages.org/). 


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