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https://issues.apache.org/jira/browse/CRUNCH-192?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13628914#comment-13628914
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Josh Wills commented on CRUNCH-192:
-----------------------------------
Thanks! I have one last nit-- could you move the SingleUseIterable into a
neutral package (e.g., o.a.c.util) so that there isn't a dependency between mem
<-> mr? After that, please commit.
> Document and enforce the semantics around reducer-based Iterables
> -----------------------------------------------------------------
>
> Key: CRUNCH-192
> URL: https://issues.apache.org/jira/browse/CRUNCH-192
> Project: Crunch
> Issue Type: Bug
> Reporter: Gabriel Reid
> Attachments: CRUNCH-192.patch, CRUNCH-192.patch.v2
>
>
> As reported on [email protected] by Chad Urso McDaniel:
> BLUF: The Iterable parameter to CombineFn.process implies you can iterate
> multiple times when you cannot and this leads to surprising behavior.
> As many of you probably know, the signature of CombineFn.process is
> ---
> process(Pair<K, Iterable<V>> input, Emitter<Pair<K, V>> emitter)
> ---
> The corresponding Hadoop Reducer signature is
> ---
> reduce(K2 key, Iterator<V2> values, OutputCollector<K3,V3> output, Reporter
> reporter)
> ---
> I assume the Crunch use of Iterable is for convenient use in "for" loops.
> Unfortunately, the behavior of this Iterable seems to return the same
> Iterator object each time Iterable.iterator() is called.
> This makes sense to me based on the underlying hadoop mapreduce, but violates
> what I think most expect from the Iterable interface.
> I understand that it's too late to change the interface, but could we at
> least have an javadoc or an exception thrown if the Iterable is used more
> than once?
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