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From "Josh Wills (JIRA)" <>
Subject [jira] [Commented] (CRUNCH-192) Document and enforce the semantics around reducer-based Iterables
Date Mon, 08 Apr 2013 21:56:17 GMT


Josh Wills commented on CRUNCH-192:

@Micah I think it's fair to say that it's off-topic, but it comes up often enough that we
should come up with some way of providing utilities to support it so that I (and others) don't
keep screwing it up.
> Document and enforce the semantics around reducer-based Iterables
> -----------------------------------------------------------------
>                 Key: CRUNCH-192
>                 URL:
>             Project: Crunch
>          Issue Type: Bug
>            Reporter: Gabriel Reid
>         Attachments: CRUNCH-192.patch
> As reported on 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
> ---
> 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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