[CRUNCH-192] Document and enforce the semantics around reducer-based Iterables

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.6.0
• Assignee: Gabriel Reid
• Reporter: Gabriel Reid
• Created: 05/Apr/13 19:39
• Updated: 13/May/13 00:18
• Resolved: 11/Apr/13 14:21

Description

As reported on [e-mail] 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?

Issue Links

• relates to: CRUNCH-194 Utilities and Documentation for handling values in reduce-Iterable