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https://issues.apache.org/jira/browse/AVRO-581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12890307#action_12890307
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Tom White commented on AVRO-581:
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A few more comments:
* It would be nice to have a unit test for map-only jobs, since they exercise a
different code path, and the output doesn't need to be a pair. For a map-only
job I get a NPE if I don't call AvroJob.setOutputSchema() - calling
AvroJob.setMapOutputSchema() was not sufficient.
* A test using Specific types would be good, to test that it works properly
with pairs.
* Should configureAvroJob() only set the input format if it hasn't already been
set? E.g. if I set AvroUtf8InputFormat before calling setInputSchema then it
will be silently changed to AvroInputFormat.
* Generating a Pair schema could be made more elegant for users by exposing
Pair#getPairSchema() as a static method.
* Not related to this patch, but it would be nice if generated specific classes
had a copy constructor, and/or a copyInto() method, since it's a common pattern
in reducers to save the state of an object since the objects are reused in the
values iterator.
> java: add reducer that separates keys and values when map output is pairs
> -------------------------------------------------------------------------
>
> Key: AVRO-581
> URL: https://issues.apache.org/jira/browse/AVRO-581
> Project: Avro
> Issue Type: New Feature
> Components: java
> Reporter: Doug Cutting
> Assignee: Doug Cutting
> Fix For: 1.4.0
>
> Attachments: AVRO-581.patch, AVRO-581.patch, AVRO-581.patch
>
>
> We should add a Pair<K,V> class, implementing SpecificRecord, that combines
> instances of two schemas (specific or generic). Pairs would be compared by
> key, ignoring value. The template for its schema would be:
> {code}
> {"type": "record", "name": "org.apache.avro.mapred.Pair", "fields":[
> {"name": "key", "type":" <<insert key schema here>>},
> {"name": "value", "order": "ignore", "type": <<insert value schema>>}
> ]}
> {code}
> When map outputs are instances of this class, a reducer may be used whose
> reduce method is something like:
> public abstract void reduce(K key, Iterable<V> values);
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