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From "Peter Cseh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-6837) Add an equivalent to Crunch's Pair class
Date Wed, 20 Sep 2017 09:59:01 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-6837?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16172967#comment-16172967
] 

Peter Cseh commented on MAPREDUCE-6837:
---------------------------------------

I'd like to pick this up. [~templedf], can you assign to me?

> Add an equivalent to Crunch's Pair class
> ----------------------------------------
>
>                 Key: MAPREDUCE-6837
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6837
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: mrv2
>            Reporter: Daniel Templeton
>              Labels: newbie++
>
> Crunch has this great {{Pair}} class (https://crunch.apache.org/apidocs/0.14.0/org/apache/crunch/Pair.html)
that saves one from constantly implementing composite writables.  It seems silly that we still
don't have an equivalent in MR.
> I would like to see a new class with the following API:
> {code}
> package org.apache.hadoop.io;
> public class CompositeWritable<P extends WritableComparable, S extends WritableComparable>
implements WritableComparable<CompositeWritable> {
>   public CompositeWritable(P primary, S secondary);
>   public P getPrimary();
>   public void setPrimary(P primary);
>   public S getSecondary();
>   public void setSecondary(S secondary);
>   // Return true if both primaries and both secondaries are equal
>   public boolean equals(CompositeWritable o);
>   // Return the primary's hash code
>   public long hashCode();
>   // Sort first by primary and then by secondary
>   public int compareTo(CompositeWritable o);
>   public void readFields(DataInput in);
>   public void write(DataOutput out);
> }
> {code}
> With such a class, implementing a secondary sort would mean just implementing a custom
grouping comparator.  That comparator could also be implemented as part of this JIRA:
> {code}
> package org.apache.hadoop.io;
> public class CompositeGroupingComparator extends WritableComparator {
>   ...
> }
> {code}
> Or some such.
> Crunch also provides {{Tuple3}}, {{Tuple4}}, and {{TupleN}} classes, but I don't think
we need to add equivalents.  If someone really wants that capability, they can nest composite
keys.
> Don't forget to add unit tests!



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