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From "Daniel Templeton (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-6837) Add an equivalent to Crunch's Pair class
Date Thu, 26 Jan 2017 15:10:24 GMT
Daniel Templeton created MAPREDUCE-6837:
-------------------------------------------

             Summary: 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


Crunch has this great {{Pair}} class (https://crunch.apache.org/apidocs/0.14.0/org/apache/crunch/Pair.html)
that save you 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 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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