Daniel Templeton created MAPREDUCE-6837:
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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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