On Apr 30, 2007, at 7:11 AM, Tahir Hashmi (JIRA) wrote:
[ https://issues.apache.org/jira/browse/HADOOP-485?
page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Tahir Hashmi updated HADOOP-485:
--------------------------------
Attachment: 485.patch
Added javadoc documentation for get and set methods in JobConf.
It would be good to document what happens if null is passed into the
setter method and what happens if the getter method is called when no
comparator has been set. Also, if the
"mapred.output.value.groupfn.class" is intended to be a public
property, then I think the javadoc should mention that these methods
control the value of this property.
allow a different comparator for grouping keys in calls to reduce
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Key: HADOOP-485
URL: https://issues.apache.org/jira/browse/HADOOP-485
Project: Hadoop
Issue Type: New Feature
Components: mapred
Affects Versions: 0.5.0
Reporter: Owen O'Malley
Assigned To: Tahir Hashmi
Attachments: 485.patch, 485.patch, 485.patch, Hadoop-485-
pre.patch, TestUserValueGrouping.java.patch
Some algorithms require that the values to the reduce be sorted in
a particular order, but extending the key with the additional
fields causes them to be handled by different calls to reduce.
(The user then collects the values until they detect a "real" key
change and then processes them.)
It would be much easier if the framework let you define a second
comparator that did the grouping of values for reduces. So your
reduce inputs look like:
A1, V1
A2, V2
A3, V3
B1, V4
B2, V5
instead of getting calls to reduce that look like:
reduce(A1, {V1}); reduce(A2, {V2}); reduce(A3, {V3}); reduce(B1,
{V4}); reduce(B2, {V5});
you could define the grouping comparator to just compare the
letters and end up with:
reduce(A1, {V1,V2,V3}); reduce(B1, {V4,V5});
which is the desired outcome. Note that this assumes that the
"extra" part of the key is just for sorting because the reduce
will only see the first representative of each equivalence class.
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