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Tom White commented on MAPREDUCE-3310: -------------------------------------- This seems like a reasonable addition to me. The implementation preserves the current behaviour by default, so it's a safe change. Regarding compatibility, JobContext is an interface but it's marked @Evolving, so adding a new method in the next minor release is permitted. I agree with Joshua's naming suggestions, so it would be good to address those. > Custom grouping comparator cannot be set for Combiners > ------------------------------------------------------ > > Key: MAPREDUCE-3310 > URL: https://issues.apache.org/jira/browse/MAPREDUCE-3310 > Project: Hadoop Map/Reduce > Issue Type: Improvement > Components: client > Affects Versions: 0.20.1 > Environment: All > Reporter: Mathias Herberts > Assignee: Alejandro Abdelnur > Attachments: MAPREDUCE-3310-branch-1.patch, > MAPREDUCE-3310-trunk.patch, MAPREDUCE-3310-trunk.patch, > MAPREDUCE-3310-trunk.patch > > > Combiners are often described as 'Reducers running on the Map side'. > As Reducers, Combiners are fed <K,{V}>, where {V} is built by grouping values > associated with the 'same' key. > For Reducers, the comparator used for grouping values can be set > independently of that used to sort the keys (using > Job.setGroupingComparatorClass). > Such a configuration is not possible for Combiners, meaning some things done > in Reducers cannot be done in Combiners (such as secondary sort). > It would be handy to have a Job.setCombinerGroupingComparatorClass method > that would allow the setting of the grouping comparator used when applying a > Combiner. -- This message was sent by Atlassian JIRA (v6.1.4#6159)