Lijie Xu created MAPREDUCE-5461: ----------------------------------- Summary: Let users be able to get latest Key in reduce() Key: MAPREDUCE-5461 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5461 Project: Hadoop Map/Reduce Issue Type: Improvement Components: task Affects Versions: 1.2.1 Environment: Any environment Reporter: Lijie Xu
Reducer generates <K, List(V)> for reduce(). In some cases such as SecondarySort, although current V and next V share the same K, their actual corresponding Ks are different. For example, in SecondarySort, map() outputs Key Value <1, 3> 3 <1, 1> 1 <2, 5> 5 <1, 8> 8 After partition by Key.getFirst(), sort and group by Key.getFirst(), reducer gets: Key Value ------Group 1------ <1, 1> 1 <1, 3> 3 <1, 8> 8 ------Group 2------ <2, 5> 5 reduce() receives: Key List<Value> <1, 1> List<1, 3, 8> <2, 5> List<5> When invoking V.next(), we can get next V (e.g, 3). But we do not have API to get its corresponding Key (e.g, <1, 3>). We can only get the first Key (e.g., <1,1>). If we let user be able to get latest key, SecondarySort does not need to emit value in map(). So that the network traffic is better. Another example is Join. If we can get latest Key, we do need to put table label in both key and value. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira