[jira] [Created] (MAPREDUCE-6712) Support grouping values for reducer on java-side

["He Tianyi (JIRA)" <ji...@apache.org>]

He Tianyi created MAPREDUCE-6712:
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             Summary: Support grouping values for reducer on java-side
                 Key: MAPREDUCE-6712
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6712
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: contrib/streaming
            Reporter: He Tianyi
            Priority: Minor


In hadoop streaming, with TextInputWriter, reducer program will receive each line representing a (k, v) tuple from {{stdin}}, in which values with identical key is not grouped.
This brings some inefficiency, especially for runtimes based on interpreter (e.g. cpython), coming from:
A. user program has to compare key with previous one (but on java side, records already come to reducer in groups),
B. user program has to perform {{read}}, then {{find}} or {{split}} on each record. even if there are multiple values with identical key,
C. if length of key is large, apparently this introduces inefficiency for caching,

Suppose we need another InputWriter. But this is not enough, since the interface of {{InputWriter}} defined {{writeKey}} and {{writeValue}}, not {{writeValues}}. Though we can compare key in custom InputWriter and group them, but this is also inefficient. Some other changes are also needed.




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