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From "Teppo Kurki (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-115) Hadoop should allow the user to use SequentialFileOutputformat as the output format and to choose key/value classes that are different from those for map output.
Date Sat, 01 Apr 2006 21:27:28 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-115?page=comments#action_12372801 ] 

Teppo Kurki commented on HADOOP-115:
------------------------------------

" I.e., the input keys and values are drawn from a different
  domain than the output keys and values. Furthermore,
  the intermediate keys and values are from the same domain
  as the output keys and values. "

The Google MapReduce paper states the Web-Link graph example where this is not the case:

"Reverse Web-Link Graph: The map function outputs
(target; source) pairs for each link to a target
URL found in a page named source. The reduce
function concatenates the list of all source URLs associated
with a given target URL and emits the pair:
(target; list(source))".



> Hadoop should allow the user to use SequentialFileOutputformat as the output format and
to choose  key/value classes that are different from those for map output.
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>          Key: HADOOP-115
>          URL: http://issues.apache.org/jira/browse/HADOOP-115
>      Project: Hadoop
>         Type: Improvement
>   Components: mapred
>     Reporter: Runping Qi

>
> When map tasks write intermediate data out, they always use SequencialFile RecordWriter
with key/value classes from the job object.
> When the reducers write the final results out, its output format is obtained from the
job object. By default, it is TextOutputFormat, and no conflicts.
> However, if one wants to use SequencialFileFormat for the final results, then the key/value
classes are also obtained from the job object, the same as the map tasks' output. Now we have
a problem. It is impossible for the map outputs and reducer outputs use different key/value
classes, if one wants the reducers generate outputs in SequentialFileFormat.
> A simple fix would be to add another two attributes to JobConf class: mapOutputLeyClass
and mapOutputValueClass. That allows the user to have different key/value classes for the
intermediate and final outputs.

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