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From "Alejandro Abdelnur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3702) add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
Date Thu, 04 Sep 2008 03:33:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628240#action_12628240
] 

Alejandro Abdelnur commented on HADOOP-3702:
--------------------------------------------

Thanks Enis,

On the use of generics in the {{addMapper/setReducer}} which one of the 3 options is the most
appropriate? I would be inclined to think that the third one. My reasoning is that on the
input the mapper/reducer must handle at least a subclass of the key/value classes, and on
the output the mapper/reducer define what the key/value classes are.

*Option 1*
{code}
public static <K1, V1, K2, V2> void addMapper(boolean isMap, JobConf jobConf,
                           Class<? extends Mapper<? extends K1,? extends  V1,? extends
 K2,? extends  V2>> klass,
                           Class<K1> inputKeyClass, Class<V1> inputValueClass,
                           Class<K2> outputKeyClass, Class<V2> outputValueClass,
                           boolean byValue, JobConf mapperConf) {
{code}

*Option 2*
{code}
public static <K1, V1, K2, V2> void addMapper(boolean isMap, JobConf jobConf,
                           Class<? extends Mapper<K1, V1, K2, V2>> klass,
                           Class<? extends K1> inputKeyClass, Class<? extends V1>
inputValueClass,
                           Class<? extends K2> outputKeyClass, Class<? extends V2>
outputValueClass,
                           boolean byValue, JobConf mapperConf) {
{code}
 
*Option 3*
{code}
public static <K1, V1, K2, V2> void addMapper(boolean isMap, JobConf jobConf,
                           Class<? extends Mapper<K1, V1, K2, V2>> klass,
                           Class<? extends K1> inputKeyClass, Class<? extends V1>
inputValueClass,
                           Class<K2> outputKeyClass, Class<V2> outputValueClass,
                           boolean byValue, JobConf mapperConf) {
{code}



> add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
> -------------------------------------------------------------------------
>
>                 Key: HADOOP-3702
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3702
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>         Environment: all
>            Reporter: Alejandro Abdelnur
>            Assignee: Alejandro Abdelnur
>             Fix For: 0.19.0
>
>         Attachments: Hadoop-3702.patch, patch3702.txt, patch3702.txt, patch3702.txt,
patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt,
patch3702.txt, patch3702.txt, patch3702.txt
>
>
> On the same input, we usually need to run multiple Maps one after the other without no
Reduce. We also have to run multiple Maps after the Reduce.
> If all pre-Reduce Maps are chained together and run as a single Map a significant amount
of Disk I/O will be avoided. 
> Similarly all post-Reduce Maps can be chained together and run in the Reduce phase after
the Reduce.

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