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https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628600#action_12628600
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Enis Soztutar commented on HADOOP-3702:
---------------------------------------
Ok, I confused option 3 as a derivative of 1, which clearly is a derivative of
2.
However the output classes can indeed be more specific than declared. An
example of this is a mapper which does not change the input key, and directly
outputs it.
The following code fragment, demonstrates that we should go with the option 2.
{code}
static class A {
}
static class B extends A {
}
static class AMapper extends MapReduceBase implements Mapper<A, Text, A,
Text> {
@Override
public void map(A key, Text value, OutputCollector<A, Text> output,
Reporter reporter) throws IOException {
}
}
static class BMapper extends MapReduceBase implements Mapper<B, Text, B,
Text> {
@Override
public void map(B key, Text value, OutputCollector<B, Text> output,
Reporter reporter) throws IOException {
}
}
static {
JobConf job = new JobConf();
addMapper(true, job, AMapper.class, A.class, Text.class, A.class,
Text.class, false, job);
addMapper(true, job, AMapper.class, B.class, Text.class, B.class,
Text.class, false, job);
addMapper(true, job, BMapper.class, A.class, Text.class, A.class,
Text.class, false, job);//should fail
addMapper(true, job, BMapper.class, B.class, Text.class, B.class,
Text.class, false, job);
}
{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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