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https://issues.apache.org/jira/browse/HADOOP-372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502890
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Runping Qi commented on HADOOP-372:
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This looks good to me.
Should we also have a method to set the default mapper?
One note here is that, it is auumed that all the mapper will genenerate
the map output values of the same class. I think this is due to the constraint
imposed by the SequenceFile.
It would be nice to relax this constrain to only require the output values are
writable.
The framework can serialize the mapout values and tag them with their actual
class.
The value class of the map output files will be simply ByteWritable.
The framework can automatically restore the values to their actual class
before they are passed to the reduce function.
> should allow to specify different inputformat classes for different input
> dirs for Map/Reduce jobs
> --------------------------------------------------------------------------------------------------
>
> Key: HADOOP-372
> URL: https://issues.apache.org/jira/browse/HADOOP-372
> Project: Hadoop
> Issue Type: New Feature
> Components: mapred
> Affects Versions: 0.4.0
> Environment: all
> Reporter: Runping Qi
> Assignee: Owen O'Malley
>
> Right now, the user can specify multiple input directories for a map reduce
> job.
> However, the files under all the directories are assumed to be in the same
> format,
> with the same key/value classes. This proves to be a serious limit in many
> situations.
> Here is an example. Suppose I have three simple tables:
> one has URLs and their rank values (page ranks),
> another has URLs and their classification values,
> and the third one has the URL meta data such as crawl status, last crawl
> time, etc.
> Suppose now I need a job to generate a list of URLs to be crawled next.
> The decision depends on the info in all the three tables.
> Right now, there is no easy way to accomplish this.
> However, this job can be done if the framework allows to specify different
> inputformats for different input dirs.
> Suppose my three tables are in the following directory respectively:
> rankTable, classificationTable. and metaDataTable.
> If we extend JobConf class with the following method (as Owen suggested to
> me):
> addInputPath(aPath, anInputFormatClass, anInputKeyClass,
> anInputValueClass)
> Then I can specify my job as follows:
> addInputPath(rankTable, SequenceFileInputFormat.class, UTF8.class,
> DoubleWritable.class)
> addInputPath(classificationTable, TextInputFormat.class, UTF8,class,
> UTF8.class)
> addInputPath(metaDataTable, SequenceFileInputFormat.class, UTF8.class,
> MyRecord.class)
> If an input directory is added through the current API, it will have the same
> meaning as it is now.
> Thus this extension will not affect any applications that do not need this
> new feature.
> It is relatively easy for the M/R framework to create an appropriate record
> reader for a map task based on the above information.
> And that is the only change needed for supporting this extension.
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