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http://issues.apache.org/jira/browse/HADOOP-372?page=comments#action_12430087 ]
Doug Cutting commented on HADOOP-372:
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> job.addInputPath(myInputDir, MyInputFormat.class, MyMapper.class, numMaps);
Why can't this simply be
job.addInputPath(myInputDir, MyInputFormat.class);
and let the mapper be determined by the inputformat?
I also don't think that users should typically be specifying the number of map
tasks. Their code should run unchanged on clusters of various sizes. The
number of map tasks should thus be determined dynamically when splitting.
I also think the above should simply be syntactic sugar for:
job.setInputFormat(MyInputFormat.class)
job.set("my.input.dirs", "foo,bar");
job.set("my.mappers", "FooMapper,BarMapper");
Then implement MyInputFormat to process my.input.dirs and my.mappers.
In other words, we should foremost first make it possible to do this in user
code, then perhaps provide some utilities that simplify things. So the first
patch I'd like to see is mappers from record readers. Then we can write some
InputFormats that use these new features and perhaps promote some of them into
the standard public APIs. Does this make sense? Am I missing something?
> should allow to specify different inputformat classes for different input
> dirs for Map/Reduce jobs
> --------------------------------------------------------------------------------------------------
>
> Key: HADOOP-372
> URL: http://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
> Assigned To: 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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