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http://issues.apache.org/jira/browse/HADOOP-372?page=comments#action_12431334 ]
Doug Cutting commented on HADOOP-372:
-------------------------------------
We don't expect most users to directly implement this API, rather this is the
API the system invokes. So the core API should be MapRunnable, and the library
should can provide a standard implementation (MapRunner) whose constructor
takes a Mapper class. Otherwise folks would be unable to use a different
MapRunnable for each task and we'd be back where we started.
I don't think we need to add JobConf methods beyond those to access the
InputFormat. The default InputFormat will remain TextInputHandler, for
back-compatibility, no? We don't want JobConf to keep growing arbitrarily, so
we should move to using static methods on library classes to access properties
specific to those classes. Thus, if one specifies an InputFormat
implementation named MetaInputHandler, then the method to set the record reader
would be MetaInputHandler.setRecordReaderClass(Class).
InputFormatBase will get a new method, getMapRunnable, whose default
implementation is something like:
MapRunnable getMapRunnable(Split) {
return new MapRunner(InputFormatBase.getMapperClass(getConf()));
}
JobConf.getMapperClass() should be thus deprecated and made to call
InputFormatBase.getMapperClass(). So a job might:
job.setInputFormatClass(SequenceFileInputFormat.class);
SequenceFileInputFormat.setMapperClass(MyMapper.class);
> 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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