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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-372) should allow to specify different inputformat classes for different input dirs for Map/Reduce jobs
Date Thu, 24 Aug 2006 06:49:05 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-372?page=comments#action_12430107 ] 
            
Doug Cutting commented on HADOOP-372:
-------------------------------------

> A very typical case is to have the same input format, but different Mappers

But, if the mapper is a function of the input format this can instead be:

job.addInputPath("foo", FooInput.class);
job.addInputPath("bar", BarInput.class);

Where FooInput is defined with something like:

public class FooInput extends TextInput {
  public void map(...) { ... };
}

In other words, if you're going to define custom mappers anyway, then it's no more work to
define custom Input formats.

> 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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