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From "Benjamin Reed (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 Mon, 28 Aug 2006 15:39:24 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-372?page=comments#action_12430992 ] 
            
Benjamin Reed commented on HADOOP-372:
--------------------------------------

We have a desperate need to be able to specify different inputformat classes, mappers, and
partition functions in the same job. Our need roughly corresponds to this issue. However,
we do not assume that a given input will need to be processed by only one inputformat, mapper,
or partition function.

I think Owen was right about directing the discussion towards HADOOP-451. If HADOOP-451 was
fixed, we (meaning my project) would not have any issues in this area. We actually use a similar
syntax to the one proposed by Michel: job.set("DispatchInputFormat.inputdirmap", "foo=org.example.FooInput
bar=org.example.BarInput"), but imagine if we get job.set("DispatchInputFormat.inputdirmap",
"foo=org.example.FooInput foo=org.example.BarInput"), now foo must be processed by both FooInput
and BarInput. When the splits are created and sent to the mappers, the DispatchInputFormat
will get {"foo", offset, length}, but it has no way of knowing wether to apply FooInput or
BarInput. With HADOOP-451 fixed, we could encode the InputFormat to use in the split.

> 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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