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MapReduce >> mail # user >> Sorting Values sent to reducer NOT based on KEY (Depending on part of VALUE)


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Vikas Jadhav 2013-04-23, 05:44
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Kai Voigt 2013-04-23, 05:54
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Re: Sorting Values sent to reducer NOT based on KEY (Depending on part of VALUE)
Hello,

Sorting is done by the SortingComparator which performs sorting based on the value of key. A possible solution would be the following:
You could write a custom Writable comparable class which extends WritableComparable (lets call it MyCompositeFieldWritableComparable), that will store your current key and the part of the value that you want your sorting to be based on. As I understand from your description, this writable class will have 2 IntWritable fields, e.g
(FieldA, fieldB)

(0,4)
(1,1)
(2,0)
Implement the methods equals, sort, hashCode, etc in your custom writable to override the defaults. Sorting before the reduce phase will be performed based on the compareTo() implementation of your custom writable, so you can write it in a way that will compare only fieldB.

Be careful in the way you will implement methods MyCompositeFieldWritableComparable.equals() -it will be used to group <key, list(values)> in the reducer-, MyCompositeFieldWritableComparable.compareTo() and MyCompositeFieldWritableComparable.hashCode().
So your new KEY class will be MyCompositeFieldWritableComparable.
As an alternative and cleaner implementation, write the MyCompositeFieldWritableComparable class and also a HashOnOneFieldPartitioner class (which extends Partitioner) that will do something like this:

@Override

public int getPartition(K key, V value,
                          int numReduceTasks) {
    if (key instanceof MyCompositeFieldWritableComparable)
         return ( ((MyCompositeFieldWritableComparable) key).hashCodeBasedOnFieldB() & Integer.MAX_VALUE) % numReduceTasks;
    else
        return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;
  }
You can also find related articles in the web, eg http://riccomini.name/posts/hadoop/2009-11-13-sort-reducer-input-value-hadoop/.

Have a nice day,
Sofia
>________________________________
> From: Vikas Jadhav <[EMAIL PROTECTED]>
>To: [EMAIL PROTECTED]
>Sent: Tuesday, April 23, 2013 8:44 AM
>Subject: Sorting Values sent to reducer NOT based on KEY (Depending on part of VALUE)
>
>
>
>Hi

>how to sort value in hadoop using standard sorting algorithm of hadoop ( i.e sorting facility provided by hadoop)

>Requirement:

>1) Values shoulde be sorted depending on some part of value

>For Exam     (KEY,VALUE)

> (0,"BC,4,XY')
> (1,"DC,1,PQ")
> (2,"EF,0,MN")

>Sorted sequence @ reduce reached should be

>(2,"EF,0,MN")
>(1,"DC,1,PQ")
>(0,"BC,4,XY')

>Here sorted depending on second attribute postion in value.

>Thanks

>
>--
>  Regards,
>   Vikas
>
>