CODEWITHSUNDEEP
hadoopdefaultreducemapper
Ronin
Ronin

Reputation: 2077

Default Mapper-Reducer class

Suppose I have two dataset :

hello world
bye world

and 

hello earth
new earth

and I want to run a map-reduce task which does not specify mapper class or reducer class, So the default mapper and reducer will be called - which both are identity function. When I run the job the output is :: 

0       hello world
0       hello earth
12      new earth
12      bye world

I am confused why the key is like 0 and 12 ?! I just used default mapper and reducer as I commented out these lines in the main() ::

//    job.setMapperClass(Map.class);
//    job.setCombinerClass(Reduce.class);
//    job.setReducerClass(Reduce.class);

So, my question is what is the output key is here ? why it looks like 0, 0 , 12, 12 ?

Upvotes: 0

Views: 1980

Answers (1)

Praveen Sripati
Praveen Sripati

Reputation: 33545

0,0,12 and 12 are the file offsets in the input data. In the case of text inputs the K to the mapper is the file offset and the value is the input line. Check this for more information.

Upvotes: 2

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