How does partitioning in MapReduce exactly work?

Question

I think I have a fair understanding of the MapReduce programming model in general, but even after reading the original paper and some other sources many details are unclear to m

Answer 1

Addendum to what Chris said,

Basically, a partitioner class in Hadoop (e.g. Default HashPartitioner)

has to implement this function,

int getPartition(K key, V value, int numReduceTasks) 

This function is responsible for returning you the partition number and you get the number of reducers you fixed when starting the job from the numReduceTasks variable, as seen for in the HashPartitioner.

Based on what integer the above function return, Hadoop selects node where the reduce task for a particular key should run.

Hope this helps.