To sum up, ‘Map’ is the step that is performed in parallel, and ‘Reduce’ is the step which
combines the intermediate results produced by the ‘Map’ phase. Each ‘Map’ phase output is
placed into intermediate state for intermediate events. These events are of three types, namely
shufing, sorting, partitioning and combining (overall, we can say ‘Group By’ with respect
to key’s). This intermediate event is handled by MapReduce framework itself and it actually
happened on each key but not on values. This Intermediate event is performed in local
le system of each DataNode, which means that the Map phase output
<key, value> ...
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