Why we cannot do aggregation (addition) in a mapper? Why we require reducer for that? | Hadoop Questions

We cannot do aggregation (addition) in a mapper because, sorting is not done in a mapper. Sorting happens only on the reducer side. Mapper method initialization depends upon each input split. While doing aggregation, we will lose the value of the previous instance. For each row, a new mapper will get initialized. For each row, input split again gets divided into mapper,  thus we do not have a track of the previous row value.