ETL with Hadoop - Big Data Made Easy: A Working Guide to the Complete Hadoop Toolset - page 297

Figure 10-11. Steps in reducer transformation
Figure 10-12. Sort Rows step of reducer transformation
Figure 10-13 shows the Group By step, which groups the data by the key value and sums it by the numeric value
to create a new summed value variable called summed_val. The aggregate value is defined as the sum function
working on the value variable.