Batch processing is the most basic type of task that HDFS and MapReduce can perform. Similar to the data parallel algorithms in Chapter 8, Multiplying Performance with Parallel Computing, the master node sends a set of instructions to the worker nodes, which execute the instructions on the blocks of data stored on them. The results are then written to the disk in HDFS.

When an aggregate result is required, both the map and reduce steps are performed on the data. For example, in order to compute the mean of a distributed dataset, the mappers on the worker nodes first compute the sum and number of elements in each local chunk of data. The reducers then add up all these results to compute the global mean.

At other times, only the map step is performed when aggregation is not required. This is common in data transformation or cleaning operations where the data is simply being transformed form one format to another. One example of this is extracting...