Customizing Input and Output in Hadoop

Hadoop allows you to modify the way data is loaded on disk in two major ways: configuring how contiguous chunks of input are generated from blocks in HDFS (or maybe more exotic sources), and configuring how records appear in the map phase. The two classes you’ll be playing with to do this are RecordReader and InputFormat. These work with the Hadoop MapReduce framework in a very similar way to how mappers and reducers are plugged in.

Hadoop also allows you to modify the way data is stored in an analogous way: with an OutputFormat and a RecordWriter.