In Module 4 you learned the basics of programming with Hadoop MapReduce. That module explains how data moves through a general MapReduce architecture, and what particular methods and classes facilitate the use of the Hadoop for processing. In this module we will look more closely at how to override Hadoop’s functionality in various ways. These techniques allow you to customize Hadoop for application-specific purposes.

Great tutorial on really getting into Hadoop. Some of the info is old, but its still worth reading.