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Writing a program in MapReduce follows a certain pattern. You start by writing your map and reduce functions, ideally with unit tests to make sure they do what you expect. Then you write a driver program to run a job, which can run from your IDE using a small subset of the data to check that it is working. If it fails, you can use your IDE’s debugger to find the source of the problem. With this information, you can expand your unit tests to cover this case and improve your mapper or reducer as appropriate to handle such input correctly.


Cover of Hadoop: The Definitive Guide, 4th Edition


this is great stuff, read entire chapter