Put jars in your code, instead of using libjars. Your jar file gets pushed to every node. Keep your mapreduce code small; don't send large libraries because you run large VMs on nodes.
master list of all java api specs
JobClient and JobConf are the Job Management interfaces. JobClient specifiesy main and args. JobConf sets memory, number of mappers, reducers, etc.
MapReduceBase, Mapper, Reducer.
What mappers and reducers write to
