Hadoop in Action Note

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MapReduce vs. Relational DataBase

scale out vs. scale up
key/value pairs vs. relational tables
functional programming vs. declarative query
offline batch processing vs. online transactions(MapReduce is more like warehouse)

New API vs. Old API

start from 0.20
classes moved from org.apache.haddop.mapred(.lib) toorg.apache.haddop.mapreduce(.lib)
introduction of context object to unitify communication with framework
- with this way, map/reduce need not to be changed for further feature addition
- only some new method need to add to context object
JobClient and JobConf replaced by Configuration and Job class
- Configuration class configures a job
- Job class defines and controls execution of job