Hadoop
* Good article on MapReduce http://www.bigsynapse.com/mapreduce-internals * org.apache.hadoop.mapreduce.Mapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT> is generic class. There is an Mapper generic interface too but it is in different package org.apache.hadoop.mapre.Mapper<K1,V1,K2,V2> and has nothing to do with the Mapper class. The Reducer is also a generic class similar to Mapper. * Hadoop also has two special annotations to indicate the audience and stability of any interface or class -InterfaceStability -InterfaceAudience * All the key value types in Hadoop Map Reduce programming must implement Writable interface, which results in type implementing efficient serialization using DataInput/DataOutput. * hadoop fs -put input/sample.txt hdfs://quickstart.cloudera:8020/sample.txt returned "ConnectException" because namenode service was not running. Used following commands to find the status and restart the service sudo service had