Map. Reduce 與<Key, Value>關係 Input Data Output Key 1 val Key 2 val … … Map Input Select key Key 1 Output val val … Key values Reducer 7
Map. Reduce: High Level 8
什麼是Map. Reduce的運作原理 Map. Reduce的特性 9
Wordcount範例 Input Mapper Cat 1 Dog 1 Cat Dog Pig FIsh Mapper Output Merge Cat 1 Pig 1 Cat [1 1] Pig [1 1] Reducer Cat 2 Pig 2 Dog FIsh 1 Dog[1] Fish [1 1] Reducer Dog 1 FIsh 2 Pig 1 FIsh 1 Cat Pig FIsh Sort/Copy Cat 1 Pig 1 11
Mapper流程 Hi Cloud say Hi (); Input Key xt) Cloud say Hi Te … … Hi Cloud say Hi … … Hi (( /user/hadoop/input/hi va lue ). t o. S tri ng String. Tokenizer itr = new String. Tokenizer( line); itr itr itr while (itr. has. More. Tokens()) { word. set(itr. next. Token()); context. write(word, one); } Input Value <word, one> <Hi, 1> <Cloud, 1> <say, 1> <Hi, 1> 12