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courses:mapreduce-tutorial:step-5 [2012/01/24 22:15] straka |
courses:mapreduce-tutorial:step-5 [2012/01/29 21:30] straka |
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====== MapReduce Tutorial : Basic reducer ====== | ====== MapReduce Tutorial : Basic reducer ====== | ||
- | The interesting part of a MR job is the reducer -- after all mappers produce the (key, value) pairs, for every unique key and all its values a '' | + | The interesting part of a Hadoop |
- | The '' | + | The '' |
- | <file perl reducer.pl> | + | <file perl> |
package Mapper; | package Mapper; | ||
use Moose; | use Moose; | ||
Line 38: | Line 38: | ||
</ | </ | ||
+ | As before, Hadoop silently handles failures. It can happen that even a successfully finished mapper needs to be executed again -- if the machine, where its output data were stored, gets disconnected from the network. | ||
+ | |||
+ | ===== Types of keys and values ===== | ||
+ | |||
+ | Currently in the Perl API, the keys and values are both strings, which are stored and loaded using UTF-8 format. If you need more complex structures, you have to serialize and deserialize them by yourselves. | ||
+ | |||
+ | The Java API offers a wide range of types, including user-defined types, to be used for keys and values. | ||
+ | |||
+ | ===== Exercise 1 ===== | ||
+ | |||
+ | Run a Hadoop job on ''/ | ||
+ | wget --no-check-certificate ' | ||
+ | rm -rf step-5-out-ex1; | ||
+ | less step-5-out-ex1/ | ||
+ | |||
+ | ==== Solution ==== | ||
+ | You can also download the solution {{: | ||
+ | wget --no-check-certificate ' | ||
+ | rm -rf step-5-out-sol1; | ||
+ | less step-5-out-sol1/ | ||
+ | |||
+ | |||
+ | ===== Exercise 2 ===== | ||
+ | |||
+ | Run a Hadoop job on ''/ | ||
+ | wget --no-check-certificate ' | ||
+ | rm -rf step-5-out-ex2; | ||
+ | less step-5-out-ex2/ | ||
+ | |||
+ | ==== Solution ==== | ||
+ | You can also download the solution {{: | ||
+ | wget --no-check-certificate ' | ||
+ | rm -rf step-5-out-sol2; | ||
+ | less step-5-out-sol2/ | ||
+ | |||
+ | ---- | ||
+ | |||
+ | < | ||
+ | <table style=" | ||
+ | <tr> | ||
+ | <td style=" | ||
+ | <td style=" | ||
+ | <td style=" | ||
+ | </tr> | ||
+ | </ | ||
+ | </ |