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courses:mapreduce-tutorial:step-8 [2012/01/25 15:00] straka |
courses:mapreduce-tutorial:step-8 [2012/01/25 15:54] straka |
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To use multiple reducers, the MR job must be executed by a cluster (even with one computer), not locally. The number of reducers is specified by '' | To use multiple reducers, the MR job must be executed by a cluster (even with one computer), not locally. The number of reducers is specified by '' | ||
- | perl script.pl [-j cluster_master | -c cluster_size [-w sec_to_wait]] [-r number_of_reducers] | + | perl script.pl |
==== Partitioning ==== | ==== Partitioning ==== | ||
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By default, (key, value) pair is sent to reducer number //hash(key) modulo number_of_reducers// | By default, (key, value) pair is sent to reducer number //hash(key) modulo number_of_reducers// | ||
- | To override the default behaviour, MR job can specify a // | + | To override the default behaviour, MR job can specify a // |
+ | <code perl> | ||
+ | package Partitioner; | ||
+ | use Moose; | ||
+ | with ' | ||
+ | |||
+ | sub getPartition { | ||
+ | my ($self, $key, $value, $partitions) = @_; | ||
+ | |||
+ | return $key % $partitions; | ||
+ | } | ||
+ | |||
+ | ... | ||
+ | package Main; | ||
+ | use Hadoop:: | ||
+ | |||
+ | my $runner = Hadoop:: | ||
+ | ... | ||
+ | partitioner => Partitioner-> | ||
+ | ...); | ||
+ | ... | ||
+ | </ | ||
+ | |||
+ | A MR job must have a reducer if it specifies a partitioner. Also, the partitioner is not called if there is only one reducer. | ||
+ | |||
+ | ===== Example ===== | ||
+ | |||
+ | Run a MR job on '/ | ||
+ | |||
+ | {{: |