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--- courses:mapreduce-tutorial:step-10 [2012/01/25 18:37]
straka
+++ courses:mapreduce-tutorial:step-10 [2012/01/31 09:38] (current)
straka Change Perl commandline syntax.
@@ Line 1: / Line 1: @@
 ====== MapReduce Tutorial : Combiners ======
+Sometimes the reduce is a binary operation, which is associative and commutative, e.g. ''+''. In that case it is inefficient to produce all the (key, value) pairs in the mappers and send them through the network.
+Instead, reducer can be executed right after the map, on //some portion// of values belonging to the same key. Only the aggregated results are then sent through the network.
+A Hadoop job can have such locally executed reducer, called a //combiner//. If a combiner is specified, the output of a mapper is processed by a combiner before sending the pairs to reducer. The combiner may be invoked 0, 1 or multiple times, usually when the data are written to disk.
+Typically, the combiner is the same as the reducer of a MR job.
+<file perl>
+package My::Mapper;
+...
+package My::Reducer;
+...
+package main;
+use Hadoop::Runner;
+my $runner = Hadoop::Runner->new(
+  mapper => My::Mapper->new(),
+  combiner => My::Reducer->new(), # Specify the combiner.
+  reducer => My::Reducer->new(),
+  input_format => 'KeyValueTextInputFormat');
+...
+</file>
+===== Exercise =====
+Compare the effect of adding the combiner to a MR job which counts occurrences of words in ''/home/straka/wiki/cs-text-medium'': {{:courses:mapreduce-tutorial:step-5-solution1.txt|step-10-wc-without-combiner.pl}} and {{:courses:mapreduce-tutorial:step-10.txt|step-10-wc-with-combiner.pl}}.
+  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-5-solution1.txt' -O 'step-10-wc-without-combiner.pl'
+  # NOW VIEW THE FILE
+  # $EDITOR step-10-wc-without-combiner.pl
+  rm -rf step-10-out-wout; time perl step-10-wc-without-combiner.pl /home/straka/wiki/cs-text-medium/ step-10-out-wout
+  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-10.txt' -O 'step-10-wc-with-combiner.pl'
+  # NOW VIEW THE FILE
+  # $EDITOR step-10-wc-with-combiner.pl
+  rm -rf step-10-out-with; time perl step-10-wc-with-combiner.pl /home/straka/wiki/cs-text-medium/ step-10-out-with
+How would you explain the results?
+----
+<html>
+<table style="width:100%">
+<tr>
+<td style="text-align:left; width: 33%; "></html>[[step-9|Step 9]]: Hadoop properties.<html></td>
+<td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td>
+<td style="text-align:right; width: 33%; "></html>[[step-11|Step 11]]: Initialization and cleanup of MR tasks, performance of combiners.<html></td>
+</tr>
+</table>
+</html>

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