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--- courses:mapreduce-tutorial:step-3 [2012/01/24 19:03]
straka vytvořeno
+++ courses:mapreduce-tutorial:step-3 [2012/01/28 11:35]
majlis Added links to previous and next chapter.
@@ Line 1: / Line 1: @@
-====== MapReduce Tutorial : ======
+====== MapReduce Tutorial : Basic mapper ======
+The simplest Hadoop job consists of a mapper only.  The input data is divided in several parts, every processed by an independent mapper, and the results are collected in one directory, one file per mapper.
+The Hadoop framework silently handles failures. If a mapper task fails, another is executed and the input of the failed attempt is discarded.
+===== Example Perl mapper =====
+<file perl>
+#!/usr/bin/perl
+package Mapper;
+use Moose;
+with 'Hadoop::Mapper';
+sub map {
+  my ($self, $key, $value, $context) = @_;
+  $context->write($key, $value);
+}
+package Main;
+use Hadoop::Runner;
+my $runner = Hadoop::Runner->new(
+  mapper => Mapper->new(),
+  input_format => 'TextInputFormat',
+  output_format => 'TextOutputFormat',
+  output_compression => 0);
+$runner->run();
+</file>
+The values ''input_format'', ''output_format'' and ''output_compression'' could be left out, because they are all set to their default value.
+Resulting script can be executed locally in a single thread using
+  perl script.pl run input_directory output_directory
+All files in input_directory are processes. The output_directory must not exist.
+===== Exercise =====
+To check that your Hadoop environment works, try running a MR job on ''/home/straka/wiki/cs-text'', which outputs only articles with names beginning with an ''A'' (ignoring the case). You can download the template {{:courses:mapreduce-tutorial:step-3-exercise.txt|step-3-exercise.pl}}  and execute it.
+  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-3-exercise.txt' -O 'step-3-exercise.pl'
+  rm -rf step-3-out-ex; perl step-3-exercise.pl run /home/straka/wiki/cs-text-medium/ step-3-out-ex
+  less step-3-out-ex/part-m-*
+==== Solution ====
+You can also download the solution {{:courses:mapreduce-tutorial:step-3-solution.txt|step-3-solution.pl}} and check the correct output.
+  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-3-solution.txt' -O 'step-3-solution.pl'
+  rm -rf step-3-out-sol; perl step-3-solution.pl run /home/straka/wiki/cs-text-medium/ step-3-out-sol
+  less step-3-out-sol/part-m-*
+----
+<html>
+<table style="width:100%">
+<tr>
+<td style="text-align:left; width: 33%; "></html>[[step-2|Step 2]]: Input and output format, testing data.<html></td>
+<td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td>
+<td style="text-align:right; width: 33%; "></html>[[step-4|Step 4]]: Counters.<html></td>
+</tr>
+</table>
+</html>

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