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courses:mapreduce-tutorial:step-4 [2012/01/24 19:04]
straka vytvořeno
courses:mapreduce-tutorial:step-4 [2012/01/31 09:40] (current)
straka Change Perl commandline syntax.
Line 1: Line 1:
 +====== MapReduce Tutorial : Counters ======
  
 +Sometimes it is useful to count events differently than outputting them as (key, value) pairs. For that reason Hadoop offers simple counter framework.
  
 +Hadoop maintains a collection of pre-defined and user-defined counters. Every counter is identified by its group name and counter name. The group name and counter name is an arbitrary string -- for example group "Documents" and name "Number of words". To increment a counter, the following code can be used:
 +
 +<code perl>
 +sub map {
 +  my ($self, $key, $value, $context) = @_;
 +
 +  $context->counter($group, $counter, $increment);
 +}
 +</code>
 +
 +At the end of computation, Hadoop prints an aggregated value of all the counters.
 +
 +===== Exercise =====
 +
 +Run a Hadoop job on ''/home/straka/wiki/cs-text-medium'', which uses counters to count the number of articles according to their first letter (ignoring the case). You can download the template {{:courses:mapreduce-tutorial:step-4-exercise.txt|step-4-exercise.pl}}  and execute it.
 +  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-4-exercise.txt' -O 'step-4-exercise.pl'
 +  # NOW EDIT THE FILE
 +  # $EDITOR step-4-exercise.pl
 +  rm -rf step-4-out-ex; perl step-4-exercise.pl /home/straka/wiki/cs-text-medium/ step-4-out-ex
 +  less step-4-out-ex/part-*
 +
 +==== Solution ====
 +You can also download the solution {{:courses:mapreduce-tutorial:step-4-solution.txt|step-4-solution.pl}} and check the correct output.
 +  wget --no-check-certificate 'https://wiki.ufal.ms.mff.cuni.cz/_media/courses:mapreduce-tutorial:step-4-solution.txt' -O 'step-4-solution.pl'
 +  # NOW VIEW THE FILE
 +  # $EDITOR step-3-solution.pl
 +  rm -rf step-4-out-sol; perl step-4-solution.pl /home/straka/wiki/cs-text-medium/ step-4-out-sol
 +  less step-4-out-sol/part-*
 +
 +----
 +
 +<html>
 +<table style="width:100%">
 +<tr>
 +<td style="text-align:left; width: 33%; "></html>[[step-3|Step 3]]: Basic mapper.<html></td>
 +<td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td>
 +<td style="text-align:right; width: 33%; "></html>[[step-5|Step 5]]: Basic reducer.<html></td>
 +</tr>
 +</table>
 +</html>

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