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--- courses:mapreduce-tutorial:step-4 [2012/01/24 21:01]
straka
+++ courses:mapreduce-tutorial:step-4 [2012/01/30 15:47]
straka
@@ Line 1: / Line 1: @@
 ====== MapReduce Tutorial : Counters ======
-Sometimes it is useful to
+Sometimes it is useful to count events differently than outputting them as (key, value) pairs. For that reason Hadoop offers simple counter framework.
-{{filename>:snippets:dokuwiki:*filelist*}}
+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:
-{{filelist>:snippets:dokuwiki:plugin-*.tar.gz&style=table&tableheader=1&tableshowdate=1&tableshowsize=1}}
+<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 run /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 run /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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