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courses:mapreduce-tutorial:step-4 [2012/01/24 19:04]
straka vytvořeno 		courses:mapreduce-tutorial:step-4 [2012/01/24 21:33]
straka
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 +====== MapReduce Tutorial : Counters ======
  
 +Sometimes it is useful to count events differently than outputting them (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. 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 counters.
 +
 +===== Exercise =====
 +
 +Run a MR job on /home/straka/wiki/cs-text, which uses counters to count the number of articles according to their first letter (ignoring the case and possible diacritics).
 +
 +{{.:step-4-solution.txt|Solution.pl}}
  

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