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:

sub map {
  my ($self, $key, $value, $context) = @_;
 
  $context->counter($group, $counter, $increment);
}

At the end of computation, Hadoop prints an aggregated value of all counters.

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).

Solution.pl