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courses:mapreduce-tutorial:step-4 [2012/01/24 21:10] straka |
courses:mapreduce-tutorial:step-4 [2012/01/24 21:31] straka |
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====== MapReduce Tutorial : Counters ====== | ====== MapReduce Tutorial : Counters ====== | ||
- | Sometimes it is useful to | + | 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-> | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | ===== Exercise ===== | ||
+ | |||
+ | Run a MR job on / | ||
+ | |||
+ | {{.: | ||
- | {{.: | ||
- | {{filelist>: |