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MapReduce Tutorial : Counters and job configuration

Counters

As in the Perl API, a mapper or a reducer can increment various counters by using context.getCounter(“Group”, “Name”).increment(value):

public void map(Text key, Text value, Context context) throws IOException, InterruptedException {
  ...
  context.getCounter("Group", "Name").increment(value);
  ...
}

The getCounter method returns a Counter object, so if a counter is incremented frequently, the getCounter method can be called only once:

public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
  ...
  Counter values = context.getCounter("Reducer", "Number of values");
  for (IntWritable value : values) {
    ...
    values.increment(1);
  }
}

Job configuration

The job properties can be set:

See the brief list of Hadoop properties.

There is one


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