An Example of Hadoop MapReduce Counter

MapReduce Counter

Hadoop MapReduce Counter provides a way to measure the progress or the number of operations that occur within MapReduce programs. Basically, MapReduce framework provides a number of built-in counters to measure basic I/O operations, such as FILE_BYTES_READ/WRITTEN
and Map/Combine/Reduce input/output records. These counters are very useful especially when you evaluate some MapReduce programs. Besides, the MapReduce Counter allows users to employ your own counters. Since MapReduce Counters are automatically aggregated
over Map and Reduce phases, it is one of the easiest way to investigate internal behaviors of MapReduce programs. In this post, I’m going to introduce how to use your own MapReduce Counter. The example sources described in this post are based on Hadoop 0.21
API.

Incrementing your counter

For your own MapReduce counter, you first define a enum type
as follow:

public static enum MATCH_COUNTER {
INCOMING_GRAPHS,
PRUNING_BY_NCV,
PRUNING_BY_COUNT,
PRUNING_BY_ISO,
ISOMORPHIC
};

And then, when you want to increment your own counter, you should call the increment method
as follows:

context.getCounter(MATCH_COUNTER.INCOMING_GRAPHS).increment(1);

You can access context instance
within setup, cleanup, map,
and reduce method
in Mapper or Reducer class. You can get a desired counter via calling context.getCounter method
with some enum value.

Finding your counter

You can get some Counters from
a finished job as follows:

Configuration conf = new Configuration();
Cluster cluster = new Cluster(conf);
Job job = Job.getInstance(cluster,conf);
result = job.waitForCompletion(true);
...
Counters counters = job.getCounters();

The instance of Counters class
contains all of the counters obtained from a job. So, when you want to get your own counter, you should call findCounter method
with a enum type
as follows:

Counter c1 = counters.findCounter(MATCH_COUNTER.INCOMING_GRAPHS);
System.out.println(c1.getDisplayName()+":"+c1.getValue());

The below example shows how to get built-in counter groups that Hadoop provides basically.

for (CounterGroup group : counters) {
System.out.println("* Counter Group: " + group.getDisplayName() + " (" + group.getName() + ")");
System.out.println("  number of counters in this group: " + group.size());
for (Counter counter : group) {
System.out.println("  - " + counter.getDisplayName() + ": " + counter.getName() + ": "+counter.getValue());
}
}

原文地址：http://diveintodata.org/2011/03/15/an-example-of-hadoop-mapreduce-counter/