Write Map Reduce code with Ruby

First is Mapper.

ARGF.each_line do |line|
        line.chomp!
        words = line.split(' ')
        words.each do |word|
                puts "#{word.upcase}\t1"
        end
end

Next is Reducer.

counter = Hash.new {|h,k| h[k] = 0 }

ARGF.each_line do |line|
        line.chomp!
        word, num = line.split(/\t/)
        counter[word] += num.to_i
end

counter.each do |word, counter|
        puts "#{word}\t#{counter}"
end

Finish to write.
Next is test.

$cat input/*
a b c
a a b c c c
$cat input/* | ruby mapper.rb | ruby reducer.rb
A       3
B       2
C       4

Done!

Do with Hadoop Streaming

$hadoop jar /usr/local/hadoop/contrib/streaming/hadoop-streaming-1.1.2.jar -D mapred.child.env='PATH=$PATH:/home/hadoop/.rvm/bin' -input input -output output -mapper 'ruby mapper.rb' -reducer 'ruby reducer.rb' -file mapper.rb -file reducer.rb
packageJobJar: [mapper.rb, reducer.rb, /tmp/hadoop-hadoop/hadoop-unjar3983648244770961497/] [] /tmp/streamjob6573145905468089718.jar tmpDir=null
13/05/28 16:18:00 INFO util.NativeCodeLoader: Loaded the native-hadoop library
13/05/28 16:18:00 WARN snappy.LoadSnappy: Snappy native library not loaded
13/05/28 16:18:00 INFO mapred.FileInputFormat: Total input paths to process : 2
13/05/28 16:18:01 INFO streaming.StreamJob: getLocalDirs(): [/tmp/hadoop-hadoop/mapred/local]
13/05/28 16:18:01 INFO streaming.StreamJob: Running job: job_201305271601_0004
13/05/28 16:18:01 INFO streaming.StreamJob: To kill this job, run:
13/05/28 16:18:01 INFO streaming.StreamJob: /usr/local/hadoop/libexec/../bin/hadoop job  -Dmapred.job.tracker=ec2-54-244-249-227.us-west-2.compute.amazonaws.com:9011 -kill job_201305271601_0004
13/05/28 16:18:01 INFO streaming.StreamJob: Tracking URL: http://ec2-54-244-249-227.us-west-2.compute.amazonaws.com:50030/jobdetails.jsp?jobid=job_201305271601_0004
13/05/28 16:18:02 INFO streaming.StreamJob:  map 0%  reduce 0%
13/05/28 16:18:08 INFO streaming.StreamJob:  map 67%  reduce 0%
13/05/28 16:18:12 INFO streaming.StreamJob:  map 100%  reduce 0%
13/05/28 16:18:17 INFO streaming.StreamJob:  map 100%  reduce 33%
13/05/28 16:18:19 INFO streaming.StreamJob:  map 100%  reduce 100%
13/05/28 16:18:21 INFO streaming.StreamJob: Job complete: job_201305271601_0004
13/05/28 16:18:21 INFO streaming.StreamJob: Output: output
$ hadoop fs -ls output
Found 3 items
-rw-r--r--   3 hadoop supergroup          0 2013-05-28 16:18 /user/hadoop/output/_SUCCESS
drwxr-xr-x   - hadoop supergroup          0 2013-05-28 16:18 /user/hadoop/output/_logs
-rw-r--r--   3 hadoop supergroup         12 2013-05-28 16:18 /user/hadoop/output/part-00000
$hadoop fs -cat output/part-00000
A       3
B       2
C       4

Done!