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2012-11-12T11:32:27.743
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Assuming you are using Hadoop Streaming, you need to use the KeyFieldBasedComparator class. <ol> <li>-D mapred.output.key.comparator.class=org.apache.hadoop.mapred.lib.KeyFieldBasedComparator should be added to streaming command</li> <li>You need to provide type of sorting required using mapred.text.key.comparator.options. Some useful ones are -n : numeric sort, -r : reverse sort</li> </ol> EXAMPLE : Create an identity mapper and reducer with the following code This is the mapper.py & reducer.py <pre><code>#!/usr/bin/env python import sys for line in sys.stdin: print "%s" % (line.strip()) </code></pre> This is the input.txt <pre><code>1 11 2 20 7 3 40 </code></pre> This is the Streaming command <pre><code>$HADOOP_HOME/bin/hadoop jar $HADOOP_HOME/hadoop-streaming.jar -D mapred.output.key.comparator.class=org.apache.hadoop.mapred.lib.KeyFieldBasedComparator -D mapred.text.key.comparator.options=-n -input /user/input.txt -output /user/output.txt -file ~/mapper.py -mapper ~/mapper.py -file ~/reducer.py -reducer ~/reducer.py </code></pre> And you will get the required output <pre><code>1 2 3 7 11 20 40 </code></pre> NOTE : <ol> <li>I have used a simple one key input. If however you have multiple keys and/or partitions, you will have to edit mapred.text.key.comparator.options as needed. Since I do not know your use case , my example is limited to this</li> <li>Identity mapper is needed since you will need atleast one mapper for a MR job to run.</li> <li>Identity reducer is needed since shuffle/sort phase will not work if it is a pure map only job.</li> </ol>
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1. COThank you so much for the code example
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