Assignment 3 - FAQ

Frequently Asked Questions

How do I specify which subset of a key to be used by the partitioner?

• Hadoop Streaming provides an option for you to modify the partitioning strategy
• Here's an example:

hadoop jar /usr/bin/hadoop/contrib/streaming/hadoop-streaming-1.0.3.16.jar -D mapred.reduce.tasks=2 -D stream.num.map.output.key.fields=2 -D num.key.fields.for.partition=2 -file wordMatrix_mapperPairs.py -mapper wordMatrix_mapperPairs.py -file wordMatrix_reducerPairs.py -reducer wordMatrix_reducerPairs.py -partitioner org.apache.hadoop.mapred.lib.KeyFieldBasedPartitioner -input /user/juliana/input -output /user/juliana/output2

• • stream.num.map.output.key.fields=2 informs Hadoop that the first 2 fields of the mapper output form the key -- in this case (word1,word2), and the third field corresponds to the value.
  • num.key.fields.for.partition=2 specifies that both fields are to be used by the partitioner.
  • Note that we also need to specify the partitioner: -partitioner org.apache.hadoop.mapred.lib.KeyFieldBasedPartitioner

• Here's another example, now using only the first field as the key:

hadoop jar /usr/bin/hadoop/contrib/streaming/hadoop-streaming-1.0.3.16.jar -D mapred.reduce.tasks=2 -D stream.num.map.output.key.fields=2 -D num.key.fields.for.partition=1 -file wordMatrix_mapperPairs.py -mapper wordMatrix_mapperPairs.py -file wordMatrix_reducerPairs.py -reducer wordMatrix_reducerPairs.py -partitioner org.apache.hadoop.mapred.lib.KeyFieldBasedPartitioner -input /user/juliana/input -output /user/juliana/output2

• • num.key.fields.for.partition=1 specifies that both fields are to be used by the partitioner.

How do I specify which subset of a key to be used by the partitioner on AWS?

• You can do this when you configure a step: