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, by adding the appropriate directives and parameters in the Arguments box:
