add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
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Key: HADOOP-3702
URL: https://issues.apache.org/jira/browse/HADOOP-3702
Project: Hadoop Core
Issue Type: New Feature
Components: mapred
Environment: all
Reporter: Alejandro Abdelnur
Assignee: Alejandro Abdelnur
Priority: Minor
On the same input, we usually need to run multiple Maps one after the other
without no Reduce. We also have to run multiple Maps after the Reduce.
If all pre-Reduce Maps are chained together and run as a single Map a
significant amount of Disk I/O will be avoided.
Similarly all post-Reduce Maps can be chained together and run in the Reduce
phase after the Reduce.
This could be done with ChainMapper and ChainReducer classes that would manage
the chain of Maps and they would override the OutputCollector to implement the
chaining.
The Maps and Reduce that are part of the Chain are unware they are executed in
a Chain, they receive records via the {{map}} and {{reduce}} methods and do the
output via the {{OutputCollector}}.
The API would look something like:
{code:java}
public class ChainMapper implements Mapper {
public static void addMapper(JobConf job, Class<? extends Mapper> klass,
Properties mapperConf);
...
}
public class ChainReducer implements Reducer {
public static void setReducer(JobConf job, Class<? extends Reducer> klass,
Properties reducerConf);
public static void addMapper(JobConf job, Class<? extends Mapper> klass,
Properties mapperConf);
...
}
{code}
The {{Properties}} configuration passed to the {{Mapper}} and {{Reducer}} when
setting them into the chain are injected into a copy of the job's
configuration. This allows maps to be configured as usual without being aware
that they are in a chain.
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