This document discusses different techniques for chaining together multiple MapReduce jobs to solve more complex problems in Hadoop - JobClient, JobControl, and ChainMapper. JobClient allows running jobs sequentially by configuring the output of one as the input to the next. JobControl provides dependencies between jobs and manages their execution. ChainMapper chains multiple mappers within a single Map task, reducing disk I/O by passing data between mappers without writing to disk.

1. Chaining

2. Simple idea is sequential call through JobClient
• Not every problem can be solved with a MapReduce
program, but fewer still are those which can be solved
with a single MapReduce job.
• Many problems can be solved with MapReduce, by
writing several MapReduce steps which run in series to
accomplish a goal:
– Map1 -> Reduce1 -> Map2 -> Reduce2 -> Map3...
• You can easily chain jobs together in this fashion by
writing multiple driver methods, one for each job.

3. JobClient in action
• Call the first driver method, which
uses JobClient.runJob() to run the job and wait for it to
complete.
• When that job has completed, then call the next driver
method, which creates a new JobConf object referring
to different instances of Mapper and Reducer, etc.
• The first job in the chain should write its output to a
path which is then used as the input path for the
second job.
• This process can be repeated for as many jobs are
necessary to arrive at a complete solution to the
problem.

4. Jobcontrol
• Hadoop provides another mechanism for managing batches of
jobs with dependencies between jobs.
• Rather than submit a JobConf to the JobClient's
– runJob() or
– submitJob() methods
• org.apache.hadoop.mapred.jobcontrol.Job objects can be
created to represent each job
• A Job takes a JobConf object as its constructor argument.
• Jobs can depend on one another through the use of
the addDependingJob() method.
• The code: x.addDependingJob(y)
• says that Job x cannot start until y has successfully completed.

5. Creating dependency using jobcontrol
• Dependency information cannot be added to a
job after it has already been started.
• Given a set of jobs, these can be passed to an
instance of the JobControl class.
• JobControl can receive individual jobs via
the addJob() method, or a collection of jobs
via addJobs().
• The JobControl object will spawn a thread in
the client to launch the jobs.

6. Jobcontrol special features
• Individual jobs will be launched when their
dependencies have all successfully completed
and when the MapReduce system as a whole
has resources to execute the jobs.
• The JobControl interface allows you to query it
to retrieve the state of individual jobs, as well
as the list of jobs waiting, ready, running, and
finished.
• The job submission process does not begin until
the run() method of the JobControlobject is
called.

7. ChainMapper
• The ChainMapper class allows to use multiple Mapper
classes within a single Map task.
• The Mapper classes are invoked in a chained (or piped)
fashion, the output of the first becomes the input of
the second, and so on until the last Mapper, the
output of the last Mapper will be written to the task's
output.
• The key functionality of this feature is that the
Mappers in the chain do not need to be aware that
they are executed in a chain.
• This enables having reusable specialized Mappers that
can be combined to perform composite operations
within a single task.

8. ChainMapper
• Special care has to be taken when creating chains that
the key/values output by a Mapper are valid for the
following Mapper in the chain.
• It is assumed all Mappers and the Reduce in the chain
use matching output and input key and value classes
as no conversion is done by the chaining code.
• Using the ChainMapper and the ChainReducer classes
is possible to compose Map/Reduce jobs that look
like [MAP+ / REDUCE MAP*].
• And immediate benefit of this pattern is a dramatic
reduction in disk IO.

9. ChainMapper usage pattern
...
Job = new Job(conf);
Configuration mapAConf = new Configuration(false);
...
ChainMapper.addMapper(job, AMap.class, LongWritable.class, Text.class,
Text.class, Text.class, true, mapAConf);
Configuration mapBConf = new Configuration(false);
...
ChainMapper.addMapper(job, BMap.class, Text.class, Text.class,
LongWritable.class, Text.class, false, mapBConf);
...
job.waitForComplettion(true);
...

10. End of session
Day – 2: Chaining