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Hadoop Interview Questions

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Yes, by using Multiple Outputs class

The hadoop job will throw an exception and exit.

This is a trick question. You cannot set it

You can either do it progamatically by using method setNumReduceTasksin the JobConfclass or set it up as a configuration setting

JobTracker is the daemon service for submitting and tracking MapReduce jobs in Hadoop. There is only One Job Tracker process run on any hadoop cluster. Job Tracker runs on its own JVM process. In a typical production cluster its run on a separate machine. Each slave node is configured with job tracker node location. The JobTracker is single point of failure for the Hadoop MapReduce service. If it goes down, all running jobs are halted. JobTracker in Hadoop performs following actions(from Hadoop Wiki:)

  • Client applications submit jobs to the Job tracker.
  • The JobTracker talks to the NameNode to determine the location of the data
  • The JobTracker locates TaskTracker nodes with available slots at or near the data
  • The JobTracker submits the work to the chosen TaskTracker nodes.
  • The TaskTracker nodes are monitored. If they do not submit heartbeat signals often enough, they are deemed to have failed and the work is scheduled on a different TaskTracker.
  • A TaskTracker will notify the JobTracker when a task fails. The JobTracker decides what to do then: it may resubmit the job elsewhere, it may mark that specific record as something to avoid, and it may may even blacklist the TaskTracker as unreliable.
  • When the work is completed, the JobTracker updates its status.
  • Client applications can poll the JobTracker for information.

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