A good understanding of Hadoop Architecture is required to understand and leverage the power of Hadoop. Here are few important practical questions which can be asked in a Hadoop Developer interview. This list primarily includes questions related to Hadoop Architecture, MapReduce, Hadoop API and HDFS. What is a JobTracker in Hadoop? How many instances of JobTracker run on a Hadoop Cluster?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.
How JobTracker schedules a task?Read more: fromDev
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