Job Tracker :
- The JobTracker is the service within Hadoop that farms out MapReduce tasks to specific nodes in the cluster, ideally the nodes that have the data, or at least are in the same rack.
- The JobTracker will communicate with NameNode to determine the location of the data ( as mention in my below post –> https://sensaran.wordpress.com/2015/11/13/hadoop-architecture-and-components/ )
- The JobTracker locates TaskTracker nodes with available slots at or near the data.
- The JobTracker submits the work to the chosen TaskTracker nodes.and monitored how i’s worked.
- If JobTracker 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.
- TaskTrackers which run on DataNodes; TaskTrackers run the tasks and report the status of task to JobTracker.
- The JobTracker runs on MasterNode aka NameNode whereas TaskTrackers run on DataNodes
- Mapper and Reducer tasks are executed on DataNodes administered by TaskTrackers.
- TaskTrackers will be assigned Mapper and Reducer tasks to execute by JobTracker.
- TaskTracker will be in constant communication with the JobTracker signalling the progress of the task in execution