Post by Admin on Mar 14, 2014 6:56:49 GMT
Ans: The TaskTrackers send out heartbeat messages to the JobTracker, usually every few minutes, to reassure the JobTracker that it is still alive. These
messages also inform the JobTracker of the number of available slots, so the JobTracker can stay up to date with where in the cluster work can be delegated.
When the JobTracker tries to find somewhere to schedule a task within the MapReduce operations, it first looks for an empty slot on the same server that
hosts the DataNode containing the data, and if not, it looks for an empty slot on a machine in the same rack.
messages also inform the JobTracker of the number of available slots, so the JobTracker can stay up to date with where in the cluster work can be delegated.
When the JobTracker tries to find somewhere to schedule a task within the MapReduce operations, it first looks for an empty slot on the same server that
hosts the DataNode containing the data, and if not, it looks for an empty slot on a machine in the same rack.