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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Update of "TaskTracker" by SteveLoughran
Date Tue, 05 Aug 2008 09:11:46 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Hadoop Wiki" for change notification.

The following page has been changed by SteveLoughran:

The comment on the change is:
creating a page

New page:
A TaskTracker is a node in the cluster that accepts tasks - Map, Reduce and Shuffle operations
- from a JobTracker.

Every TaskTracker is configured with a set of ''slots'', these indicate the number of tasks
that it can accept. 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

The TaskTracker spawns a separate JVM processes to do the actual work; this is to ensure that
process failure does not take down the task tracker. The TaskTracker monitors these spawned
processes, capturing the output and exit codes. When the process finishes, successfully or
not, the tracker notifies the JobTracker. The TaskTrackers also send out heartbeat messages
to the JobTracker, usually every few minutes, to reassure the JobTracker that it is still
alive. These message 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.

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