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JobTracker is a daemon which runs on Apache Hadoop's MapReduce engine. JobTracker is an essential service which farms out all MapReduce tasks to the different nodes in the cluster, ideally to those nodes which already contain the data, or at the very least are located in the same rack as nodes containing the data.
JobTracker is the service within Hadoop that is responsible for taking client requests. It assigns them to TaskTrackers on DataNodes where the data required is locally present. If that is not possible, JobTracker tries to assign the tasks to TaskTrackers within the same rack where the data is locally present. If for some reason this also fails, JobTracker assigns the task to a TaskTracker where a replica of the data exists. In Hadoop, data blocks are replicated across DataNodes to ensure redundancy, so that if one node in the cluster fails, the job does not fail as well.