One of the master components, it is responsible for managing the overall execution of a job. It performs functions such as scheduling child tasks (individual Mapper and Reducer) to individual nodes, keeping track of the health of each task and node, and even rescheduling failed tasks. As we will soon demonstrate, like the NameNode, the Job Tracker becomes a bottleneck when it comes to scaling Hadoop to very large clusters. The JobTracker daemon is responsible for launching and monitoring MapReduce jobs.