The Task Tracker is a service daemon. It Runs on individual DataNodes. It is responsible for starting and managing individual Map/Reduce tasks. It communicates with the JobTracker. It runs on each compute node of the Hadoop cluster, accepts requests for individual tasks such as Map, Reduce, and Shuffle operations. which are present on every node of the cluster. The actual execution of the tasks is controlled by TaskTrackers. It is responsible to start map jobs.
Each TaskTracker is configured with a set of slots that is usually set up as the total number of cores available on the machine. When a request is received from the JobTracker to launch a task, the TaskTracker initiates a new JVM for the task. The TaskTracker is assigned a task depending on how many free slots it has (total number of tasks = actual tasks running).
The TaskTracker is responsible for sending heartbeat messages to the JobTracker. Apart from telling the JobTracker that it is healthy, these messages also tell the JobTracker about the number of available free slots.