TaskTrackers are MapReduce daemon processes that run on slave nodes. They accept tasks assigned by the JobTracker on the master node and fork JVM processes/threads to run the tasks. TaskTracker is also responsible for reporting the progress of the tasks as well as its health status using heartbeat.
Hadoop maintains three lists for TaskTrackers: blacklist , gray list, and excluded list. TaskTracker black listing is a function that can blacklist a TaskTracker if it is in an unstable state or its performance has been downgraded. For example, when the ratio of failed tasks for a specific job has reached a certain threshold, the TaskTracker will be blacklisted for this job. Similarly, Hadoop maintains a gray list of nodes by identifying potential problematic nodes.
Sometimes, excluding certain TaskTrackers from the cluster is desirable. For example, when we debug or upgrade a slave node, we want to separate this node from the cluster in case it affects the cluster. Hadoop supports...