What is a Heartbeat in HDFS?
Heartbeat is the signals that NameNode receives from the DataNodes to show that it is functioning (alive). NameNode and DataNode do communicate using Heartbeat. If after the certain time of heartbeat NameNode do not receive any response from DataNode, then that Node is dead. The NameNode then schedules the creation of new replicas of those blocks on other DataNodes.
Heartbeats from a Datanode also carry information about total storage capacity. Also, carry the fraction of storage in use, and the number of data transfers currently in progress.
The default heartbeat interval is 3 seconds. One can change it by using dfs.heartbeat.interval in hdfs-site.xml.
