What is heartbeat in HDFS?

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Heartbeat is a signal which is used between a data node and name node, and between task tracker and job tracker. If the name node or job tracker doesn’t respond to the signal then it is considered that there is some issue with data node or task tracker.

In Hadoop, the term “heartbeat” typically refers to the mechanism by which DataNodes in the Hadoop Distributed File System (HDFS) communicate with the NameNode to confirm their liveliness and availability.

Here’s how it works:

  1. Heartbeat Messages: DataNodes periodically send heartbeat messages to the NameNode. These messages serve as a way for the DataNodes to inform the NameNode that they are still alive and functioning properly.
  2. Liveness Confirmation: The NameNode uses these heartbeat messages to confirm that the DataNodes are alive. If the NameNode stops receiving heartbeats from a particular DataNode within a specified time frame, it assumes that the DataNode is no longer available or has failed.
  3. Block Report: Along with the heartbeat, DataNodes also send a block report to the NameNode. This report includes information about the blocks stored on that DataNode. This helps the NameNode keep track of the block locations and overall cluster health.
  4. Handling Node Failures: If the NameNode determines that a DataNode has failed (due to a lack of heartbeat), it will initiate block replication to maintain data availability. This involves copying the blocks from the failed DataNode to other healthy DataNodes in the cluster.

In summary, the heartbeat mechanism in HDFS is a crucial aspect of the overall system’s health monitoring, helping to identify and respond to failures in a timely manner.