seniorHadoop
What is HDFS block report and heartbeat mechanism?
Updated May 16, 2026
Short answer
DataNodes send heartbeats and block reports to NameNode for health and metadata synchronization.
Deep explanation
Heartbeats are frequent signals sent by DataNodes to confirm they are alive. Block reports are less frequent and contain full lists of stored blocks. NameNode uses this information to manage replication, detect failures, and maintain cluster integrity.
Real-world example
Detecting failed storage servers in a large distributed cluster.
Common mistakes
- Assuming block reports are sent frequently like heartbeats.
Follow-up questions
- What happens if heartbeat is missed?
- Why block reports are expensive?