How does the NameNode communicate with the DataNode?

1.	How does the NameNode communicate with the DataNode?
Answer» NameNode communicates and gets INFORMATION from DataNode via MESSAGES or signals. There are TWO types of messages/signals that are USED for this communication across the channel: Block report signals: These are the list of all HDFS data blocks stored on DataNode. They correspond to all local files and send this report to NameNode. Heartbeat signals: These signals sent between DataNode and NameNode are taken as sign of vitality. They are used to check whether the DataNode is alive and functional. It acts as a periodic report to check whether to use NameNode or not. If this signal is not sent, it implies DataNode has some technical issues or health issues and it has stopped working. The default heartbeat signal is 3 seconds.

Answer»

NameNode communicates and gets INFORMATION from DataNode via MESSAGES or signals.

There are TWO types of messages/signals that are USED for this communication across the channel:

Block report signals: These are the list of all HDFS data blocks stored on DataNode. They correspond to all local files and send this report to NameNode.
Heartbeat signals: These signals sent between DataNode and NameNode are taken as sign of vitality. They are used to check whether the DataNode is alive and functional. It acts as a periodic report to check whether to use NameNode or not. If this signal is not sent, it implies DataNode has some technical issues or health issues and it has stopped working. The default heartbeat signal is 3 seconds.

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