DataNodes are the workhorses of the Hadoop cluster. Their main function is to store and retrieve data in the form of blocks. They always communicate their status to the NameNode in the form of heartbeats. That's how NameNode keeps track of any DataNodes, whether they are alive or dead. DataNodes keep three copies of the blocks known and the replication factor. DataNodes communicate with other DataNodes to copy data blocks to maintain data replication.
DataNode
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