DATA NODE

3.  DATA NODE

-  DATANODES are the workhorse of the HADOOP CLUSTER.

-  All the Data resides on top of DN.

-  DN can read or write a HDFS file (Actually the file is broken into blocks and the Name  Node will tell your client which Data Node each block resides in).

- Upon initialization, each of the Data Nodes informs the Name Node of the blocks it is currently storing.

- After this mapping is complete, the Data Nodes continually poll the Name Node to provide information regarding local changes as well as receive instructions to create, move, or delete blocks from the local disk.

- Your client communicates directly with the Data Node daemons to process the local files corresponding to the blocks.  

- Data Node may communicate with other Data Nodes to replicate its data blocks for redundancy. 

- The Data Nodes provide backup store of the blocks and constantly report to the Name Node to keep the metadata current. 

- Backup store of the blocks ensures that if any one Data Node crashes or becomes inaccessible over the network, you will still be able to read the files.