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HADOOP - How NameNode Handles data node failures?




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asked SRVMTrainings February 11, 2014 05:24 AM  

How NameNode Handles data node failures?


           

3 Answers



 
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NameNode periodically receives a Heartbeat and a Blockreport from each of the DataNodes in the cluster. Receipt of a Heartbeat implies that the DataNode is functioning properly. A Blockreport contains a list of all blocks on a DataNode. When NameNode notices that it has not recieved a hearbeat message from a data node after a certain amount of time, the data node is marked as dead. Since blocks will be under replicated the system begins replicating the blocks that were stored on the dead datanode. The NameNode Orchestrates the replication of data blocks from one datanode to another. The replication data transfer happens directly between datanodes and the data never passes through the namenode.

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Data node passes a heartbeat signal to Name node in an interval of specified time (Usually 2 minutes), which helps the Name node to determine that the data node is alive & functional.

When Name node does not receive heartbeat signals from Data node, it assumes that the data node is either dead or non-functional.

As soon as the data node is declared dead/non-functional all the data blocks it hosts are transferred to the other data nodes with which the blocks are replicated initially.

In this process of transferring the data blocks from dead name node to other data nodes, Name node is not involved.
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Through checksums. every data has a record followed by a checksum. if checksum doesnot match with the original then it reports an data corrupted error
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