How Namenode handles data node failures?

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I am working on a Hadoop cluster and in my cluster I've 5 datanodes but one of my data node gets down. Now my question is how Namenode handles the datanode failures?

Can someone please explain?

Thanks in advance!
Jul 11, 2018 in Big Data Hadoop by Shubham
• 13,310 points
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1 answer to this question.

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Let me explain you the whole scenario.

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. 
 
NoteIf the Name Node stops receiving heartbeats from a Data Node it presumes it to be dead and any data it had to be gone as well. Based on the block reports it had been receiving from the dead node, the Name Node knows which copies of blocks died along with the node and can make the decision to re-replicate those blocks to other Data Nodes.It will also consult the Rack Awareness data in order to maintain the two copies in one rack, one copy in another rack replica rule when deciding which Data Node should receive a new copy of the blocks.

image

Hope this will help you.

answered Jul 11, 2018 by nitinrawat895
• 10,730 points

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