Hadoop Interview Questions

1. What is the responsibility of NameNode in HDFS?

  - NameNode is a master daemon for creating metadata for blocks, stored on Datanodes.

  - Every DataNode sends heartbeat and block report to NameNode. 

  - If NameNode not recieves any heartbeat then its simply identifies that the DataNode is Dead. This NamaNode is the single Point of Failover. 

  - If NameNode goes down Entire HDFS Cluster is Inaccessiable. 

2. How NameNode Handles Data Node Failures? 

  - NameNode periodically receives a Heartbeat and a Blockreport from each of the DataNode in the cluster. Receipt of the Heartbeat implies that the DataNode is functioning properly ..

  - When NameNode notices that it has not received a heartbeat message (typically every 10 sec ) from a DataNode after a certain amount of time, that DataNode is identified as dead.

  - Since blocks will be under replicated the system NameNode begins replicating the blocks that were stored on the dead DataNode.

  - The NameNode takes responsibility of the replication of the data blocks from one DataNode to another. The Replication data transfer happens directly between DataNodes and the data never passes thorugh the NameNode. 

3. What is Fault Tolerance?

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