Monday, 17 August 2015

What is NameNode and what it does?


NameNode is a very important server in Hadoop Distributed File System. It is the master node that keeps track of the metadata , references, sequence, permissions etc of the actual  blocks of data. DataNodes  send signal to the NameNode in every few seconds, so that NameNode can detect that the datanode is working properly. If NameNode don’t get signal from any DataNode then the DataNode is considered as dead. Namenode will decide which part of the data will be stored in which datanode.  It takes care of  replicating data file into blocks and distributing it across the cluster. The Namenode keeps an image of all this information in a local file or preferably on NFS mount. It also maintains a log, called the ‘edit log’, where all the changes are recorded. If NameNode goes down The HDFS goes down.

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