Namenode v s Secondary namenode

Question

Hadoop is Consistent and partition tolerant, i.e. It falls under the CP category of the CAP theoram.

Hadoop is not available because all the nodes are dependent on the name node. If the name node falls the cluster goes down.

But considering the fact that the HDFS cluster has a secondary name node why cant we call hadoop as available. If the name node is down the secondary name node can be used for the writes.

What is the major difference between name node and secondary name node that makes hadoop unavailable.

Thanks in advance.

Answer 1

Name Node is a primary node in which all the metadata is stored into fsimage and edit log files periodically. But, when name node down secondary node will be online but this node only has the read access to the fsimage and edit log files and don't have the write access to them. All the secondary node operations will be stored to the temp folder. when name node back to online this temp folder will be copied to name node and the namenode will update the fsimage and edit log files.

Comments

Good Answer Ravikiran..Also could you please let us know what is the temp folder name which the fs image and editlogs of secondary namenode will save .SPOF is overcomed by High availability is that accurate?

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The name of the directory where you will find fs image and editlogs will be in the directory mentioned to the dfs.name.dir option in the hdfs-site.xml file. 

Yes, SPOF can be solved by High Availability. You can read more on High Availability here: https://www.edureka.co/blog/how-to-set-up-hadoop-cluster-with-hdfs-high-availability/