NameNode

The NameNode is responsible for managing the metadata of the HDFS cluster, such as lists of files and folders that exist in a cluster, the number of splits each file is divided into, and their replication and storage at different DataNodes. It also maintains and manages the namespace and file permission of all the files available in the HDFS cluster. Apart from bookkeeping, NameNode also has a supervisory role that keeps a watch on the replication factor of all the files and if some block goes missing, then it issue commands to replicate the missing block of data. It also generates reports to ascertain cluster health. It is important to note that all the communication for a supervisory task happens from DataNode to NameNode; that is, DataNode sends reports (block reports) to NameNode and it is then that NameNode responds to them by issuing different commands or instructions as the need may be.