What is NameNode in Hadoop?

NameNode is the foundation of the HDFS system. It stores all the directory tree of the files in a single file system and keeps track of where the data file is kept. It does not store the data within itself. The NameNode responds to the successful requests by returning the lists of the relevant DataNode servers.

The NameNode is also considered as the single point of failure for the HDFS cluster. The file system goes down when there is a failure of NameNode. The NameNode can be configured to store a single transaction log on a separate disk image.