In Hadoop, the NameNode is a component of the Hadoop Distributed File System (HDFS). It is a central component that manages the file system namespace and regulates access to files by clients.
The NameNode stores metadata about the files and directories in the file system, including file names, permissions, and the locations of data blocks. It also tracks which data nodes store each block of data, and it regulates access to the file system by clients, allowing multiple clients to access and modify the same files simultaneously.
The NameNode is a single point of failure in a Hadoop cluster, which means that if it fails, the entire HDFS cluster becomes unavailable. To address this issue, Hadoop includes a secondary NameNode, which periodically checkpoints the NameNode's metadata and can be used to restore the file system in the event of a NameNode failure.
Overall, the NameNode is a critical component of HDFS that manages the file system namespace and regulates access to files by clients. Its metadata management capabilities enable Hadoop to efficiently store and manage large amounts of data across a distributed cluster of commodity hardware.