Name Node

HDFS uses a master/slave architecture. An HDFS cluster consists of a single NameNode, a master server that manages the file system namespace and regulates access to files by clients. In addition, there are a number of DataNodes, usually one per node in the cluster, which manage storage attached to the nodes that they run on. HDFS exposes a file system namespace and allows user data to be stored in files. Internally, a file is split into one or more blocks and these blocks are stored in a set of DataNodes.

The NameNode is the arbitrator and repository for all HDFS metadata. Data never flows through the NameNode and it is primarily used to execute file system namespace operations like opening, closing, and renaming files and directories. It also determines the mapping of blocks to DataNodes.