function of hadoop namenode

The namenode node is very important, it maintains the filesystem tree (filesystem tree) and the metadata of all files and folders in the file tree (metadata)
      a: Manage the namespace of the file system (the most important work) The name, size, metadata of the file, the situation of the file in the node
      b: Record the location and replica information of each file data block in each Datanode
   When a node wants to access a file, it will first access the namenode, obtain the location information of the file, and then communicate directly with the dataNode to obtain the data block, (similar to the role of a directory)
      c: Coordinate client access to files
      d: record changes in the namespace or changes to the properties of the space itself
      e:Namenode uses transaction logs to record changes in HDFS metadata, and uses the namespace of the image file storage system, including file mapping, file attributes, etc.
The transaction log is used to record changes to hdfs metadata
Image file: The namespace where the file system is stored, including the mapping of data blocks