NameNode- It is also known as Master node. It maintains the file SYSTEM tree and the metadata for all the files and directories present in the system. NameNode is a very highly available server that manages the File System Namespace and controls access to files by clients. It records the metadata of all the files stored in the cluster i.e. location of blocks stored, size of the files, HIERARCHY,permissions ETC .

NameNode is the master daemon that manages and maintains all the DataNodes (slave nodes).

There are two files associated with the metadata:

• FsImage: It is the snapshot of the file system when Name Node is started.
• EditLogs: It is the sequence of changes made to the file system after the Name Node is started.

Checkpoint node- Checkpoint node is the new implementation of Secondary NameNode . It is used to create periodic checkpoints of file system metadata by merging edits file with fsimage file and finally it uploads the new image back to the active NameNode 

It is structured in the same directory as the NameNode and stores the latest checkpoint .

Backup Node - Backup Node is an extended checkpoint node that performs checkpointing and also supports online streaming of file system edits.

Its main role is to act as the dynamic Backup for the Filesystem Namespace (Metadata )in the Primary Namenode of the Hadoop Ecosystem.

The Backup node keeps an in-memory, up-to-date copy of the file system namespace which is always SYNCHRONIZED with the active NameNode state.

Backup node does not need to download fsimage and edits files from the active NameNode to create a checkpoint, as it already has an up-to-date state of the namespace in it’s own main memory.  So, creating checkpoint in backup node is just saving a copy of file system meta-data (namespace) from main-memory to its LOCAL files system.