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Let us first study about Namenode:
NameNode in Hadoop stores Metadata. Two files associated with metadata are FsImage and EditLogs.FsImage stores inode details like modification time, access time etc.
EditLogs contains all the recent modifications made to the file system about the most recent.Backup node provides the same checkpointing functionality as the Checkpoint node (Checkpoint node is a node which periodically creates checkpoints of the namespace. Checkpoint Node downloads fsimage and edits from the active NameNode merges them locally, and uploads the new image back to the active NameNode).
In Hadoop, Backup node keeps an in-memory, up-to-date copy of the file system namespace, which is always synchronized with the active NameNode state. There is no need for this node to download fsimage and edits files from the active NameNode in order to create a checkpoint, as would be required with a Checkpoint node or Secondary Namenode, because it already has an up-to-data state of the namespace state in memory.
The Backup node checkpoint process is more efficient as it only needs to save the namespace into the local fsimage file and reset edits. One Backup node is supported by the NameNode at a time. No checkpoint nodes may be registered if a Backup node is in use.
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Backup node as the name states 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 implements the Checkpointingfunctionality along with the online streaming of the File system edits transaction in the Primary Namenode.Prior to Hadoop 2.0, the Checkpoint Node functionality comprises the task of creating the periodic checkpoints of the filesystem metadata in the Primary node by merging the edits file with the fsimage file in the local memory. Each fsimage file merging transaction is termed as the Checkpoint.Then this fsimage file will be uploaded in the Secondary node Memory.
In the Backup node the purpose of uploading the fsimage file to its Memory is not required since the Backup node maintains the in-memory and up-to-date copy of the Filesystem namespace and also accepts and applies the real time online stream of the transactions(edits) on its own namespace copy in its main memory from the Primary Namenode . Here the Checkpointing refers to the the saving the copy of file from the Main-Memory to its Local memory .
The Backup node should have the main memory specifications similar to the Primary node since its maintaining the backup for the Namespace and only one Backup node is supported by the NameNode at a time.
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