NameNode

NameNode is the gatekeeper for all HDFS-related queries. It serves as a single point for all types of coordination on HDFS data, which is distributed across multiple nodes. NameNode works as a registry to maintain data blocks that are spread across Data Nodes in the cluster. Similarly, the secondary NameNodes keep a backup of active Name Node data periodically (typically every four hours). In addition to maintaining the data blocks, NameNode also maintains the health of each DataNode through the heartbeat mechanism. In any given Hadoop cluster, there can only be one active name node at a time. When an active NameNode goes down, the secondary NameNode takes up responsibility. A filesystem in HDFS is inspired from Unix-like filesystem data structures. Any request to create, edit, or delete HDFS files first gets recorded in journal nodes; journal nodes are responsible for coordinating with data nodes for propagating changes. Once the writing is complete, changes are flushed and a response is sent back to calling APIs. In case the flushing of changes in the journal files fails, the NameNode moves on to another node to record changes.