What is HA in a NameNode?

The implementation of Standby and Secondary NameNodes from Hadoop 2.x ensures High Availability in Hadoop clusters, which was not present in Hadoop 1.x. In the case of Hadoop 1.x clusters (one NameNode, multiple DataNodes), a NameNode was a single point of failure. If a NameNode went down owing to lack of backup, then the entire cluster would become unavailable. Hadoop 2.x solved this problem of a single point of failure by including an additional Standby/Secondary NameNode to the cluster. In Hadoop, a pair of NameNodes is in an active-standby configuration. The standby NameNode acts as a backup for the NameNode metadata. The standby NameNode also receives block reports from the DataNodes and maintains a synced copy of edit logs with the active NameNode, and in case the NameNode is down, the standby NameNode takes charge and ensures cluster availability.