Chapter 3. Namenode High Availability
The HDFS NameNode High Availability feature enables you to run redundant NameNodes in the same cluster in an Active/Passive configuration with a hot standby. This eliminates the NameNode as a potential single point of failure (SPOF) in an HDFS cluster.
Formerly, if a cluster had a single NameNode, and that machine or process became unavailable, the entire cluster would be unavailable until the NameNode was either restarted or started on a separate machine. This situation impacted the total availability of the HDFS cluster in two major ways:
In the case of an unplanned event such as a machine crash, the cluster would be unavailable until an operator restarted the NameNode.
Planned maintenance events such as software or hardware upgrades on the NameNode machine would result in periods of cluster downtime.
HDFS NameNode HA avoids this by facilitating either a fast failover to the new NameNode during machine crash, or a graceful administrator-initiated failover during planned maintenance.
This guide provides an overview of the HDFS NameNode High Availability (HA) feature, instructions on how to deploy Hue with an HA cluster, and instructions on how to enable HA on top of an existing HDP cluster using the Quorum Journal Manager (QJM) and ZooKeeper Failover Controller for configuration and management. Using the QJM and ZooKeeper Failover Controller enables the sharing of edit logs between the Active and Standby NameNodes.
![[Note]](../common/images/admon/note.png) | Note |
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This guide assumes that an existing HDP cluster has been manually installed and deployed. If your existing HDP cluster was installed using Ambari, configure NameNode HA using the Ambari wizard, as described in the Ambari User's Guide. |