To enable High Availability (HA) on a vRealize Operations cluster, the following two options are requirements:
The cluster must have at least one Primary node: The primary node is the first node that is deployed in the cluster and it is responsible for managing the cluster configuration, user interface, and administration functions. The primary node also stores data and performs analytics like any other data node. Without a primary node, the cluster cannot function properly1.
The cluster must have at least one Data node: A data node is a node that collects data from various sources, stores data in its local file system, and performs analytics and alerting functions. A data node also participates in the cluster load balancing and data replication. To activate HA, you must have at least one data node deployed, in addition to the primary node. This allows the creation of a replica node that can take over the primary node functions in case of a failure2. References: 1: About vRealize Operations High Availability 2: High Availability (HA)