A vRealize Operations cluster in a Continuous Availability configuration consists of three node types: primary node, primary replica node, and witness node. These node types are deployed across two fault domains, which are separate physical locations that can tolerate failures of an entire rack or site. The witness node is deployed in a third location to monitor the network connectivity between the two fault domains and prevent split-brain scenarios1.
The primary node is the first node that is deployed in the cluster and acts as the master node. It is responsible for managing the cluster configuration, user interface, alerting, and reporting. The primary node is deployed in fault domain 11.
The primary replica node is the second node that is deployed in the cluster and acts as the backup node for the primary node. It is responsible for replicating and synchronizing the data and configuration from the primary node. The primary replica node can take over the functions of the primary node in case of a failure. The primary replica node is deployed in fault domain 21.
The witness node is the third node that is deployed in the cluster and acts as the arbitrator node. It is responsible for monitoring the network connectivity and availability of the primary node and the primary replica node. The witness node can detect a split-brain situation, which occurs when the network connection between the two fault domains is lost and both nodes assume the role of the master node. The witness node can resolve the split-brain situation by making one of the nodes offline to avoid data inconsistency. The witness node is deployed in a third location that is independent of the two fault domains1.
The other options are not node types that are configured in a Continuous Availability configuration. A secondary node is a node that is added to the cluster to increase the capacity and performance of the cluster. A secondary node can be deployed in either fault domain, but it is not required for Continuous Availability1. A remote collector node is a node that is deployed outside the cluster to collect data from remote or isolated data sources. A remote collector node can be deployed in any location, but it is not part of the Continuous Availability configuration2. A secondary replica node is not a valid node type in vRealize Operations.
References: 1: About vRealize Operations Continuous Availability 2: Remote Collector Nodes
