vCenter HA cluster spanning multiple datacenters, our design does not assume a shared storage–based deployment. As a result, one node in the vCenter HA cluster is permanently designated as a quorum node, or a Witness node. The other two nodes in the cluster dynamically assume the roles of Active and Passive nodes. vCenter Server availability is assured as long as there are two nodes running inside a cluster. However, a cluster is considered to be running in a degraded state if there are only two nodes in it. A subsequent failure in a degraded cluster means vCenter services are no longer available. A vCenter Server appliance is stateful and requires a strong, consistent state for it to work correctly. The appliance state (configuration state or runtime state) is mainly composed of: • Database data (stored in the embedded PostgreSQL database) • Flat files (for example, configuration files). The appliance state must be backed up in order for VCHA failover to work properly. For the state to be stored inside the PostgreSQL database, we use the PostgreSQL native replication mechanism to keep the database data of the primary and secondary in sync. For flat files, a Linux native solution, rsync, is used for replication. Because the vCenter Server appliance requires strong consistency, it is a strong requirement to utilize a synchronous form of replication to replicate the appliance state from the Active node to the Passive node.
