All OPC UA interactions within a given Session shall be supported by one Server and the Client is able to identify which Server that is, allowing a complete audit trail for the data. It is the responsibility of the Servers to ensure that information is synchronised between the Servers. A functional Server will take over the Session and Subscriptions from the Failed Server. Failover may require a reconnection of the Client’s SecureChannel but the EndpointUrl of the Server and the ServerUri shall not change. The Client shall be able to continue communication with the Sessions and Subscriptions created on the previously used Server.
Figure 26 provides an abstract view of a transparent Redundant Server Set. The two or more Servers in the Redundant Server Set share a virtual network address and therefore all Servers have the identical EndpointUrl. How this virtual network address is created and managed is vendor specific. There may be special hardware that mediates the network address displayed to the rest of the network. There may be custom hardware, where all components are redundant and Failover at a hardware level automatically. There may even be software based systems where all the transparency is governed completely by software.