All OPC UA interactions within a given Sessionshall be supported by one Serverand the Clientis able to identify which Serverthat is, allowing a complete audit trail for the data. It is the responsibility of the Serversto ensure that information is synchronized between the Servers. A functional Serverwill take over the Sessionand Subscriptionsfrom the Failed Server. Failovermay require a reconnection of the Client’s SecureChannelbut the EndpointUrlof the Serverand the ServerUrishall not change. The Clientshall be able to continue communication with the Sessionsand Subscriptionscreated on the previously used Server.

Figure 26provides an abstract view of a transparent Redundant Server Set. The two or more Serversin the Redundant Server Setshare a virtual network address and therefore all Servershave 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 Failoverat a hardware level automatically. There may even be software based systems where all the transparency is governed completely by software.