To a Client the transparent Redundant Server Set appears as if it is just a single Server and the Client has no Failoveractions to perform. All Serversin the Redundant Server Sethave an identical ServerUriand an identical EndpointUrl.

Figure 26shows a typical transparent Redundancysetup.


Figure 26– Transparent Redundancy setup example

For transparent Redundancy, OPC UA provides data structures to allow Clientsto identify which Serversare in the Redundant Server Set, the ServiceLevelof each Server, and which Server is currently responsible for the Client Session. This information is specified in TransparentRedundancyType ObjectTypedefined in OPC 10000-5. Since the ServerUriis identical for all Serversin the Redundant Server Set, the Serversare identified with a ServerIdcontained in the information provided in the TransparentRedundancyType Object.

In transparent Redundancy, a Clientis not able to control which physical Serverit actually connects to. Failoveris controlled by the Redundant Server Setand a Clientis also not able to actively Failoverto another Serverin the Redundant Server Set.

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.