To a Client the transparent Redundant Server Set appears as if it is just a single Server and the Client has no Failover actions to perform. All Servers in the Redundant Server Set have an identical ServerUri and an identical EndpointUrl.
Figure 26 shows a typical transparent Redundancy setup.
Figure 26 – Transparent Redundancy setup example
For transparent Redundancy, OPC UA provides data structures to allow Clients to identify which Servers are in the Redundant Server Set, the ServiceLevel of each Server, and which Server is currently responsible for the Client Session. This information is specified in TransparentRedundancyType ObjectType defined in OPC 10000-5. Since the ServerUri is identical for all Servers in the Redundant Server Set, the Servers are identified with a ServerId contained in the information provided in the TransparentRedundancyType Object.
In transparent Redundancy, a Client is not able to control which physical Server it actually connects to. Failover is controlled by the Redundant Server Set and a Client is also not able to actively Failover to another Server in the Redundant Server Set.
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.