For non-transparent Redundancy, OPC UA provides the data structures to allow the Client to identify what Servers are available in the RedundantServerSet and also Server information which tells the Client what modes of Failover the Server supports. This information allows the Client to determine what actions it may need to take in order to accomplish Failover. This information is specified in NonTransparentRedundancyType ObjectType defined in OPC 10000-5.

The Servers in the non-transparent RedundantServerSet shall use the ServerCapability NTRS defined in OPC 10000-12 for discovery including the registration with a GlobalDiscoveryServer.

Figure 27 shows a typical non-transparent Redundancy setup.


Figure 27 – Non-Transparent Redundancy setup

For non-transparent Redundancy, the Servers will have unique IP addresses and unique ApplicationUris. The Server also has additional Failover modes of Cold, Warm, Hot and HotAndMirrored. The Client shall be aware of the RedundantServerSet and shall be required to perform some actions depending on the Failover mode. These actions are described in Table 111 and additional examples and explanations are provided in Cold, for Warm, for Hot and for HotAndMirrored.

A Client needs to be able to expect that the SourceTimestamp associated with a value is approximately the same from all Servers in the RedundantServerSet for the same value.