OPC UA provides the data structures and services by which Redundancy may be achieved in a standardized manner. Redundancy may be used for high availability, fault tolerance and load balancing. Part 4 formally defines Client, Server and Network Redundancy. Only some Profiles Part 7 will require redundancy support, but not the base Profile.
Required client and server behaviours are associated with two distinct modes of Server Redundancy, transparent and non-transparent. The Client and Server responsibilities when using either transparent or non-transparent redundancy are defined in Part 4.
Servers that support non-transparent redundancy can also support client controlled load balancing. The health of a Server including its ability to Service requests is collectively defined as ServiceLevel. See Part 5 for a formal definition of ServiceLevel. Part 4 defines four distinct ServiceLevel sub-ranges and example usage.