OPC UA provides the data structures and services by which Redundancymay be achieved in a standardized manner. Redundancy may be used for high availability, fault tolerance and load balancing. OPC 10000-4formally defines Client, Serverand Network Redundancy. Whether and what Redundancyis supported by an OPC UA Applicationis defined by its Profiles. Profilesare described in OPC 10000-7.
Required client and server behaviours are associated with two distinct modes of Server Redundancy, transparent and non-transparent. The Clientand Serverresponsibilities when using either transparent or non-transparent redundancy are defined in OPC 10000-4.
Serversthat support non-transparent redundancy can also support client controlled load balancing. The health of a Serverincluding its ability to Servicerequests is collectively defined as ServiceLevel. See OPC 10000-5for a formal definition of ServiceLevel. OPC 10000-4defines four distinct ServiceLevelsub-ranges and example usage.