OPC UA Servers that are part of a Redundant Server Set have certain AddressSpace requirements. These requirements allow a Client to consistently access information from Servers in a Redundant Server Set and to make intelligent choices related to the health and availability of Servers in the Redundant Server Set.

Servers in the Redundant Server Set shall have an identical AddressSpace including:

The only Nodes that can differ between Servers in a Redundant Server Set are the Nodes that are in the local Server namespace like the Server diagnostic Nodes. A Client that fails over shall not be required to translate browse paths or otherwise resolve NodeIds. Servers are allowed to add and delete Nodes as long as all Servers in the Redundant Server Set will be updated with the same Node changes.

All Servers in a Redundant Server Set shall be synchronised with respect to time. This may mean installing a NTP service or a PTP service.

There are other important considerations for a redundant system regarding synchronization: