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:
- identical NodeIds
- identical browse paths and structure of the AddressSpace
- identical logic for setting the ServiceLevel
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:
- EventIds:Each UA Server in a Transparent and HotAndMirrored Redundant Server Set shall synchronize EventIds to prevent a Client from mistakenly processing the same event multiple times simply because the EventIds are different. This is very important for Alarms & Conditions. For Cold, Warm, and Hot Redundant Server Sets Clients must be able to handle EventIds that are not synchronised. Following any Failover the Client must call ConditionRefresh defined in OPC 10000-9.
- Timestamp (Source/Server):If a Server is exposing data from a downstream device (PLC, DCS etc.) then the SourceTimestamp and ServerTimestamp reported by all redundant Servers should match as closely as possible. Clients should favour the use of the SourceTimestamp.
- ContinuationPoints:Behaviour of continuation points does not change, in that Clients must be prepared for lost continuation points. Servers in Transparent and HotAndMirrored Redundancy sets shall synchronize continuation points and they may do so in other modes.