OPC UA Servers that are part of a RedundantServerSet have certain AddressSpace requirements. These requirements allow a Client to consistently access information from Servers in a RedundantServerSet and to make intelligent choices related to the health and availability of Servers in the RedundantServerSet.
Servers in the RedundantServerSet 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 RedundantServerSet 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 RedundantServerSet will be updated with the same Node changes.
All Servers in a RedundantServerSet shall be synchronized 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 (6.6.2.3) and HotAndMirrored (6.6.2.4.5.5) RedundantServerSet 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 RedundantServerSets Clients shall be able to handle EventIds that are not synchronized. Following any Failover the Client shall 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 shall 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.