The Failovermode of a Serveris provided in the ServerRedundancy Objectdefined in OPC 10000-5. The different Failovermodes for non-transparent Redundancy are described in Table 110.

Table 110– Server Failover modes




Cold Failovermode is where only one Servercan be active at a time. This may mean that redundant Serversare unavailable (not powered up) or are available but not running (PC is running, but application is not started)


Warm Failovermode is where the backup Server(s)can be active, but cannot connect to actual data points (typically, a system where the underlying devices are limited to a single connection). Underlying devices, such as PLCs, may have limited resources that permit a single Server connection. Therefore, only a single Server will be able to consume data. The ServiceLevel Variabledefined in OPC 10000-5indicates the ability of the Serverto provide its data to the Client.


Hot Failovermode is where all Serversare powered-on, and are up and running. In scenarios where Serversacquire data from a downstream device, such as a PLC, then one or more Serversare actively connected to the downstream device(s) in parallel. These Servershave minimal knowledge of the other Serversin their group and are independently functioning. When a Serverfails or encounters a serious problem then its ServiceLeveldrops. On recovery, the Serverreturns to the Redundant Server Setwith an appropriate ServiceLevelto indicate that it is available.


HotAndMirrored Failovermode is where Failoversare for Serversthat are mirroring their internal states to all Serversin the Redundant Server Setand more than one Servercan be active and fully operational. Mirroring state minimally includes Sessions, Subscriptions, registered Nodes, ContinuationPoints, sequence numbers, and sent Notifications. The ServiceLevel Variabledefined in OPC 10000-5should be used by the Clientto find the Serverswith the highest ServiceLevelto achieve load balancing.