The Failover mode of a Server is provided in the ServerRedundancy Object defined in OPC 10000-5. The different Failover modes for non-transparent Redundancy are described in Table 110.

Table 110 – Server Failover Modes




Cold Failover mode is where only one Server can be active at a time. This may mean that redundant Servers are unavailable (not powered up) or are available but not running (PC is running, but application is not started)


Warm Failover mode 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 Variable defined in OPC 10000-5 indicates the ability of the Server to provide its data to the Client.


Hot Failover mode is where all Servers are powered-on, and are up and running. In scenarios where Servers acquire data from a downstream device, such as a PLC, then one or more Servers are actively connected to the downstream device(s) in parallel. These Servers have minimal knowledge of the other Servers in their group and are independently functioning. When a Server fails or encounters a serious problem then its ServiceLevel drops. On recovery, the Server returns to the Redundant Server Set with an appropriate ServiceLevel to indicate that it is available.


HotAndMirrored Failover mode is where Failovers are for Servers that are mirroring their internal states to all Servers in the Redundant Server Set and more than one Server can be active and fully operational. Mirroring state minimally includes Sessions, Subscriptions, registered Nodes, ContinuationPoints, sequence numbers, and sent Notifications. The ServiceLevel Variable defined in OPC 10000-5 should be used by the Client to find the Servers with the highest ServiceLevel to achieve load balancing.