A Warm Failover mode is where the Client should connect to one or more Servers in the Redundant Server Set primarily to monitor the ServiceLevel. A Client can connect and create Subscriptions and MonitoredItems on more than one Server, but sampling and publishing can only be active on one Server. However, the active Server will return actual data, whereas the other Servers in the Redundant Server Set will return an appropriate error for the MonitoredItems in the Publish response such as Bad_NoCommunication. The one Active Server can be found by reading the ServiceLevel Variable from all Servers. The Server with the highest ServiceLevel is the Active Server. For Failover the Client activates sampling and publishing on the Server with the highest ServiceLevel. Figure 30 illustrates the steps a Client would perform when communicating with a Server using Warm Failover mode.


Figure 30 – Warm Failover

Note: There may be a temporary loss of data from the time the connection to the Active Server is interrupted until the time the Client gets Publish Responses from the backup Server.