A Hot Failover mode is where the Client should connect to two or more Servers in the RedundantServerSet and to subscribe to the ServiceLevel variable defined in OPC 10000-5 to find the highest ServiceLevel to achieve load balancing; this means that Clients should issue Service requests such as Browse, Read, Write to the Server with the highest ServiceLevel. Subscription related activities will need to be invoked for each connected Server. Clients have the following choices for implementing Subscription behaviour in a Hot Failover mode:

1. The Client connects to multiple Servers and establishes Subscription(s) in each where only one is Reporting; the others are Sampling only. The Client should setup the queue size for the MonitoredItems such that it can buffer all changes during the Failover time. The Failover time is the time between the connection interruption and the time the Client gets Publish Responses from the backup Server. On a Failover the Client shall enable Reporting on the Server with the next highest availability.
2. The Client connects to multiple Servers and establishes Subscription(s) in each where all Subscriptions are Reporting. The Client is responsible for handling/processing multiple Subscription streams concurrently.

Figure 31 illustrate the functionality a Client would perform when communicating with a Server using Hot Failover mode (the figure include both (a) and (b) options)

Figure 31 – Hot Failover

Clients are not expected to automatically switch over to a Server that has recovered from a failure, but the Client should establish a connection to it.