A Hot Failovermode is where the Clientshould connect to two or more Serversin the Redundant Server Setand to subscribe to the ServiceLevelvariable defined in OPC 10000-5to find the highest ServiceLevelto achieve load balancing; this means that Clientsshould issue Servicerequests such as Browse, Read, Writeto the Serverwith the highest ServiceLevel. Subscriptionrelated activities will need to be invoked for each connected Server. Clientshave the following choices for implementing Subscriptionbehaviour in a Hot Failovermode:

  1. The Clientconnects to multiple Serversand establishes Subscription(s) in each where only one is Reporting; the others are Samplingonly. The Clientshould setup the queue size for the MonitoredItemssuch that it can buffer all changes during the Failovertime. The Failovertime is the time between the connection interruption and the time the Clientgets Publish Responsesfrom the backup Server. On a Failoverthe Clientshall enable Reportingon the Serverwith the next highest availability.
  2. The Clientconnects to multiple Servers and establishes Subscription(s) in each where all Subscriptionsare Reporting. The Clientis responsible for handling/processing multiple Subscriptionstreams concurrently.

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

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Figure 31– Hot Failover

Clientsare not expected to automatically switch over to a Serverthat has recovered from a failure, but the Clientshould establish a connection to it.