A HotAndMirrored Failover mode is where a Client only connects to one Server in the Redundant Server Set because the Server will share this session/state information with the other Servers. In order to validate the capability to connect to other redundant Servers it is allowed to create Sessions with other Servers and maintain the open connections by periodically reading the ServiceLevel. A Client shall not create Subscriptions on the backup Servers for status monitoring (to prevent excessive load on the Servers). This mode allows Clients to fail over without creating a new context for communication. On a Failover the Client will simply create a new SecureChannel on an alternate Server and then call ActivateSession; all Client activities (browsing, subscriptions, history reads, etc.) will then resume. Figure 32 illustrate the behaviour a Client would perform when communicating to a Server in HotAndMirrored Failover mode.
Figure 32 – HotAndMirrored Failover
This Failover mode is similar to the transparent Redundancy. The advantage is that the Client has full control over selecting the Server. The disadvantage is that the Client needs to be able to handle Failovers.