Local survivability refers to the continuation of Messaging service in case an application server is unable to reach the storage server. Therefore, local survivability only applies to locations in which an application server is physically present. Local survivability does not apply to single server configurations.
The connection between the application server and the AxC/Directory server
The connection between the AxC/Directory server and the third-party message store
When an application server is unable to reach the storage server, the application server operates in the offline mode. The offline handling of messages and user directory transactions is the mechanism that Messaging uses to achieve local survivability. Because offline handling is session based and because an application server only uses offline handling when the application server cannot reach the storage server, individual users who do not have an account on the affected application server experience no change in service.
In a distributed topology, all locations have a local application server and therefore, local survivability applies. All application servers use a WAN to connect to the storage server or to the third-party message store through the AxC/Directory server, regardless of their location. If an application server loses this connection, the application server operates in the offline mode and Messaging continues to function.
In a centralized topology with multiple locations, users at locations that do not have an application server do not have local survivability. These users are connected to Messaging by the network connection between two telephony servers. If there is an outage in this network connection, users at the location that has no application server lose Messaging service. However, if the outage is in the connection between an application server and the storage server, the affected application server operates in the offline mode and service continues for all users.
When an application server operates in offline mode, Messaging continues to operate almost as the application server does when the application server can interact with the storage server. For more information, see Limitations of local survivability.