Sometimes a session's counterparty may be out of service, causing session logon failure. Some systems provide secondary servers, which can be connected while primary servers are down. There are the following options to implement the failover in such cases:

The manual option is to handle exceptions when you try to connect to the primary server by the OnixS::FIX::Session::logonAsInitiator method. For example, if there are OnixS::FIX::LinkErrorException or OnixS::FIX::TimeoutException exceptions then this can be considered as a reason to connect to a secondary server. Otherwise, if there is the OnixS::FIX::FirstMessageNotLogonException exception you can analyze the text from the tag 58 of the received message and also decide to connect to a secondary server (if there is the corresponding reason).
The automatic option is to use the Sessions Scheduler component and specify the secondary server to the OnixS::FIX::Scheduling::InitiatorConnectionSettings class. Scheduler will automatically connect to this server in case of the connection failure to the primary server, please see the Session Connection Settings page for details.