EMA C++ Developers Guide : 7 Warm Standby Feature : 7.2 Warm Standby Modes
 
7.2 Warm Standby Modes
The Enterprise Message API Value Add layer supports two Warm Standby modes:
Login based Warm Standby
Service based Warm Standby
The login based Warm Standby uses the connection lost event to switch from a primary server to a standby server from the standby server list. The service based Warm Standby uses the service down event OR connection lost event to switch all subscribe items from a primary service to a standby service.
The service based Warm Standby mode offers better resiliency than the login based mode as it can switch from primary to standby if an upstream service is down but the connection to both servers remains intact. A particular server may be the primary for one service and standby for another service as a result. This ability to failover in the event of service down or channel down events makes the service based Warm Standby the recommended mode.
The following figure illustrates the sequence of events when using the Login Based Warm Standby feature:
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Figure 5. Login Based Warm Standby Order of Events in a Cutover from Active to Standby
The following figure illustrates the sequence of events when using the Service Based Warm Standby feature:
Figure 6. Service Based Warm Standby Order of Events in a Cutover from Active to Standby