EMA C++ Developers Guide : 7 Warm Standby Feature : 7.1 Overview
 
7.1 Overview
The Warm Standby feature, a client-side feature, is implemented at the Value Add Watchlist layer of Enterprise Transport API (ETA) and made available via Enterprise Message API with configuration. This feature works by providing the application the capability to failover from an active to one or more standby server(s) in the event that the primary/active fails. Application must configure the active and standby servers to use this API feature. After the connections are established with the provided servers which form a Warm Standby group, the client-side or consumer sends messages to the standby server connections to change their mode to Standby. Requested items are opened on all servers by the consumer but the active server responds with messages such as refresh, updates, status, etc. to the consumer. Standby servers respond with blank/empty refreshes. When primary fails, consumer notifies the next server in standby list that it is now Active. The new active server responds with refresh as needed resumes updates for all open items. This process of cut-over is transparent to the application.
A server qualifies to be a standby only if it advertises support for Warm Standby, supports similar features over login and offers an identical service (supported domains, quality of service, etc.) as the active server.
Warm Standby not only reduces overall recovery time, but also network traffic by not inducing a “packet storm” with a flurry of re-requests to a standby server. Because the standby server is already aware of items an application has subscribed for, during a failover Enterprise Enterprise Message API does not need to re-subscribe open items between a provider and consumer.