To use HotStandby (HSB), your application design must determine how your application connects to the HSB server and how the application handles failures and failovers.
The way that an application connects to the HSB server pair might vary depending on where the application resides (compared to the database it is using), and the preferred level of automation in failover situations.
In an environment with no automatic failover handling, an application connects to a single database and uses that database for all the queries, reads, and writes. If that database becomes unavailable, the connection is broken and the application must wait until it can reconnect again to the same database. With a pair of HSB servers, there are two databases, both live and running, mirroring each other and having the same content. With HSB, more options are available to make sure that the service that an application provides is able to continue even if one of the database servers fails.