Connection failover allows an application to connect to an alternate, or backup, database server if the primary database server is unavailable, for example, because of a hardware failure or traffic overload. Connection failover provides failover protection for new connections only and does not provide protection for lost connections to the database, nor does it preserve states for transactions or queries.
You can customize the drivers for connection failover by configuring a list of alternate database servers that are tried if the primary server is not accepting connections. Connection attempts continue until a connection is successfully established or until all the alternate database servers have been tried the specified number of times.
For example, suppose you have the environment with multiple database servers as shown in the following figure. Database Server A is designated as the primary database server, Database Server B is the first alternate server, and Database Server C is the second alternate server.
First, the application attempts to connect to the primary database server, Database Server A (1). If connection failover is enabled and Database Server A fails to accept the connection, the application attempts to connect to Database Server B (2). If that connection attempt also fails, the application attempts to connect to Database Server C (3).
In this scenario, it is probable that at least one connection attempt would succeed, but if no connection attempt succeeds, the driver can retry each alternate database server (primary and alternate) for a specified number of attempts. You can specify the number of attempts that are made through the connection retry feature. You can also specify the number of seconds of delay, if any, between attempts through the connection delay feature. See Using connection retry for more information about connection retry.
A driver fails over to the next alternate database server only if a successful connection cannot be established with the current alternate server. If the driver successfully establishes communication with a database server and the connection request is rejected by the database server because, for example, the login information is invalid, then the driver generates an exception.
