What happens when an idle connection loses its physical connection to the database? For example, suppose the database server is rebooted or the network experiences a temporary interruption. An application that attempts to connect could receive errors because the physical connection to the database has been lost.
The driver handles this situation transparently. The application does not receive any errors on the connection attempt because the driver simply returns a connection from a connection pool. The first time the connection handle is used to execute a SQL statement, the driver detects that the physical connection to the server has been lost and attempts to reconnect to the server before executing the SQL statement. If the driver can reconnect to the server, the result of the SQL execution is returned to the application; no errors are returned to the application.
The driver uses connection failover option values, if they are enabled, when attempting this seamless reconnection; however, it attempts to reconnect even if these options are not enabled. See Configuring Connection Failover for information about configuring the driver to connect to a backup server when the primary server is not available.
Note: If the driver cannot reconnect to the server (for example, because the server is still down), an error is returned indicating that the reconnect attempt failed, along with specifics about the reason the connection failed.
The technique that the driver uses for handling dead connections in connection pools allows for maximum performance of the connection pooling mechanism. Some drivers periodically test the server with a dummy SQL statement while the connections sit idle. Other drivers test the server when the application requests the use of the connection from the connection pool. Both of these approaches add round trips to the database server and ultimately slow down the application during normal operation.
