Using the driver : Using failover : Select connection failover

Select connection failover

Select connection failover provides failover protection for the following types of connections:

New connections (in the same way as described in "Connection failover")

Lost connections (in the same way as described in "Extended connection failover")

In addition, the driver can recover work in progress because it keeps track of the last Select statement the application executed on each Statement handle, including how many rows were fetched to the client. For example, if the database had only processed 500 of 1,000 rows requested by a Select statement when the connection was lost, the driver would reestablish the connection to an alternate server, re-execute the Select statement, and position the cursor on the next row so that the driver can continue fetching the balance of rows as if nothing had happened.

Performance, however, is a factor when considering whether to use Select mode. Select mode incurs additional overhead when tracking what rows the application has already processed.

Note: The driver only recovers work requested by Select statements. You must explicitly restart the following types of statements after a failover occurs:

Insert, Update, or Delete statements

Statements that modify the connection state, for example, SET or ALTER SESSION statements

Objects stored in a temporary tablespace or global temporary table

Partially executed stored procedures and batch statements

When in manual transaction mode, no statements are rerun if any of the operations in the transaction were Insert, Update, or Delete. This is true even if the statement in process at the time of failover was a Select statement.

By default, the driver verifies that the rows that are restored match the rows that were originally fetched and, if they do not match, generates an exception warning your application that the Select statement must be reissued. By setting the FailoverGranularity connection property, you can configure the driver to fail the entire failover process if the rows do not match.