XA Affinity and Failover When accessed within a global transaction, the member data source from which the JDBC connection was obtained is pinned to the global transaction for the life of the transaction. This ensures that all database operations performed on connections obtained from the Multi Data Source, for a particular transaction, all execute on the same RAC instance. XA affinity results in improved performance and is even a requirement for older versions of RAC, such as prior to 11g. The XA failover is also supported by the Multi Data Source and transaction manager implementations. If a pinned RAC instance suffers a failure, then a global transaction can complete utilizing a different RAC instance using a connection obtained one of the other member data sources.
Reference: Oracle WebLogic Server Active GridLink for Oracle Real Application Clusters(RAC)