A global (XA) transaction can span Oracle RAC instances by default, allowing any application that uses the Oracle XA library to take full advantage of the Oracle RAC environment to enhance the availability and scalability of the application.
GTXn background processes support XA transactions in an Oracle RAC environment. The GLOBAL_TXN_PROCESSES initialization parameter, which is set to 1 by default, specifies the initial number of GTXn background processes for each Oracle RAC instance. Use the default value for this parameter clusterwide to allow distributed transactions to span multiple Oracle RAC instances. Using the default value allows the units of work performed across these Oracle RAC instances to share resources and act as a single transaction (that is, the units of work are tightly coupled). It also allows 2PC requests to be sent to any node in the cluster.
Before Oracle RAC 11g release 1 (11.1), the way to achieve tight coupling in Oracle RAC was to use distributed transaction processing (DTP) services, that is, services whose cardinality (one) ensured that all tightly-coupled branches landed on the same instance—regardless of whether load balancing was enabled. If the XA application does not use suspend and resume on the same transaction branch, and does not issue savepoints that span branches, then tightly coupled XA transactions no longer require the special type of singleton services to be deployed on Oracle RAC databases. If your application cannot determine whether a transaction branch has been suspended and resumed, then the application must continue to use DTP services or preferably use XA affinity.
XA affinity (placing all branches of the same XA transaction at the same Oracle RAC instance) is a requirement when suspending and resuming the same XA branch or if using savepoints across branches. It also provides much better performance because different transactions can be balanced. XA affinity is available with Oracle WebLogic Server Active GridLink for Oracle RAC, JDBC Universal Connection Pool, and Oracle Tuxedo.
Note:
Many transaction processing monitors continue to require either XA affinity or DTP services because the applications that use them suspend and resume the same branch or use savepoints.
An external transaction manager, such as Oracle Services for Microsoft Transaction Server (OraMTS), coordinates DTP/XA transactions. However, an internal Oracle transaction manager coordinates distributed SQL transactions. Both DTP/XA and distributed SQL transactions must use the DTP service in Oracle RAC.
See Also:
Oracle Database Development Guide for complete information about using Oracle XA with Oracle RAC
Oracle Database Reference for information about the GLOBAL_TXN_PROCESSES initialization parameter