Relational databases

User application programs can access RDBMs by using embedded structured query language (SQL) commands. They can also use CICS® API commands to access files and queues that are stored in DB2® or Oracle. (See Figure 1.)
Figure 1. Relational database services
Relational database services
If you use the X/Open XA interface to XA-enabled relational databases, the CICS region, instead of the RDBMS, coordinates the transactional updating of data. For example, a CICS region issues syncpoints to ensure that two-phase commit protocol is used to update the relational database and the CICS region. The benefits are that the CICS region can do the following:
  • Implement a two-phase commit process
  • Restore shared resources to a consistent state after a failure
  • Roll back transactions if a failure occurs in the first phase of a two-phase commit

CICS also provides optimized single-phase commit support that improves performance for updates to DB2, Oracle, Sybase, and Informix®.

XA-enabled databases support the X/Open XA interface, as defined by the X/Open distributed transaction processing standard (X/Open DTP). That standard defines the concept of a functional model that consists of an application program, resource managers, and transaction managers. For information about the X/Open DTP standard, see X/Open Distributed Transaction Processing Reference Model ISBN 1 872630 16 2.

CICS TX on Cloud supports the XA resiliency feature, that the CICS TX on Cloud region remains available even after a resource manager failure during interaction of a CICS TX on Cloud region with an XA compliant resource manager (failure could be detected either during the region startup or while executing the transaction). You need to ensure that SQL errors that result due to unavailability of RDBMS are handled appropriately in the application programs to avoid any data integrity issues.

If XA resiliency feature is disabled (XAResiliency attribute of Region Definition (RD) set as no), the CICS region might crash when CICS application server detects XA failures due to unavailability of Resource Managers (RM).

When WebSphere® MQ is used as a Resource Manager, you might not see the CICS region failure, if the Queue Manager is unavailable after region startup. WMQ registers to XA transaction only when the CICS-MQ application uses MQPUT with SYNCPOINT option. Hence any outage of the Queue manager before executing MQPUT with SYNCPOINT option will not be detected by the CICS region.

If the RDBMS that you are using is not compliant with the X/Open XA interface, or you are not accessing that RDBMS with the XA interface, you cannot use two-phase commit between the resources that are coordinated by CICS.

DB2 supports XA dynamic registration; that is, the CICS region can drive a syncpoint in the database when the database is updated for a given transaction. Dynamic registration can reduce the time it takes to syncpoint when you are running in an environment with multiple XA databases and file managers. It is particularly useful if the majority of your transactions do not need to access a database.

For more details about the use of relational databases, see the following topics: