CICS® intercommunication facilities allow different CICS systems to communicate and share resources with each other. These facilities consist of the following components:
For details of the CICS intercommunication facilities, see the CICS Intercommunication Guide. See also Splitting online systems: virtual storage, and Splitting online systems to improve availability.
If there are a number of intercommunication requests for each transaction, function shipping generally incurs the most overhead. The number of requests per transaction that constitutes the break-even point depends on the nature of the requests.
Both distributed transaction processing (DTP) and asynchronous processing are, in many cases, the most efficient method of intercommunication because a variety of requests can be batched in one exchange. DTP, however, requires an application program specifically designed to use this facility. For information about designing and developing DTP, see the CICS Distributed Transaction Programming Guide.
Transaction routing, in most cases, involves one input and one output between systems, and the overhead is minimal.
Multiregion operation (MRO), in general, causes less processor overhead than intersystem communication (ISC) because the SVC pathlength is shorter than that through the multisystem networking facilities of VTAM®. This is particularly true with CICS MRO, which provides a long-running mirror transaction and fastpath transformer program.
Some SVC-processing overhead can be eliminated from MRO in CICS with the use of MVS™ cross-memory services. Cross-memory services use the MVS common system area (CSA) storage for control blocks, not for data transfer. This can also be of benefit. Note, however, that MVS requires that an address space using cross-memory services be nonswappable.
For situations where ISC is used across MVS images, consider using XCF/MRO. XCF/MRO consumes less processor overhead than ISC.
ISC mirror transactions can be prioritized. The CSMI transaction is for data set requests, CSM1 is for communication with IMS/ESA® systems, CSM2 is for interval control, CSM3 is for transient data and temporary storage, and CSM5 is for IMS/ESA DB requests. If one of these functions is particularly important, it can be prioritized above the rest. This prioritization is not effective with MRO because any attached mirror transaction services any MRO request while it is attached.
If ISC facilities tend to flood a system, this can be controlled with the VTAM VPACING facility. Specifying multiple sessions (VTAM parallel sessions) increases throughput by allowing multiple paths between the systems.
CICS also allows you to specify a VTAM class of service (COS) table with LU6.2 sessions, which can prioritize ISC traffic in a network. Compare the performance of CICS function shipping with that of IMS/ESA data sharing.
See the CICS Transaction Server for z/OS® Installation Guide for information about resetting the system for MRO or ISC. See also Splitting online systems: virtual storage.
CICS ISC/IRC statistics (see topic ISC/IRC system and mode entry statistics) show the frequency of use of intercommunication sessions and mirror transactions. The VTAM trace, an SVC trace, and RMF™ give additional information.