Sample system configurations

These figures illustrate sample IMS Intersystem Communication (ISC) configurations. The first two figures use IBM® CICS® Transaction Server for z/OS® as an example of an ISC node.

Existing or new IMS transactions invoked from CICS

The following figure illustrates ISC's distributed transaction processing capability—the ability to invoke existing or new IMS transactions from a CICS application or from a CICS application on behalf of a terminal attached to CICS (using transaction routing). The links in the figure can be either ISC TCP/IP links or ISC VTAM® links.

Figure 1. Existing or new IMS transactions invoked from CICS
Transaction flow: CICS terminal to front-end CICS, across ISC link to back-end IMS, accessing IMS databases, back across ISC link to CICS, and back to the CICS terminal.

In figure above, ISC provides transaction-to-transaction capability, because an application program can be written as two complementary transactions: one executing in an IMS system, the other in a CICS system. User functions can be distributed between systems as required. If ISC TCP/IP links are used, only asynchronous CICS transactions can be used and other limitations apply.

IMS MFS DPM mapping function distributed to CICS's BMS

The following figure illustrates ISC VTAM's ability to distribute device mapping function from an IMS Message Format Service (MFS) system to a CICS Basic Mapping Support (BMS) system. MFS partially maps the data stream that is sent to a CICS application. The application is responsible for processing the input data stream to a form that is acceptable to BMS. BMS can then be used to complete the device mapping.

MFS does not support ISC TCP/IP communication.

Figure 2. IMS MFS DPM mapping function distributed to CICS's BMS
Transaction Flow: Device input is sent to BMS. Partially mapped data is sent to MFS. After transaction processing, MFS sends partially mapped data to BMS. BMS sends device output to terminal.

IMS-to-IMS with ISC VTAM

The following figure illustrates how two IMS systems can be connected using ISC VTAM links. Interconnection of two IMS systems allows a new or modified transaction (TRAN1) in one IMS system to invoke an existing transaction (TRAN2) in another IMS system. Using MFS, replies can be routed to a new transaction (TRAN3) in the initiating IMS system, or to a new instance of the originating transaction (TRAN1). Without MFS, replies are treated as message switches and routed to the source terminal.

Figure 3. IMS-to-IMS with ISC VTAM
Transaction flow: TRAN1 program in IMS A receives message and sends a message to TRAN2 program in IMS B, which sends a message, through MFS, to TRAN3 program on IMS A, which sends output to terminal.

CICS-to-IMS MSC using ISC

The following figure illustrates how a CICS subsystem can communicate with an IMS subsystem that has MSC links to other IMS subsystems. In this case, MSC is used to couple the IMS subsystems, while the CICS-to-IMS session is supported by ISC. The CICS subsystem has a single-system view of the multiple IMS subsystems with which it can communicate. MSC distributes the load between the multiple IMS subsystems.

Figure 4. CICS-to-IMS MSC using ISC
Begin figure description: a terminal on the left connects to CICS; an ISC link connects CICS to IMS A on the right; MSC links connect IMS A to IMS B and IMS C. End figure description.