Introduction to the external CICS interface

The external CICS® interface is an application programming interface that enables a non-CICS program (a client program) running in MVS™ to call a program (a server program) running in a CICS region and to pass and receive data by means of a communications area. The CICS application program is invoked as if linked-to by another CICS application program.

This programming interface allows a user to allocate and open sessions, or pipes ( a one-way communication path between a sending process and a receiving process) to a CICS region, and to pass distributed program link (DPL) requests over them. The multiregion operation (MRO) facility of CICS interregion communication (IRC) facility supports these requests, and each pipe maps onto one MRO session, where the client program represents the sending process and the CICS server region represents the receiving process. Start of changeThere is a default limit of 100 pipes per EXCI address space; the limit can be changed when MVS is IPLed.End of change

This limit is implemented to avoid EXCI clients monopolising MRO resources, which could prevent CICS systems from using MRO. The limit is applied in both MRO and cross system MRO (XCF/MRO) environments. An ALLOCATE_PIPE request results in an MRO LOGON request being issued and there is a limit on the total number of MRO LOGON requests allowed from all address spaces. This is particularly critical when using XCF/MRO, where the limit on the number of members in a XCF group also limits the total number of MRO LOGONs

Start of changeThe external CICS interface identifies the CICS region to communicate with by the CICS region's applid, as defined in the SIT APPLID parameter. (In an XRF environment, it is the generic applid that must be used). You can specify the applid either on an EXCI API call or by means of the DFHXCURM user-replaceable program. (For more information about DFHXCURM, see The EXCI user-replaceable module.)

Note:
Do not confuse the term "generic applid" with "generic resource name". Generic resource names apply only to VTAM generic resource groups, which are not supported by EXCI.
End of change

The client program and the CICS server region (the region where the server program runs or is defined) must be in the same MVS image unless:

Alternatively, if there is no local CICS region in the MVS image, you must specify the SVC parameter that the external CICS interface is to use, by coding a CICSSVC parameter in the DFHXCOPT table.

Although the external CICS interface does not support the cross-memory access method, it can use the XCF access method provided by the CICS XCF/MRO facility. SeeUsing the EXCI options table, DFHXCOPT for information about XCF/MRO.

A client program that uses the external CICS interface can operate multiple sessions for different users (either under the same or separate TCBs) all coexisting in the same MVS address space without knowledge of, or interference from, each other.

Where a client program attaches another client program, the attached program runs under its own TCB.

Related concepts
The EXCI programming interfaces
Resource recovery
EXCI concepts
Requirements for the external CICS interface
Related tasks
The EXCI CALL interface
The EXCI EXEC CICS interface
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