APPC

The two scenarios differ in the subsystem that manages the updating and synchronization of protected resources that the application program accesses.

In the APPC/IMS scenario, when SYNCLVL = NONE or SYNCLVL = CONFIRM, IMS is the synchronization-point manager. When SYNCLVL = SYNCPT, z/OS® Resource Recovery Services (RRS) is the synchronization-point manager.

In the CPI-C driven scenario:

• The sync point manager is the RRS function of z/OS.
• The resource manager is IMS.
• The program that accesses and updates the protected resources is the APPC/MVS application program.

APPC/IMS defines the formats and protocols for program-to-program communication. APPC/IMS enables applications to be distributed throughout the network and to communicate with each other regardless of the underlying hardware architectures and software environments. APPC/IMS provides a facility for implementing logical unit type 6.2 (LU 6.2) support.

APPC/MVS is used for all interaction with remote LU 6.2 devices or subsystems. IMS accesses the session through APPC/MVS services. Using APPC/IMS, IMS and LU 6.2 devices access each other without requiring coding changes to existing application programs. With slight modifications to IMS application programs, Common Programming Interface (CPI) communications-driven application programs can communicate with IMS application programs (and can execute as IMS application programs).

A restriction exists, however, on LU 6.2 synchronous conversations with implicit transactions. If a transaction creates more than one daughter transaction, which, in turn, might create other transactions, and one of the daughter transactions provides the response, then the result is unpredictable. In some cases, depending on the execution sequence of the transactions, the LU receives a DFS2082 message and the response is sent to the default TP name DFSASYNC. In other cases, the LU receives the response and no DFS2082 message is issued.

APPC/IMS flood control

APPC/IMS includes an optional flood control function that queues APPC requests in 64-bit storage if the number of active APPC conversations exceeds a flood threshold that is defined on the APPCMAXC= parameter in the DFSDCxxx PROCLIB member. The default threshold is 5,000 active APPC conversations.

IMS stops all APPC input from z/OS, including input for both APPC/IMS and CPI-C, if either the number of APPC requests that are queued in 64-bit storage reaches the maximum number that is defined in the second value position of the APPCMAXC= parameter or if queueing in 64-bit storage is disabled and the maximum number of active APPC conversations is reached.