Overview of an APPC/MVS Inbound Request

When a request to establish communications comes from a remote node in the network into the local z/OS system, it is called an “inbound” request. An inbound request could also come from the same LU.

An illustration of inbound processing follows.

Figure 1. APPC/MVS Communication Services (Inbound)

An installation can use Resource Access Control Facility (RACF) or an equivalent security product to check that the inbound request is authorized to access the local LU. A security environment can then be established to validate access to other resources.

The inbound request contains the 1- to 64-character name of the local TP that is to be attached. When an inbound request enters the system, APPC/MVS first checks to see whether any address spaces on the local system had previously requested to serve the request (that is, whether an APPC/MVS server has registered for the request through the Register_For_Allocates service). If so, APPC/MVS places the request on an allocate queue from which the server can later select it for processing. When the server selects the request from the allocate queue, it receives the conversation ID, and a conversation with the issuer of the request starts.

If the server used a symbolic destination name to register for the request, APPC/MVS uses the side information file to translate the symbolic destination name into the required routing information.

If no servers have registered for the request, APPC/MVS attempts to schedule the request to a transaction scheduler. APPC/MVS maps the name of the TP targeted by the request to a TP profile that contains information necessary to set up the appropriate z/OS environment that will be required to run the TP. All inbound TPs processed by the APPC/MVS transaction scheduler must have a TP profile associated with them. The TP profile contains information such as:

Transaction program capabilities and status
Transaction scheduler information:
- MVS job name
- MVS program name (for example, “IEBMAIL”)
- Data set allocation environment
- Execution class.

The APPC/MVS transaction scheduler is responsible for maintaining pools of address spaces into which TPs are scheduled. These address spaces can receive the services of all MVS components, and are called subordinate address spaces. An APPC transaction initiator is the program that runs in each of the APPC/MVS transaction scheduler's subordinate address spaces, and is responsible for setting up the appropriate environment (as specified in the TP profile) and managing the processing of the TPs. The APPC/MVS transaction initiator is similar to the MVS initiator that provides a processing environment for traditional types of work on z/OS (such as batch jobs). The term transaction initiator is used throughout this document to mean an APPC/MVS transaction scheduler subordinate address space. Figure 1 shows these initiators on the right-hand side.