This topic discusses two exit routines that apply to the TPF environment: a USERVAR exit routine and a control logical unit (CLU) search exit routine. The USERVAR routine is an exit from VTAM® that enables the logon manager to provide a single-system image to PLUs initiating sessions with TPF. This allows the user to request a session with a TPF application program using a generic name. The CLU search routine is an exit from the logon manager that allows you to customize the logon manager's load balancing.

A sample USERVAR exit routine is stored in SYS1.SAMPLIB under the VTAM module name ISTEXCUV. The sample USERVAR exit routine is an implementation of a VTAM exit routine that is specific to the TPF environment. A sample CLU search exit routine is also stored in SYS1.SAMPLIB under the module name ELMCLUEX. You can use the samples as they are coded. However, if you want logic different from the sample exit routines, you can modify the samples, or write your own USERVAR or CLU search exit routine. Although the CLU search exit is a user-replaceable module, the logon manager cannot operate without it. Likewise, the logon manager must use a USERVAR exit to process any session requests other than those from dependent SLUs.

This topic describes the function of the sample exit routines. It also contains interface information for the CLU search exit. See USERVAR exit routine for interface information for that exit. You might also see Sample USERVAR exit routine for TPF sessions and Sample CLU search exit routine for TPF sessions.

The logon manager defines a USERVAR for each application program it supports whenever it opens an ACB for the application program. The logon manager defines the USERVAR in the host where the logon manager is running. The logon manager initializes the USERVAR value to the name of the logon manager ACB for the application program and specifies TYPE=VOLATILE. Being volatile, TPF USERVARs are not stored by the SSCP that owns the OLU and the USERVAR table is not updated. Instead, the USERVAR exit at the DLU SSCP provides a new value for the USERVAR translation each time VTAM drives the exit. The USERVAR exit is driven for every TPF session request so that the exit can assign a TPF destination logical unit based on current load conditions. The USERVAR exit does this by using the REQTAIL macro to invoke the logon manager for PLU-initiated sessions. The logon manager invokes the CLU search exit routine, which is responsible for choosing the best CLU for a session.