Planning for VTAM generic resource groups

This topic describes the planning and administration tasks associated with using VTAM® generic resource groups.

Requirements for using VTAM generic resource groups
There are several requirements for balancing sessions using VTAM generic resource groups.
VTAM generic resource group restrictions
When creating a VTAM generic resource group, system programmers should be aware of the restrictions.
VTAM generic resource affinity
After VTAM selects a specific IMS subsystem in a generic resource group to perform the work of a specific terminal, VTAM generic resource (VGR) affinity is established, and all of the terminal's subsequent sessions connect to the same IMS subsystem until IMS or VTAM resets the terminal's affinity.
Creating a VTAM generic resource group
You create a VTAM generic resource group by specifying the same generic resource name for all the IMS systems participating in the VTAM generic resource group.
Specifying APPC generic resource names
Specifying an APPC generic resource name enables LU 6.2 devices to participate in the session balancing provided by a generic resource group. Specify the APPC generic resource name on either the GRNAME parameter of the LUADD statement in the APPC/MVS APPCPMxx member or with the SET APPC z/OS® operator command.
Initiating sessions with IMS in a VTAM generic resource group
Terminal sessions can be initiated either from IMS or from a terminal. All sessions initiated by IMS are generic sessions; that is, sessions in which affinity is established. MSC sessions are initiated with an /RSTART LINK command.
XRF and VTAM generic resources
Extended Recovery Facility (XRF) systems help ensure session persistence and the availability of IMS resources. There are two types of XRF systems: XRF systems that use VTAM multinode persistent sessions (MNPS) and XRF systems that use a USERVAR. XRF with MNPS systems can participate in a VTAM generic resource group, but XRF with USERVAR systems cannot.
Changing the logon procedure
You can provide the option of logging on using either a generic resource name or an IMS APPLID name; therefore, plan to make one or both of these names available in the logon procedure.
Overriding the APPLID field
Each IMS identifies itself to VTAM using the application name you specify in the APPLID= keyword of the IMS COMM system definition macro and in the VTAM APPL definition statement.
Determining when affinities are terminated
When you terminate a session by logging off, IMS terminates the affinity between your node and the IMS VTAM generic resource member. However, in some situations, affinities persist.
Terminating affinities that persist
A terminal cannot participate in session balancing if it has a persisting affinity with a particular IMS from a previous session. When VTAM manages the VGR affinity, affinity is automatically reset at session termination.
Dropping an IMS system from a generic resource group
You can use the /STOP VGRS command to drop an IMS out of a generic resource group. Similarly, you can use the /START VGRS command to bring that IMS back into the generic resource group.
Resetting terminal status
You can reset terminal status (and thus terminate affinities) using commands, exit routines or parameters.
Logging on after IMS failure with affinity
Suppose you are operating in a shared-queues environment, and your terminal is logged onto an IMS in a generic resource group. If that IMS fails, the affinity between that IMS and your terminal might persist.
Controlling VTAM generic resource member selection
You can control VTAM's selection of a generic resource member for a terminal when the terminal logs on. To control the selection process, use either the VTAM Generic Resource Resolution exit routine (ISTEXCGR) or the z/OS Workload Manager.
Ensuring consistency across the IMSplex
Inconsistencies in definitions, operator commands and procedures, and exit routines can create problems within a generic resource group.
Bypassing affinity management during the IMS ESTAE process
If you use IMS to manage generic resource affinities, you can control whether or not IMS uses or bypasses the VTAM generic resource logic in the IMS ESTAE exit. To use the IMS ESTAE process, indicate GRESTAE=Y for terminal sessions and GRMESTAE=Y for MSC link sessions in the DFSDCxxx IMS.PROCLIB member data set.

Parent topic: IMS system administration considerations and tasks