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.

IMS assigns VGR affinity management to IMS or VTAM for each session depending on the status recovery mode of that session. IMS ignores the GRAFFIN= parameter, which is obsolete for VGR. Multiple Systems Coupling (MSC) links use local affinity only.

Note: For ISC terminals and MSC links, VTAM and IMS do not release VGR affinities until all parallel sessions have been terminated.

VGR affinity management

Either IMS or VTAM can manage VGR affinities. An important difference between the two is the ability of each to reset affinities for terminals in the event of an IMS failure. If a terminal or MSC link has an affinity with an IMS that fails, the terminal cannot continue work until its affinity is reset.

  • IMS-managed – IMS tells VTAM when to delete VGR affinity when a session (or IMS) terminates. IMS deletes the affinity unless significant status for a terminal exists on a local IMS system.
  • VTAM-managed – VTAM deletes VGR affinity when a session (or IMS) terminates.

When VTAM manages affinities, if IMS fails, VTAM can reset the affinities of terminals or MSC links that had sessions open with the failed IMS. With their affinities reset, the terminals can open a new session with another IMS in the generic resource group.

When IMS manages affinities, if IMS fails, IMS might not be able to reset the affinities between the terminals and the failed IMS. In this case, the terminals cannot open new sessions until the failed IMS is restarted. In the case of MSC links, the sessions cannot be moved to cloned links on another IMS when the affinity is set.

The type of terminal and the status recovery mode you set for the terminal determine whether IMS or VTAM manages VGR affinities.

IMS-managed affinity

IMS manages VGR affinity for the following:

  • Static terminals with recovery mode of LOCAL, where IMS sets VGR affinity because the terminal must not be allowed to access other IMS systems while significant status exists on the local IMS system. IMS ensures that VGR affinity matches RM affinity.
  • Dynamic STSN terminals with status recovery mode of LOCAL, where IMS sets VGR affinity because the terminal must not be allowed to access other IMS systems while significant status exists on the local IMS system. IMS ensures that VGR affinity matches RM affinity because the user name and status recovery mode must be provided at logon.
  • MSC links use local mode and IMS managed affinity only, and do not have RM support.