Balancing sessions with generic resources

If you are operating in a sysplex environment and have multiple IMS systems, you can initiate a session by using the name of a generic resource group. VTAM® balances the sessions among generic resource members in a generic resource group.

If you do not require the services of a specific IMS, initiate the session using a generic resource name, rather than the APPLID name of a specific IMS. VTAM Generic Resources is intended to run in a sysplex environment, but a sysplex environment is not required.

This topic gives an overview of the benefits and terminology of VTAM Generic Resources.

Benefits of generic resource groups

The benefits of using generic resource groups include:

Automatic session workload balancing: Using generic resources is complementary to using shared queues; generic resources distributes network traffic among multiple IMS systems, while shared queues distributes back-end application workload.
Single-image resources: You can use a single generic resource name to access multiple IMS systems, offering a single-system image while you are using the resources of many IMS systems.
Enhanced IMS system availability: In general, if one IMS fails, you can log on to another IMS in that generic resource group.
Exception: You might not be able to logon if the terminal has LOCAL status recovery mode.
Share global messages: You can obtain messages on shared queues from any IMS in the generic resource group.
Recommendation: Before attempting to obtain messages from a terminal that is in either conversation or fast-path response mode, you may need to re-logon to the system from which the input was originally submitted. If the status recovery mode is LOCAL, you must re-logon to the system from which the input was originally submitted. If the status recovery mode is GLOBAL or NONE, you can logon to any IMS within the generic resource group.

Generic resource group definitions

Definitions:

A generic resource group is a set of IMS subsystems that have the same generic resource name, enabling VTAM to distribute terminal sessions among them.
A generic resource member is an IMS subsystem that belongs to a generic resource group.
A generic resource name is the common name by which VTAM knows all the IMS subsystems that belong to a generic resource group.
An APPLID name is a unique application program name by which VTAM knows an IMS subsystem. In a generic resource group, VTAM uses the APPLID name to differentiate each IMS subsystem.
In the context of generic resources, an affinity is an association between a VTAM logical unit and a specific IMS subsystem in a generic resource group.

Generic resource affinity

VTAM establishes an affinity between a terminal and a specific IMS in the following circumstances:

For all sessions initiated by IMS
When logging onto IMS using a generic resource name

Until the affinity is reset, subsequent logons resolve to the same IMS.

Affinity is automatically reset in one of two ways:

By VTAM at session termination if VTAM-managed affinity is established for the session.
By IMS at session termination if IMS-managed affinity is established for the session, unless the terminal has end-user significant status in the local IMS (status recovery mode of LOCAL).