Initiating sessions using the generic resource name

When an LU initiates a session using a generic resource name, VTAM® performs generic resource resolution to identify a specific application name. VTAM determines whether the LU has an affinity, or mapping, with a generic resource member. If an affinity exists, VTAM establishes the session with the same generic resource member, regardless of current workload. (A session can be started with one of the other generic resource members using its application program network name.) If an affinity does not exist, VTAM uses the default or defined generic resource preferences from the generic resources preferences table to determine how to resolve the generic name. You can define a default generic resource preference or a generic resource preference for specific generic resources using the generic resources preference table (VBUILD type GRPREFS). See z/OS Communications Server: SNA Resource Definition Reference for the generic resource preference table.

The default generic resource resolution preferences for selecting a generic resource (unless overridden by defining a nameless entry in the GRPREFS table) are as follows:

If a local application initiates a session to a generic resource, VTAM prefers generic resource members on the same VTAM node over those on other VTAM nodes. If no local generic resource members are active, then all generic resource members are eligible for resolution. This preference avoids the overhead associated with routing a session to other nodes. In many situations, you might want this local generic resource preference, but it can create a temporary or minor imbalance of session distribution. However, if the origin logical unit (OLU) is a session manager application or a TN3270 server through which most of the sessions in your network are started, VTAM always selects a local instance of a generic resource; this can significantly overload a single instance of a generic resource. By setting the LOCAPPL generic resource preference in the GRPREFS table, you can control the preference for picking local instances of a generic resource.
If a local LU that is part of a local SNA or local non-SNA major node initiates a session to a generic resource, VTAM prefers generic resource members on the same VTAM node over those on other VTAM nodes. If no local generic resource members are active, then all generic resource members are eligible for resolution. This preference avoids the overhead associated with routing a session to other nodes. In many situations you might want this local generic resource preference, but it can create a temporary or minor imbalance in session distribution. However, if you have a large number of local LUs on the same host as the destination generic resource, VTAM always selects a local instance of a generic resource; this can significantly overload a single instance of a generic resource. By setting the LOCLU generic resource preference in the GRPREFS table, you can control the preference for picking local instances of a generic resource.
If a generic resource resolution is from an LU that is not a local application or a local LU, then all generic resource members are eligible for resolution. However in the case of a third-party-initiated session (CLSDST-PASS) it might be preferable to select from generic resource members at the OLU CP for the session. If, for example, workload balancing had previously established the OLU of the session (for example, TN3270 using sysplex distributor), but the session setup path includes a CLSDST-PASS to a generic resource by a session manager on a non-OLU CP, you might prefer that the generic resource be resolved to a generic resource member on the OLU CP. This would avoid the overhead associated with routing the final session path to other nodes. By setting the PASSOLU generic resource preference in the GRPREFS table, you can control the preference for resolving to generic resource instances at the OLU CP of the session during CLSDST-PASS.
VTAM calls the Workload Manager (WLM) to select an instance of the generic resource from all generic resource members that were previously determined to be eligible for this session. In most conditions, the workload at the time of the session initiation is the best way to determine how to resolve a generic name. However, you can set the WLM generic resource preference in the GRPREFS table to avoid calling the Workload Manager and instead resolve to a generic resource based on the fewest total number of active and pending sessions.
IBM® provides a default installation-wide generic resource exit routine (ISTEXCGR) that you can modify to select the member based on user-specified criteria. By default VTAM does not call the generic resource exit to select an instance of the generic resource. In most conditions, you can set generic resource preferences to customize generic resource resolution. However, you can cause the generic resource exit to be called using the GREXIT generic resource preference in the GRPREFS table.
The generic resource exit can use the information passed in the generic resource exit parameter list to identify an instance of the generic resource. This information includes:
- OLU name
- Generic resource name
- Eligible generic resource instances
- Session counts
- WLM's best generic resource instance
- Best session load balanced generic resource instance
The exit is called only if no affinity exists and after the other generic resource preferences have been used to identify eligible generic resource instances. Only the previously determined eligible generic resource instances are passed to the exit. For more information, see the generic resource resolution exit routine in z/OS Communications Server: SNA Customization.