Group-generic SNA access

With group-generic access, a requester uses the generic LU name of the group to make an initial connection to any member of the group.

VTAM® then chooses one of the group members and establishes a session with that member on behalf of the requester. The requester uses the LU name of the chosen member when it makes subsequent requests.

VTAM provides workload balancing at the session level by allocating each connection request to the member with the fewest number of sessions at the time of connection. VTAM determines which member has the most capacity based on either the number of each member's active DDF sessions or the result of a user-written VTAM or z/OS® Workload Manager exit routine.

After a connection is established between a requester and a member, all requests from the same requester that are made during the session are directed to the same member. VTAM does not reassess a member's capacity at the time of subsequent requests. Only after all connections between the requester and member are closed, can VTAM choose a different member to process future requests from the requester.

Exception: If the connection between the requester and the member is enabled for two-phase commit processing, VTAM continues to direct all requests from this requester to the same member. The mapping between the requester LU and the member LU is preserved until the Db2 command RESET GENERICLU is issued.

If a requester is active at the time of a communication or system failure, VTAM handles the reconnection in one of two ways:
  • If the pre-failure session was enabled for two-phase commit support, VTAM reconnects the requester to the same member.

    If that member is unavailable, a communication error is returned.

  • If the pre-failure session was not enabled for two-phase commit support, VTAM can connect the requester to any member.

Because VTAM balances the workload based on the number of currently established sessions rather than on true capacity, you should carefully evaluate this access method before implementing it. If the number of sessions is an accurate reflection of your workload, this method can be a good choice. It has the added benefit of being relatively easy to set up.