Planning for XRF and APPC

All LU 6.2 conversation messages that are sent to a failing IMS are broken at the time of an XRF takeover. New conversations must be allocated on the new active IMS after takeover processing completes. XRF support for LU 6.2 application programs is Class 3 service.

The application program uses the USERVAR parameter as the LU name parameter on the ALLOCATE verb. This allows the same LU name to allocate the conversation on the new active IMS. This is the same concept as the LU usage for non-LU 6.2 IMS terminals, but a different USERVAR name is used. At takeover time, the alternate IMS issues a VTAM® MODIFY USERVAR command to redirect the USERVAR from the failed active IMS to the alternate IMS. Also, a VTAM VARY TERM command is issued against the failed active IMS to terminate any conversations held during takeover. Then VTAM propagates the USERVAR changes to other VTAM instances as needed.

Any APPC transactions that are enqueued to a remote IMS are handled by the MSC protocols. After takeover in the local IMS, the queued irrecoverable messages are lost. If the same message is queued to a remote IMS, it is recovered when the local IMS takes over. Processing continues and the reply is sent to the active IMS.

VTAM, z/OS®, Tivoli® NetView for z/OS, and any other product requirements should be carefully reviewed. APPC/IMS requires correct operation of these other products.