Managing MSC links in a TCP/IP generic resource group

When MSC is used with TCP/IP generic resources, each link that connects to the TCP/IP generic resource group has affinity to a specific IMS system in the group.

Affinity restricts all of the logical link parallel sessions on a physical link to one IMS system. Affinity is cleared in IMS when a logical link is terminated normally.

When a link has affinity, a /DISPLAY AFFIN LINK or QUERY MSLINK displays the link with a status of AFFIN. The QUERY MSLINK command also shows the IMS ID of the IMS system that has affinity.

IMS Connect manages the routing of link requests to the TCP/IP generic resource group. The definitions of the MSC links in IMS Connect correlate the shared generic IMS ID of the TCP/IP generic resource group to the individual participating IMS systems in the group.

When a link to a TCP/IP generic resource group is started by the remote IMS system, the start request is routed by IMS Connect to all available IMS systems in the group. IMS Connect passes the link request to the first IMS system to accept the link and the link affinity is set. IMS Connect then routes all logical link parallel sessions on the same physical link to that IMS system.

If a TCP/IP generic resource group contains only one IMS system, then the initial link request is always accepted by that system, regardless of which side a link is started from.

Restriction: In a TCP/IP generic resource group, where a physical link is defined in multiple IMS systems, if a logical link is in a PSTOPPED ERE state on any IMS system in the group, do not start a logical link that uses the same physical link on any other IMS system in the group. Before you can start a logical link on the same physical link on another system, you must clear the affinity of the ERE link either by shutting it down normally or by resetting it to a COLD state.

As a precaution, display the affinity status of an MSC link on its current IMS system before moving the link by restarting it on another IMS system. If the affinity is still active, shutdown the link normally, which resets the affinity status. If the link cannot be shut down normally, set the link to COLD status by issuing either the type-1 command /CHANGE LINK linknum FORCSESS | SYNCSESS COLDSESS or the type-2 command UPDATE MSLINKNAME(linkname) SET(SYNCOPT(COLDSESS)

Attention: Changing the link status from ERE to COLD or moving a link in ERE status and restarting it on another IMS system prevents the synchronization of the message sequence numbers during restart, which can cause the duplication or loss of messages.

If multiple IMS systems participate in a TCP/IP generic resource group, you can use the following methods to control which IMS system gets affinity when you start the first logical link on a physical link:

Start the link from the IMS system in the TCP/IP generic resource group with which you need affinity to be established.
In IMS Connect, stop the MSC physical link paths to the IMS systems in the generic resource group that do not need affinity.
1. Issue any one of the following IMS Connect commands to stop the physical link path in IMS Connect:
  - The IMS type-2 format command UPDATE IMSCON TYPE(MSC) NAME(lclPlkid) STOP(COMM)
  - The WTOR format command STOPMSC lclPlkid
  - The z/OS® MODIFY command UPDATE MSC NAME(lclPlkid) STOP(COMM)
2. After the links have been stopped to all but the IMS system that needs affinity, start the link from the remote IMS system.
In each IMS system in the link connection is to be prevented, stop logons to the generic resource group on the physical link.
1. You can stop logons in an IMS system by issuing either one of the following IMS commands for each IMS system with which the link connection needs to be prevented:
  - The IMS type-2 command UPDATE MSPLINK NAME(msplink_name) STOP(GENLOGON)
  - The IMS type-1 command /PSTOP MSPLINK msplink_name
2. After logons to the generic resource group have been stopped for all IMS systems in the group except the IMS system that the link will connect to, start the link from the remote IMS system.