Persistent sessions support improves the availability of CICS®. It benefits from VTAM® 3.4.1 persistent LU-LU session improvements to provide restart-in-place of a failed CICS without rebinding. VTAM 4.3 introduced multi node persistent sessions which allows for VTAM failures and MVS™ failures as well as CICS failures. See the VTAM Network Implementation Guide for how to set up VTAM for MNPS and how and when it can be used.
CICS support of persistent sessions includes the support of all LU-LU sessions except LU0 pipeline and LU6.1 sessions. CICS determines for how long the sessions should be retained from the PSDINT system initialization parameter. This is a user-defined time interval. If a failed CICS is restarted within this time, it can use the retained sessions immediately--there is no need for network flows to rebind them.
You can change the interval using the CEMT SET VTAM command, or the EXEC CICS SET VTAM command, but the changed interval is not stored in the CICS global catalog, and therefore is not restored in an emergency restart.
If CICS is terminated through CEMT PERFORM SHUTDOWN IMMEDIATE, or if CICS fails, its sessions are placed in "recovery pending" state.During emergency restart, CICS restores those sessions pending recovery from the CICS global catalog and the CICS system log to an "in session" state. This happens when CICS opens its VTAM ACB.
Subsequent processing is LU dependent: cleanup and recovery for non-LU6 persistent sessions are similar to that for non-LU6 backup sessions under XRF. Cleanup and recovery for LU6.2 persistent sessions maintain the bound session when possible but there are cases where it is necessary to unbind and rebind the sessions, for example, where CICS fails during a session resynchronization.
The end user of a terminal sees different symptoms of a CICS failure following a restart, depending on whether VTAM persistent sessions, or XRF, are in use:
Sessions held by VTAM in a recovery pending state are not always reestablished by CICS. CICS (or VTAM) unbinds recovery pending sessions in the following situations:
In all these situations, the sessions are unbound, and the result is as if CICS has restarted following a failure without VTAM persistent session support.
There are some other situations where APPC sessions are unbound. For example, if a bind was in progress at the time of the failure, sessions are unbound.
There are some circumstances in which VTAM does not retain LU-LU sessions:
Without persistent sessions support, all sessions existing on a CICS system are lost when that CICS system fails. In any subsequent restart of CICS, the rebinding of sessions that existed before the failure depends on the terminal’s AUTOCONNECT option. If AUTOCONNECT is specified for a terminal, the user of that terminal waits until the GMTRAN transaction has run before being able to continue working. If AUTOCONNECT is not specified for a terminal, the user of that terminal has no way of knowing (unless told by support staff) when CICS is operational again unless the user tries to log on. In either case, users are disconnected from CICS and need to reestablish a session, or sessions, to regain their working environment.
For CICS persistent session support, you need the VTAM persistent LU-LU session enhancements in VTAM 3.4.1 or later. CICS Transaction Server for z/OS®, Version 3 Release 1 functions with releases of VTAM earlier than 3.4.1, but in the earlier releases sessions are not retained in a bound state in the event of a CICS failure.
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