Persistent sessions support improves the availability of CICS®. It benefits from VTAM® persistent LU-LU session support to provide restart-in-place of a failed CICS without re-binding.
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 time on 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 re-bind 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 by means of a CEMT PERFORM SHUTDOWN IMMEDIATE command or if CICS fails, the CICS sessions are held by VTAM in "recovery pending" state, and may be recovered during startup by a newly starting CICS region.
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.
Before specific terminal types and levels of service are discussed, note that many factors can affect the performance of a terminal at takeover, including:
Subsequent processing is LU-dependent: cleanup and recovery for non-LU6 persistent sessions are similar to those 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 re-bind 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 CICS is initialized with VTAM persistent sessions support or XRF support:
If APPC sessions are active at the time CICS fails, persistent sessions recovery appears to APPC partners as CICS "hanging". VTAM saves requests issued by the APPC partner, and passes them to CICS when the persistent recovery is complete. After a successful emergency restart, recovery of terminal sessions is determined by the options defined in PSRECOVERY of the CONNECTION definition and RECOVOPTION of the session definition. If the appropriate recovery options have been selected (see the CICS Resource Definition Guide), and the APPC sessions are in the correct state, CICS performs an ISSUE ABEND to inform the partner that the current conversation has been abnormally terminated.
If you are using multi-node persistent sessions (MNPS) support, sessions can be retained across a VTAM failure. After VTAM has been restarted, sessions are restored when the ACB is reopened (either automatically by the COVR transaction or by a CEMT, or EXEC CICS, SET VTAM OPEN command). To ensure that CICS retains its sessions and restores them when the ACB is reopened, code PSTYPE=MNPS as a system initialization parameter.
APPC synclevel 2 sessions are not restored. They are unbound when the ACB is opened.
When the VTAM failure occurs and the TPEND failure exit is driven, the autoinstalled terminals that are normally deleted at this point are retained by CICS. If the session is not restored and the terminal is not reused within the AIRDELAY interval, CICS deletes the TCTTE when the AIRDELAY interval expires after the ACB is re-opened successfully.
Table 1 indicates whether sessions are restored or unbound in a number of recovery scenarios: (1) a CICS emergency restart with persisting sessions; (2) a VTAM restart after a VTAM failure, where CICS continues running and the ACB is reopened; (3) CICS cold and warm starts; and (4) the dynamic open of the VTAM ACB by a CICS operator command.
Recovery scenario | Terminal | LU62 (synclevel 1) | LU62 (synclevel 2) | LU61 |
---|---|---|---|---|
CICS emergency restart | Restored | Restored | Restored | Unbound |
VTAM_RESTART (MNPS only) | Restored | Restored | Unbound | Unbound |
CICS COLD start | Unbound | Unbound | Unbound | Unbound |
CICS WARM start | Unbound | Unbound | Unbound | Unbound |
Dynamic open of VTAM ACB | Unbound | Unbound | Unbound | Unbound |
If you are not running with VTAM MNPS then you do not need a PSTYPE system initialization parameter and sessions are not retained or recovered after a VTAM abend and subsequent opening of the VTAM ACB (CEMT SET VTAM OPEN).
CICS does not always reestablish sessions held by VTAM in a recovery pending state. 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 sessions 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:
For more information about persistent sessions support, see the CICS System Definition Guide.
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