Many countries operate a policy of adjusting clocks by one hour at the beginning and end of Summer to effect what is commonly referred to as ‘day-light saving’. These time changes are also applied to the local time held in computers. Generally, most hardware (TOD) clocks are set to Greenwich Mean Time (GMT), with an offset value to indicate local time. It is this offset value that is adjusted when making day-light saving time changes, while the hardware clock remains unaltered.
When setting clocks forward or back an hour to adjust for Summer and Winter time while a CICS® region is running, use the CEMT PERFORM RESET (or EXEC CICS RESETTIME) command to ensure that CICS immediately resynchronizes its local time with that of the MVS™ TOD clock.
A local time change, forwards or backwards, has no effect on CICS logging or journaling, or on CICS restarts, but could affect the operation of utility programs such as DFHJUP.
CICS time-stamps the data it writes to the system log as follows:
For general logs, in addition to time-stamping as in system logs, CICS also includes local time in the journal records.
During a restart, for system recovery purposes, CICS reads the youngest--most recently written--record from the primary log stream. Thereafter, CICS uses only direct reads using block ids and does not rely upon time stamps. CICS also uses direct read with block ids to retrieve the logged data for transaction backout purposes, again without any dependence on time stamps.
Operating a recovery process that is independent of time-stamps in the system log data ensures that CICS can restart successfully after an abnormal termination, even if the failure occurs shortly after local time has been put back.
Changing the local time forward has no effect on the processing of system log streams or general log streams by the CICS utility program, DFHJUP.
Changing local time backwards will not affect the operation of DFHJUP provided you specify the GMT option on the SUBSYS parameter of the log stream DD statement in the DFHJUP JCL.
However, if you use local time on the SUBSYS parameter to specify the partitioning of a log stream for processing by DFHJUP, you must take steps to ensure the chronological sequence of time-stamps when adjusting clocks backwards. You can do this by stopping CICS regions until the new local time passes the old time at which the change was made.
User- or vendor-written journal utilities and DFHJUP exit programs may also be sensitive to local time changes. These should be checked to ensure that there are no problems posed by backwards time changes.
Forward recovery utilities (but not CICS VSAM Recovery 2.3) may also be sensitive to the time sequence of forward recovery log data. If you are not using CICSVR 2.3, check that your forward recovery utility can handle discontinuities in logged records.
The only circumstances in which forward recovery is jeopardized, while using CICSVR 2.3, are when you:
If you use a backup taken earlier than the new local time, or if you specify GMT, CICSVR handles forward recovery successfully.
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