IBM SC34-7012-01 manual Model log streams for CICS system logs, Recovery considerations

Model log streams for CICS system logs

If CICS fails to connect to its system log streams because they have not been defined, CICS attempts to have them created dynamically using model log streams.

To create a log stream dynamically, CICS must specify to the MVS system logger all the log stream attributes needed for a new log stream. To determine these otherwise unknown attributes, CICS requests the MVS system logger to create the log stream using attributes of an existing model log stream definition. If you decide to allow CICS to create log streams dynamically, you are responsible for creating the required model log stream definitions to ensure that dynamic creation succeeds.

It is generally worthwhile setting up model log streams only if:

vEach model log stream will be used to create several log streams

vEach log stream created using a model log stream has similar characteristics consistent with the use of the same coupling facility structure

vYou don't know in advance how many CICS regions are going to run in a given MVS image (for example, in a development and test environment).

Otherwise, it is probably less work to define the log streams explicitly using IXCMIAPU. Generally, dynamic creation using model log streams is best suited for test and development purposes, and explicit definition for production regions.

The model log stream names that CICS uses for dynamic creation of its system log streams are of the form &sysname.LSN_last_qualifier.MODEL, where:

v&sysname is the MVS symbol that resolves to the system name of the MVS image

vLSN_last_qualifier is the last qualifier of the log stream name as specified on the JOURNALMODEL resource definition.

If you do not provide a JOURNALMODEL resource definition for DFHLOG and DFHSHUNT, or if you use the CICS-supplied definitions in group DFHLGMOD, the model names default to &sysname.DFHLOG.MODEL and &sysname.DFHSHUNT.MODEL.

Example: If a CICS region issues a request to create a log stream for its primary system log, and CICS is running in an MVS image with a sysid of MV10 and using the default JOURNALMODEL definition, the system logger expects to find a model log stream named MV10.DFHLOG.MODEL.

CICS invokes the XLGSTRM global user exit immediately before calling the MVS system logger to create a log stream. If you don't want to use CICS default values for the creation of a log stream, you can write an XLGSTRM global user exit program to modify the request details, including the model log stream name (in parameter UEPMLSN).

Recovery considerations

If you are using coupling facility log streams, sharing structures between MVS images provides some recovery advantages. If an MVS image or logger address space fails, another surviving MVS image using the same log stream structures (not necessarily the same log streams) is notified of the failure and can start immediate log stream recovery for the log streams used by the failing MVS. Otherwise, recovery is delayed until the next time a system connects to a log stream in the affected structures, or until the failed system logger address space restarts.

110CICS TS for z/OS 4.1: Recovery and Restart Guide