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DB2 performance enhancements

The CICS® DB2® adapter is enhanced to improve the performance of user transactions that issue DB2 requests. This improvement is achieved by exploiting the open transaction environment (OTE) to minimize (or avoid altogether) TCB switching for DB2 requests. Instead of the CICS DB2 adapter having to manage its own private pool of TCBs, CICS provides an L8 mode open TCB (see the CICS System Definition Guide system initialization parameter). Exploiting OTE is particularly important for enterprise beans that make DB2 requests, as it means they need only two, rather than four, TCB switches to process a DB2 request.

To obtain the performance benefits for CICS DB2 applications, the user application programs must be threadsafe. In this case, no TCB switches are needed until the task terminates, unless it issues a non-threadsafe CICS request. If the user application programs are not threadsafe, TCB switching occurs, as in earlier releases, for every DB2 request.

You can use the CICS-supplied utility DFHEISUP to assist you in determining if an application program is threadsafe. See New utility programs for a summary of DFHEISUP, or the CICS Operations and Utilities Guide for more information about it.

The OPENAPI option for the DB2 task-related user exit is set by CICS automatically. See the CICS System Programming Reference manual for information about the OPENAPI option.

Effect on some external interfaces

The introduction of the L8 mode TCB for the DB2 task-related user exit program affects the following external interfaces:

CSD resource definitions for DB2

The interpretation of TCBLIMIT on the DB2CONN resource definition has changed. The change in meaning is as follows:

Table 27.
Attribute CICS TS 1.3 Now
TCBLIMIT (on DB2CONN) Specifies the number of subtask TCBs that CICS can create to process DB2 requests. Specifies the number of L8 mode TCBs that can be used to process DB2 requests. The L8 mode TCBs are allocated from the pool of open TCBs up to the maximum set by system initialization parameter.

INQUIRE DB2CONN TCBS command

The interpretation of the number of TCBs returned on the CEMT, or EXEC CICS, INQUIRE DB2CONN command has changed.

CICS TS 1.3
The subtask TCBs are created and managed by the CICS DB2 adapter to service DB2 requests, and remain permanently associated with a DB2 connection (a command, pool or DB2ENTRY thread). In this case, the TCBS option returns the high-water mark of TCBs created to access DB2.
Now
The L8 mode subtask TCBs are allocated by CICS from the pool of open TCBs. In this case, a DB2 connection is not permanently assigned to the same L8 TCB, and between CICS tasks can move from one L8 mode TCB to another. In this environment, the TCBS option returns the number of L8 mode TCBs that are using a DB2 connection at the time of the inquiry, and this value varies depending on workload.
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