Hi
Mohammad,
I hope that the following (snipped from the CICS DB2 Guide) will
help:
<snip>
The CICS DB2 attachment facility includes a CICS DB2 task-related user exit,
DFHD2EX1, that is invoked when an application program makes an SQL request. It
manages the process of acquiring a thread connection into DB2, and of returning
control to the application program when the DB2 processing is
complete.
When CICS is connected to DB2 Version 5 or earlier, the CICS DB2
task-related user
exit operates as a quasi-reentrant task-related user exit program. It runs
on the CICS
main TCB (the QR TCB) and uses its own subtask thread TCBs to run threads,
switching to and from the subtask thread TCBs for each DB2 request.
However, when
CICS is connected to DB2 Version 6 or later, the CICS DB2 attachment
facility exploits
the open transaction environment (OTE), to enable the CICS DB2 task-related
user exit
to invoke and return from DB2 without switching TCBs. In the open transaction
environment, the CICS DB2 task-related user exit operates as a threadsafe
and open
API task-related user exit program--it is automatically enabled using the
OPENAPI
option on the ENABLE PROGRAM command during connection processing. This
enables it to receive control on an open L8 mode TCB. Requests to DB2 are also
issued on the L8 TCB, so it acts as the thread TCB, and no switch to a
subtask TCB is
needed. For full details of the CICS DB2 configuration needed to support
the open
transaction environment, see "Migrating to a different release of DB2" in
topic 2.3.
In the open transaction environment, if the user application program that
invoked the
task-related user exit conforms to threadsafe coding conventions and is
defined to
CICS as threadsafe, it can also run on the L8 TCB. Before its first SQL
request, the ap
plication program runs on the CICS main TCB, the QR TCB. When it makes an SQL
request and invokes the task-related user exit, control passes to the L8
TCB, and DB2
processing is carried out. On return from DB2, if the application program
is threadsafe,
it now continues to run on the L8 TCB. Where the correct conditions are
met, the use
of open TCBs for CICS DB2 applications decreases usage of the QR TCB, and
avoids
TCB switching. An ideal CICS DB2 application program for the open transaction
environment is a threadsafe program, containing only threadsafe EXEC CICS
commands, and using only threadsafe user exit programs. An application like
this will
move to an L8 TCB when it makes its first SQL request, and then continue to
run on the
L8 TCB through any amount of DB2 requests and application code, requiring
no TCB
switching. This situation produces a significant performance improvement
where an
application program issues multiple SQL calls. The gains are also
significant when
using an enterprise bean, because when enterprise beans make DB2 requests, they
require additional TCB switches to and from the enterprise bean's own TCB
(see "Using
JDBC and SQLJ in enterprise beans: special considerations" in topic 8.9).
If the
application program does not issue many SQL calls, the performance benefits
might not
be as significant.
</snip>
Regards,
Don
******
Don Deese, Computer Management Sciences,
Inc.
Voice: (804) 776-7109 Fax: (804)
776-7139
http://www.cpexpert.org
******
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to [email protected] with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
