The open transaction environment (OTE) is an environment
where threadsafe application code can run on its own TCB inside the CICS® address space, without interference
from other transactions. Applications that use the OTE can run on
a class of TCB called an open TCB, as well as on the quasi-reentrant
(QR TCB).
Using an open TCB provides the following advantages over using
the QR TCB:
- There is no dispatching of other CICS tasks
on an open TCB.
- Multiple open TCBs can run concurrently.
- An application that runs under an open TCB can issue non-CICS API requests that can involve
the TCB being blocked. If blocking occurs only this open TCB is halted,
and not the whole of CICS.
- When the CICS task ends,
the open TCB can be reused by another CICS task.
TCB modes
CICS manages open TCBs in separate pools, with
each pool containing a different type, or mode, of open TCB. Each
mode has a 2-character identifier to indicate its specific purpose,
and is handled by CICS in a
different way:
- L8 mode TCBs and L9 mode TCBs
- These TCBs are used to run threadsafe programs; that is, programs
defined as CONCURRENCY(THREADSAFE) or CONCURRENCY(REQUIRED) in the
PROGRAM resource definition.
- L8 mode TCBs are used by CONCURRENCY(THREADSAFE) and CONCURRENCY(REQUIRED)
CICSAPI application programs because CICS services
do not require TCB key matching.
- L8 mode TCBs are used for CICSKEY OPENAPI application programs.
- L9 mode TCBs are used for USERKEY OPENAPI application programs.
- SP mode TCB and S8 mode TCBs
- These TCBs are used by CICS to
manage SSL connections and requests to LDAP by using the DFHDDAPX
XPI interface. The S8 TCBs run in a single enclave, which is owned
by the SP TCB and also contains the SSL cache.
- TP mode TCB and T8 mode TCBs
- A TP mode TCB is used by a JVM server to perform system processing.
T8 TCBs are used to run Java™ programs
in a JVMSERVER resource.
- X8 mode TCBs and X9 mode TCBs
- These TCBs are used to run C and C++ programs compiled with the
XPLINK option. X8 TCBs are used for programs in CICS key, and X9 mode TCBs are used for programs
in user key. Each instance of an XPLINK program uses one X8 or X9
TCB. Programs compiled with the XPLINK option must also be defined
in the PROGRAM resource definition as threadsafe. This indication,
and the XPLINK “signature” in the load module, are the only things
required to put the task on an X8 or X9 TCB.
A CICS task
is allowed as many X8 and X9 TCBs as it requires, and these TCBs are
kept only until the program finishes. Each CICS task is allowed at most one L8 and one
L9 TCB, and it keeps an L8 and an L9 TCB from the time it is allocated
to the end of the task. The TCBs then become free, and CICS can allocate them to another task or destroy
them.
TCB storage requirements
The maximum number
of TCBs allowed in each pool is specified by a MAXxxxxTCBS parameter.
When
specifying any of the MAXxxxxTCBS parameters, take
into account TCB storage requirements. All TCBs use real storage,
and virtual storage below 16 MB, so the number of open TCBs that a CICS region
can support is restricted by the amount of storage available both
above and below 16 MB.
For more information about the MAXxxxxTCBS
parameters, see Open TCB pools.
Last updated: Thursday, 27 June 2019