Only one application can allocate and access the
CICS BAC control
file at a time, and access is serialized by means of an exclusive
systems-level enqueue. In this context, the application could be one
of the following:
- A CICS® region in which CICS BAC is active
(until CICS BAC is shut down in the region)
- A CICS BAC batch request utility job step
- The CICS BAC communication server when it is processing requests
from the workstation client
When the
CICS BAC startup procedure runs in the CICS region,
it starts the
CICS BAC subtask, which obtains the enqueue and holds
it until the CICS region is shut down, either normally or abnormally.
If the
CICS BAC batch request utility cannot obtain a systems-level
enqueue using the same resource name (the region applid) used by the
CICS BAC request
server, it knows that the subtask is active and that its batch request
utility
input commands must be processed by EXCI
requests passed to the CICS region. If it succeeds in obtaining
the enqueue, it means that the
CICS BAC request server subtask is not
active and the batch request utility can allocate and open the control
file itself and process itself those
input commands
that can be applied to the control file. This could be any of the SET commands but not the
DEFAULT command, which is applicable, and local, only to the
CICS BAC batch
request utility.