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The backout fails because a duplicate key is detected in the AIX indicated by
message DFHFC4701, with a failure code of X'F0'. There is no locking on the
AIX
®
key to prevent the second task taking the key before the end of the first
task’s unit of work. If there is an application requirement for this sort of
operation, you should use the CICS enqueue mechanism to reserve the key
until the end of the unit of work.
5. To ensure that the data being read is up-to-date, the application program
should:
v For files accessed in non-RLS mode, issue a READ UPDATE command
(rather than a simple READ), thus locking the data until the end of the unit
of work
v For files accessed in RLS mode, use the consistent read integrity option.
Implicit enqueuing on logically recoverable TD destinations
CICS provides an enqueuing protection facility for logically recoverable (as distinct
from physically recoverable) transient data destinations in a similar way to that for
recoverable files.
There is one minor difference, however: CICS regards each recoverable destination
as two separate recoverable resources—one for writing and one for reading.
Transient data control commands that invoke implicit enqueuing are:
v WRITEQ TD
v READQ TD
v DELETEQ TD
Thus, for example:
v If a task issues a WRITEQ TD command to a particular destination, the task is
enqueued upon that write destination until the end of the task (or unit of work).
While the task is thus enqueued:
– Another task attempting to write to the same destination is suspended.
– Another task attempting to read from the same destination is allowed to read
only committed data (not data being written in a currently incomplete unit of
work).
v If a task issues a READQ TD command to a particular destination, the task is
enqueued upon that read destination until the end of task (or unit of work).
While the task is thus enqueued:
– Another task attempting to read from the same destination is suspended.
– Another task attempting to write to the same destination is allowed to do so
and will itself enqueue on that write destination until end of task (or unit of
work).
– If a task issues a DELETEQ TD request, the task is enqueued upon both the
read and the write destinations. While the task is thus enqueued, no other
task can read from, or write to, the queue.
Implicit enqueuing on recoverable temporary storage queues
CICS provides the enqueuing protection facility for recoverable temporary storage
queues in a similar way to that for recoverable files on VSAM data sets.
There is one minor difference, however: CICS enqueuing is not invoked for READQ
TS commands, thereby making it possible for one task to read a temporary storage
queue record while another is updating the same record. To avoid this, use explicit
Chapter 13. Programming for recovery 157