COBOL Manual for TNS and TNS/R Programs

Fault-Tolerant Processes
HP COBOL Manual for TNS and TNS/R Programs—522555-006
32-16
Concepts
Each transaction has a unique transaction identifier. The identifier includes information
about the starting time of the transaction.
If your transaction involves more than one file or if the single file it involves is available
to other transactions, you can introduce inconsistency into the database.
Multiple-File Transactions
Suppose your process is maintaining a corporate personnel system in which each
department has its own file of information. You want to move an employee’s record
from one file to another. It does not matter whether the process writes the record in the
new file before deleting it from the old file or the reverse. During the interval between
the two events, the database is inconsistent. The employee record is either absent
from the database or present in two places.
Multiple Changes to a Single File
Suppose that the corporate personnel system does not use multiple files, but a single
file. If you want to move an employee’s record from one department’s portion of the file
to another department’s portion of the file, the same problem arises as that with
multiple files. If the record is deleted from one place in the file and written to another
place in the file, there is an interval during which the database is inconsistent.
Defining Transactions
Because you, as the application programmer, are in the best position to define what
constitutes a transaction, TMF gives you the authority to specify the beginning and the
end of the transaction.
In a Pathway application, the SCREEN COBOL terminal program uses the BEGIN-
TRANSACTION and END-TRANSACTION statements for this purpose.
In TAL, FORTRAN, and HP COBOL programs outside of the Pathway environment,
you must call the operating system routines BEGINTRANSACTION and
ENDTRANSACTION explicitly.
If a terminal program determines that a transaction cannot or must not complete, it
uses the SCREEN COBOL ABORT-TRANSACTION statement to instruct TMF to back
it out and not retry it. If a process written in TAL, FORTRAN, or HP COBOL needs to
abort a transaction, it calls the ABORTTRANSACTION routine.
If a terminal program determines that a transaction should be aborted and retried as a
new transaction (because of a locking contention problem, for example), it can use the
SCREEN COBOL RESTART-TRANSACTION statement to initiate the operation. The
TAL, FORTRAN, or HP COBOL process must explicitly use the
ABORTTRANSACTION routine and handle its own restarting.
After TMF is informed of the boundaries of a transaction, it can assure that either the
entire transaction is performed or none of it is performed. TMF does this by using audit
trails.