TMF Introduction (G06.26+)

Transaction Protection and Database Recovery
HP NonStop TMF Introduction—522414-002
3-8
Transaction Backout
Transaction Backout
The TMF backout process uses the before-images in the audit trails to undo the effects
of an aborted transaction. TMF can back out transactions without affecting database
consistency because the record locks and two-phase commit feature prevent
transaction interaction, maintaining locks on data until backout completes.
During transaction backout, the backout process writes the before-images (saved in
the audit trails) back to the files that were affected by a failed transaction. After the
effects of a transaction are undone, the locks for the transaction are released.
Initiating Transaction Backout
A transaction is aborted when an event prevents the transaction from being committed.
Possible causes for transaction backout include the following:
•
Under the Pathway environment, a SCREEN COBOL program encounters a
RESTART-TRANSACTION or ABORT-TRANSACTION statement.
•
A SCREEN COBOL program is suspended or aborted because of errors or specific
Pathway environment commands.
•
The CPU of a Pathway environment server that has work in progress fails. The
transaction is aborted, then the terminal control process (TCP) restarts it
automatically.
•
The primary TCP that started a transaction is disabled. The transaction’s changes
are aborted, and the backup TCP restarts the transaction. If there is no backup
TCP, the transaction is aborted but not restarted.
•
The CPU of the primary disk process controlling the disk where an affected data
file resides fails before the transaction is committed.
•
The TMF operator aborts the transaction.
•
The application aborts the transaction.
•
The application terminates because of errors.
•
Communication is lost between the participants of a distributed transaction before
the transaction is committed.
Because the effect of backout is the same as if the transaction had never started, an
application program can recover by simply restarting the transaction from the
beginning, without having to undo the effects of the transaction.