Availability Guide for Problem Management

Problem Management Tools
Availability Guide for Problem Management–125509
9-23
How TM/MP Works
Transaction Protection
Because transactions usually consist of a series of operations, more than one transaction
at a time can threaten database consistency and concurrency, making transaction
management complex.
The TMF subsystem protects the transaction as a single unit, making sure that either all
or none of the changes in a transaction are applied to the database. For example, in a
money transfer transaction, if a power failure occurs just before the credit (operation 2)
and interrupts the system, the TMF subsystem ensures that when the system is restarted,
the debit (operation 1) is not applied to the database. The TMF subsystem protects the
database by backing out any partial transactions applied to the database.
Database Recovery
The TMF subsystem maintains the consistency of the database if a transaction fails to
complete successfully, even if any of the following problems occur:
•
A TMF operator aborts a transaction that an application had previously initiated.
•
Program logic determines that an initiated transaction cannot complete and,
therefore, aborts the transaction.
•
Some single-component failure interrupts a transaction, causing it to be aborted, for
example, the processor processing the transaction fails.
The TMF subsystem provides database recovery using the following methods:
•
Logging updates in audit trails—records information about a transaction’s changes
to a database in a series of files known as audit trails.
•
Backing out transactions—reapplies before-images (copied portions of the data
records before the transaction changed them) to database records to undo the effects
of the aborted transaction.
•
Recovering volumes—ensures that the effects of all committed transactions are
applied to the database files, and that the effects of uncommitted transactions are
backed out.
•
Performing online dumps and audit dumps—copies of database files or audit trail
files are dumped, or copied, to tape or disk while the application is running and the
database is online.
•
Recovering files—recovers database files after a disk media failure or incorrect
program update. TMF uses online and audit dumps to restore consistency to
damaged files.