Introduction to Data Management

Ensuring Database Consistency With TMF
15873 Tandem Computers Incorporated 7-9
The online dump contains the most recent backup copy of the files in your database.
The operator produces these dumps for your archives on a regular basis, by copying
the database to tape or an offline disk where it is safe from the effects of any massive
failure. The rollforward facility requires the latest dump because any disk failure
destroys the current version of the database that would otherwise be used as the
starting point for the recovery.
The rollforward facility also typically requires the use of audit dumps. These dumps
are tape copies of the files that contain the TMF audit trails; the operators create the
tape copies periodically to recover space on disk.
To begin the rollforward process, the operator restores the database under TMF
supervision. First, the operator copies the most recent online dump back to disk.
Then, if any required audit trails were dumped to tape, the operator copies the audit
dumps back to disk. Next, the operator begins rollforward recovery on the archive
copy of the database. Rollforward then reapplies the after-images of all committed
transactions in the audit trails to the archive copy, and backs out the incomplete
transactions. When this operation is complete, the database is restored to the
condition it was in before the disk failure occurred. The rollforward process is
illustrated in Figure 7-6.
Concurrency Control TMF allows you to code each transaction as if it were the only activity in the system.
TMF ensures that while a transaction is being processed, all records modified,
inserted, or deleted by that transaction are locked to prevent concurrent access by
other transactions. This record locking remains in force until a transaction is either
committed or backed out. During this period, all database changes in progress remain
invisible to other concurrent transactions.