COBOL Manual for TNS and TNS/R Programs

Fault-Tolerant Processes
HP COBOL Manual for TNS and TNS/R Programs—522555-006
32-15
Concepts
Before TMF was available, the answer to the first question was to write application
code to back out partial transactions and restore the affected parts of the database to
the condition they were in when the failed transaction began. Further, this application
code had to be fault tolerant to overcome possible single points of failure.
The answer to question two was to stop the system periodically and make tape or disk
backups. Then if a catastrophic failure occurred, you could restore the database from
the tape or disk and redo the transactions that had been done since the backup. This
required considerable design and coding effort, particularly if you had to do it for
several applications.
TMF has packaged the solution to these two questions of backout and recovery. TMF
automatically backs out incomplete transactions and provides a mechanism for
periodically taking online dumps and a roll-forward procedure to automate the
repetition of post-backup transactions.
Topics:
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Concepts
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Designing Programs for the TMF Subsystem
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The TMF Subsystem and Requester Screen Transactions
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The TMF Subsystem for Batch Jobs
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Debugging Programs That Use the TMF Subsystem
Concepts
The basic concepts of TMF are:
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Transactions in General
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Multiple-File Transactions
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Multiple Changes to a Single File
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Defining Transactions
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Audit Trails
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Online Dump and Roll-Forward Recovery
•
Record Locking
Transactions in General
A transaction is a multistep operation that affects the consistency of a database by
transforming the database from one consistent state to a new consistent state. A
transaction is also a unit of recovery. During a transaction, the database can be
inconsistent. But either the entire transaction must happen, or none of it must happen.