HP 3000 Manuals HP 3000 Manuals
User Logging Applications
An application, which uses multiple files, may require multiple actions
in order to record information for a single event. For example, an order
entry system, where an order is placed for an item must be reflected in
files containing the following types of data:
* Shipping data
* The inventory
* The cost of goods sold
* The accounts receivable
If this data is in separate files, each file must be updated with a
separate call to FWRITE. If the shipping data was updated, but a program
abort or system failure occurred before the accounts receivable was
updated, then the order might be shipped but never billed. Depending
upon the point at which the failure occurred, other files may have
inconsistencies in their data such as:
* The item may be shipped, but still be included in inventory.
* The inventory may be reduced, but cost of goods sold was not
increased.
The collection of all actions (such as, calls to FWRITE) which must be
complete, in order for all data involved to be consistent, will be
referred to as a single transaction. A transaction is complete only
after all of these actions are completed.
This type of application may utilize User Logging to record information
about each action which makes up the transaction. The log record could
contain information about the transaction and the user could then write a
recovery program to reapply to the backup copies of the data structures,
the changes for those transactions which completed. This recovery
program would not reapply changes which were part of transactions, that
did not fully complete. This ensures the preservation of data integrity
through a program abort or system failure.