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Programming the Perl DBI by Alligator Descartes, Tim Bunce

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Concurrent Database Access and Locking

Before we start looking at DBM file storage management, we should discuss the issues that were flagged earlier regarding concurrent access to flat-file databases, as these problems affect all relatively low-level storage managers.

The basic problem is that concurrent access to files can result in undefined, and generally wrong, data being stored within the data files of a database. For example, if two users each decided to delete a row from the megalith database using the program shown in the previous section, then during the deletion phase, both users would be operating on the original copy of the database. However, whichever user’s deletion finished first would be overwritten as the second user’s deletion copied their version of the database over the first user’s deletion. The first user’s deletion would appear to have been magically restored. This problem is known as a race condition and can be very tricky to detect as the conditions that cause the problem are difficult to reproduce.

To avoid problems of multiple simultaneous changes, we need to somehow enforce exclusive access to the database for potentially destructive operations such as the insertion, updating, and deletion of records. If every program accessing a database were simply read-only, this problem would not appear, since no data would be changed. However, if any script were to alter data, the consistency of all other processes accessing the data for reading or writing could ...

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