Practical mod_perl

Chapter 19. DBM and mod_perl

Some of the earliest databases implemented on Unix were Database Management (DBM) files, and many are still in use today. As of this writing, the Berkeley DB is the most powerful DBM implementation. Berkeley DB is available at http://www.sleepycat.com/. If you need a light database with an easy API, using simple key-value pairs to store and manipulate a relatively small number of records, DBM is the solution that you should consider first.

With DBM, it is rare to read the whole database into memory. Combine this feature with the use of smart storage techniques, and DBM files can be manipulated much faster than flat files. Flat-file databases can be very slow when the number of records starts to grow into the thousands, especially for insert, update, and delete operations. Sort algorithms on flat files can also be very time-consuming.

The maximum practical size of a DBM database depends on many factors, such as your data, your hardware, and the desired response times. But as a rough guide, consider 5,000 to 10,000 records to be reasonable.

We will talk mostly about Berkeley DB Version 1.x, as it provides the best functionality while having good speed and almost no limitations. Other implementations might be faster in some cases, but they are limited either in the length of the maximum value or the total number of records.

There are a number of Perl interfaces to the major DBM implementations, such as DB_File, NDBM_File, ODBM_File, GDBM_File, and ...