Section 4.1.  Beyond Flat Files

4.1. Beyond Flat Files

The word database might conjure up thoughts of the DBI and big expensive servers running expensive software packages,^[*] but a database is really just anything you can get data in to and back out of.

^[*] Or more likely, these days, commodity PCs running free software packages.

Just a step up from the comma-separated text file is the humble DBM database. This exists as a C library in several incarnationsthe most well known being the Sleepycat Berkeley DB, available from http://www.sleepycat.com/download.html, and the GNU libgdbm, from http://www.gnu.org/order/ftp.html. When Perl is compiled and installed, it supplies Perl libraries to interface with the C libraries that it finds and to the SDBM library, which is shipped along with Perl. I prefer to use the Berkeley DB, with its Perl interface DB_File.

DBMs store scalar data in key-value pairs. You can think of them as the on-disk representation of a hash, and, indeed, the Perl interfaces to them are through a tied hash:

    use DB_File;
    tie %persistent, "DB_File", "languages.db" or die $!;
    $persistent{"Thank you"} = "arigatou";

    # ... sometime later ...

    use DB_File;
    tie %persistent, "DB_File", "languages.db" or die $!;
    print $persistent{"Thank you"} # "arigatou"

DBMs, however, have a serious limitationsince they only store key-value pairs of scalar data, they cannot store more complex Perl data structures, such as references, objects, and the like. The other problem with key-value structures like DBMs is that they're very bad at expressing relationships between data. For this, we need a relational database such as Oracle or MySQL. We'll return to this subject later in the chapter to see a way of dealing with the limitations.