[Chapter 7] 7.2.1 AnyDBM_File - Provide Framework for Multiple DBMs

7.2.1 AnyDBM_File - Provide Framework for Multiple DBMs

use AnyDBM_File;

This module is a "pure virtual base class" - it has nothing of its own. It's just there to inherit from the various DBM packages. By default it inherits from NDBM_File for compatibility with earlier versions of Perl. If it doesn't find NDBM_File, it looks for DB_File, GDBM_File, SDBM_File (which is always there - it comes with Perl), and finally ODBM_File.

Perl's dbmopen function (which now exists only for backward compatibility) actually just calls tie to bind a hash to AnyDBM_File. The effect is to bind the hash to one of the specific DBM classes that AnyDBM_File inherits from.

You can override the defaults and determine which class dbmopen will tie to. Do this by redefining @ISA:

@AnyDBM_File::ISA = qw(DB_File GDBM_File NDBM_File);

Note, however, that an explicit use takes priority over the ordering of @ISA, so that:

use GDBM_File;

will cause the next dbmopen to tie your hash to GDBM_File.

You can tie hash variables directly to the desired class yourself, without using dbmopen or AnyDBM_File. For example, by using multiple DBM implementations, you can copy a database from one format to another:

use Fcntl;         # for O_* values
use NDBM_File;
use DB_File;
tie %oldhash, "NDBM_File", $old_filename, O_RDWR;
tie %newhash, "DB_File",   $new_filename, O_RDWR|O_CREAT|O_EXCL, 0644;
while (($key,$val) = each %oldhash) {
    $newhash{$key} = $val;
}

7.2.1.1 DBM comparisons

Here's a table of the features that the different DBMish packages offer:

Feature	ODBM	NDBM	SDBM	GDBM	BSD-DB
Linkage comes with Perl	Yes	Yes	Yes	Yes	Yes
Source bundled with Perl	No	No	Yes	No	No
Source redistributable	No	No	Yes	GPL	Yes
Often comes with UNIX	Yes	Yes[1]	No	No	No
Builds OK on UNIX	N/A	N/A	Yes	Yes	Yes[2]
Code size	Varies[3]	Varies[3]	Small	Big	Big
Disk usage	Varies[3]	Varies[3]	Small	Big	OK[4]
Speed	Varies[3]	Varies[3]	Slow	OK	Fast
FTPable	No	No	Yes	Yes	Yes
Easy to build	N/A	N/A	Yes	Yes	OK[5]
Block size limits	1k	4k	1k[6]	None	None
Byte-order independent	No	No	No	No	Yes
User-defined sort order	No	No	No	No	Yes
Wildcard lookups	No	No	No	No	Yes

[1] On mixed-universe machines, may be in the BSD compatibility library, which is often shunned.
[2] Providing you have an ANSI C compiler.
[3] Depends on how much your vendor has "tweaked" it.
[4] Can be trimmed if you compile for one access method.
[5] See the DB_File library module. Requires symbolic links.
[6] By default, but can be redefined (at the expense of compatibility with older files).

7.2.1.2 See also

Relevant library modules include: DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File. Related manpages: dbm(3), ndbm(3). Tied variables are discussed extensively in Chapter 5, and the dbmopen entry in Chapter 3, Functions, may also be helpful. You can pick up the unbundled modules from the src/misc/ directory on your nearest CPAN site. Here are the most popular ones, but note that their version numbers may have changed by the time you read this:

http://www.perl.com/CPAN/src/misc/db.1.85.tar.gz
http://www.perl.com/CPAN/src/misc/gdbm-1.7.3.tar.gz