DESCRIPTION
This is the Cookbook for DBM::Deep. It contains useful tips and tricks, plus some examples of how to do common tasks.RECIPES
Unicode data
If possible, it is highly recommended that you upgrade your database to version 2 (using the utils/upgrade_db.pl script in the CPAN distribution), in order to use Unicode.If your databases are still shared by perl installations with older DBM::Deep versions, you can use filters to encode strings on the fly:
my $db = DBM::Deep->new( ... );
my $encode_sub = sub { my $s = shift; utf8::encode($s); $s };
my $decode_sub = sub { my $s = shift; utf8::decode($s); $s };
$db->set_filter( 'store_value' => $encode_sub );
$db->set_filter( 'fetch_value' => $decode_sub );
$db->set_filter( 'store_key' => $encode_sub );
$db->set_filter( 'fetch_key' => $decode_sub );
A previous version of this cookbook recommended using "binmode $db->_fh, ":utf8"", but that is not a good idea, as it could easily corrupt the database.
Real-time Encryption Example
NOTE: This is just an example of how to write a filter. This most definitely should NOT be taken as a proper way to write a filter that does encryption. (Furthermore, it fails to take Unicode into account.)Here is a working example that uses the Crypt::Blowfish module to do real-time encryption / decryption of keys & values with DBM::Deep Filters. Please visit <http://search.cpan.org/search?module=Crypt::Blowfish> for more on Crypt::Blowfish. You'll also need the Crypt::CBC module.
use DBM::Deep;
use Crypt::Blowfish;
use Crypt::CBC;
my $cipher = Crypt::CBC->new({
'key' => 'my secret key',
'cipher' => 'Blowfish',
'iv' => '$KJh#(}q',
'regenerate_key' => 0,
'padding' => 'space',
'prepend_iv' => 0
});
my $db = DBM::Deep->new(
file => "foo-encrypt.db",
filter_store_key => \&my_encrypt,
filter_store_value => \&my_encrypt,
filter_fetch_key => \&my_decrypt,
filter_fetch_value => \&my_decrypt,
);
$db->{key1} = "value1";
$db->{key2} = "value2";
print "key1: " . $db->{key1} . "\n";
print "key2: " . $db->{key2} . "\n";
undef $db;
exit;
sub my_encrypt {
return $cipher->encrypt( $_[0] );
}
sub my_decrypt {
return $cipher->decrypt( $_[0] );
}
Real-time Compression Example
Here is a working example that uses the Compress::Zlib module to do real-time compression / decompression of keys & values with DBM::Deep Filters. Please visit <http://search.cpan.org/search?module=Compress::Zlib> for more on Compress::Zlib.
use DBM::Deep;
use Compress::Zlib;
my $db = DBM::Deep->new(
file => "foo-compress.db",
filter_store_key => \&my_compress,
filter_store_value => \&my_compress,
filter_fetch_key => \&my_decompress,
filter_fetch_value => \&my_decompress,
);
$db->{key1} = "value1";
$db->{key2} = "value2";
print "key1: " . $db->{key1} . "\n";
print "key2: " . $db->{key2} . "\n";
undef $db;
exit;
sub my_compress {
my $s = shift;
utf8::encode($s);
return Compress::Zlib::memGzip( $s ) ;
}
sub my_decompress {
my $s = Compress::Zlib::memGunzip( shift ) ;
utf8::decode($s);
return $s;
}
Note: Filtering of keys only applies to hashes. Array ``keys'' are actually numerical index numbers, and are not filtered.
Custom Digest Algorithm
DBM::Deep by default uses the Message Digest 5 (MD5) algorithm for hashing keys. However you can override this, and use another algorithm (such as SHA-256) or even write your own. But please note that DBM::Deep currently expects zero collisions, so your algorithm has to be perfect, so to speak. Collision detection may be introduced in a later version.You can specify a custom digest algorithm by passing it into the parameter list for new(), passing a reference to a subroutine as the 'digest' parameter, and the length of the algorithm's hashes (in bytes) as the 'hash_size' parameter. Here is a working example that uses a 256-bit hash from the Digest::SHA256 module. Please see <http://search.cpan.org/search?module=Digest::SHA256> for more information.
The value passed to your digest function will be encoded as UTF-8 if the database is in version 2 format or higher.
use DBM::Deep;
use Digest::SHA256;
my $context = Digest::SHA256::new(256);
my $db = DBM::Deep->new(
filename => "foo-sha.db",
digest => \&my_digest,
hash_size => 32,
);
$db->{key1} = "value1";
$db->{key2} = "value2";
print "key1: " . $db->{key1} . "\n";
print "key2: " . $db->{key2} . "\n";
undef $db;
exit;
sub my_digest {
return substr( $context->hash($_[0]), 0, 32 );
}
Note: Your returned digest strings must be EXACTLY the number of bytes you specify in the hash_size parameter (in this case 32). Undefined behavior will occur otherwise.
Note: If you do choose to use a custom digest algorithm, you must set it every time you access this file. Otherwise, the default (MD5) will be used.
PERFORMANCE
Because DBM::Deep is a conncurrent datastore, every change is flushed to disk immediately and every read goes to disk. This means that DBM::Deep functions at the speed of disk (generally 10-20ms) vs. the speed of RAM (generally 50-70ns), or at least 150-200x slower than the comparable in-memory datastructure in Perl.There are several techniques you can use to speed up how DBM::Deep functions.
-
Put it on a ramdisk
The easiest and quickest mechanism to making DBM::Deep run faster is to create a ramdisk and locate the DBM::Deep file there. Doing this as an option may become a feature of DBM::Deep, assuming there is a good ramdisk wrapper on CPAN.
-
Work at the tightest level possible
It is much faster to assign the level of your db that you are working with to an intermediate variable than to re-look it up every time. Thus
# BAD while ( my ($k, $v) = each %{$db->{foo}{bar}{baz}} ) { ... } # GOOD my $x = $db->{foo}{bar}{baz}; while ( my ($k, $v) = each %$x ) { ... } -
Make your file as tight as possible
If you know that you are not going to use more than 65K in your database, consider using the "pack_size => 'small'" option. This will instruct DBM::Deep to use 16bit addresses, meaning that the seek times will be less.