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DBM::Deep::Cookbook - Cookbook for DBM::Deep


This is the Cookbook for DBM::Deep. It contains useful tips and tricks, plus some examples of how to do common tasks.


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;

  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;

  sub my_compress {
      my $s = shift;
      return Compress::Zlib::memGzip( $s ) ;
  sub my_decompress {
      my $s = Compress::Zlib::memGunzip( shift ) ;
      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;

  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.


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.


DBM::Deep(3), Digest::MD5(3), Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)