=head1 NAME

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

=head1 DESCRIPTION

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

=head1 RECIPES

=head2 Unicode data

If possible, it is highly recommended that you upgrade your database to
version 2 (using the F<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
C<binmode $db-E<gt>_fh, ":utf8">, but that is I<not> a good idea, as it
could easily corrupt the database.

=head2 Real-time Encryption Example

B<NOTE>: This is just an example of how to write a filter. This most
definitely should B<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 I<Crypt::Blowfish> module to
do real-time encryption / decryption of keys & values with DBM::Deep Filters.
Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
on I<Crypt::Blowfish>. You'll also need the I<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] );
  }

=head2 Real-time Compression Example

Here is a working example that uses the I<Compress::Zlib> module to do real-time
compression / decompression of keys & values with DBM::Deep Filters.
Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
more on I<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;
  }

B<Note:> Filtering of keys only applies to hashes. Array "keys" are
actually numerical index numbers, and are not filtered.

=head1 Custom Digest Algorithm

DBM::Deep by default uses the I<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 I<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
I<Digest::SHA256> module. Please see
L<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 );
  }

B<Note:> Your returned digest strings must be B<EXACTLY> the number
of bytes you specify in the hash_size parameter (in this case 32). Undefined
behavior will occur otherwise.

B<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.

=head1 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.

=over 4

=item * 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.

=item * 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 ) {
    ...
  }

=item * Make your file as tight as possible

If you know that you are not going to use more than 65KB in your database,
consider using the C<pack_size =E<gt> 'small'> option. This will instruct
DBM::Deep to use 16bit addresses, meaning that the seek times will be less.

=back

=head1 SEE ALSO

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

=cut
