This file is indexed.

/usr/share/perl5/DBM/Deep/Cookbook.pod is in libdbm-deep-perl 2.0014-1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
=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 65K 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