7 # Multi-level database module for storing hash trees, arrays and simple
8 # key/value pairs into FTP-able, cross-platform binary database files.
10 # Type `perldoc DBM::Deep` for complete documentation.
14 # tie %db, 'DBM::Deep', 'my_database.db'; # standard tie() method
16 # my $db = new DBM::Deep( 'my_database.db' ); # preferred OO method
18 # $db->{my_scalar} = 'hello world';
19 # $db->{my_hash} = { larry => 'genius', hashes => 'fast' };
20 # $db->{my_array} = [ 1, 2, 3, time() ];
21 # $db->{my_complex} = [ 'hello', { perl => 'rules' }, 42, 99 ];
22 # push @{$db->{my_array}}, 'another value';
23 # my @key_list = keys %{$db->{my_hash}};
24 # print "This module " . $db->{my_complex}->[1]->{perl} . "!\n";
27 # (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
28 # This program is free software; you can redistribute it and/or
29 # modify it under the same terms as Perl itself.
37 our $VERSION = q(0.99_01);
39 use Fcntl qw( :DEFAULT :flock :seek );
43 use DBM::Deep::Engine;
47 # Setup constants for users to pass to new()
49 sub TYPE_HASH () { DBM::Deep::Engine->SIG_HASH }
50 sub TYPE_ARRAY () { DBM::Deep::Engine->SIG_ARRAY }
58 $proto->_throw_error( "Odd number of parameters to " . (caller(1))[2] );
63 unless ( eval { local $SIG{'__DIE__'}; %{$_[0]} || 1 } ) {
64 $proto->_throw_error( "Not a hashref in args to " . (caller(1))[2] );
69 $args = { file => shift };
77 # Class constructor method for Perl OO interface.
78 # Calls tie() and returns blessed reference to tied hash or array,
79 # providing a hybrid OO/tie interface.
82 my $args = $class->_get_args( @_ );
85 # Check if we want a tied hash or array.
88 if (defined($args->{type}) && $args->{type} eq TYPE_ARRAY) {
89 $class = 'DBM::Deep::Array';
90 require DBM::Deep::Array;
91 tie @$self, $class, %$args;
94 $class = 'DBM::Deep::Hash';
95 require DBM::Deep::Hash;
96 tie %$self, $class, %$args;
99 return bless $self, $class;
102 # This initializer is called from the various TIE* methods. new() calls tie(),
103 # which allows for a single point of entry.
108 $args->{fileobj} = DBM::Deep::File->new( $args )
109 unless exists $args->{fileobj};
111 # locking implicitly enables autoflush
112 if ($args->{locking}) { $args->{autoflush} = 1; }
114 # These are the defaults to be optionally overridden below
117 base_offset => undef,
124 $self->{engine} = DBM::Deep::Engine->new( { %{$args}, obj => $self } );
126 # Grab the parameters we want to use
127 foreach my $param ( keys %$self ) {
128 next unless exists $args->{$param};
129 $self->{$param} = $args->{$param};
132 $self->_engine->setup_fh( $self );
134 $self->{fileobj}->set_db( $self );
141 require DBM::Deep::Hash;
142 return DBM::Deep::Hash->TIEHASH( @_ );
147 require DBM::Deep::Array;
148 return DBM::Deep::Array->TIEARRAY( @_ );
152 my $self = shift->_get_self;
153 return $self->_fileobj->lock( $self, @_ );
157 my $self = shift->_get_self;
158 return $self->_fileobj->unlock( $self, @_ );
162 my $self = shift->_get_self;
163 my ($spot, $value) = @_;
168 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
169 ${$spot} = $value->_repr;
170 $value->_copy_node( ${$spot} );
173 my $r = Scalar::Util::reftype( $value );
174 my $c = Scalar::Util::blessed( $value );
175 if ( $r eq 'ARRAY' ) {
176 ${$spot} = [ @{$value} ];
179 ${$spot} = { %{$value} };
181 ${$spot} = bless ${$spot}, $c
189 die "Must be implemented in a child class\n";
193 die "Must be implemented in a child class\n";
198 # Recursively export into standard Perl hashes and arrays.
200 my $self = shift->_get_self;
202 my $temp = $self->_repr;
205 $self->_copy_node( $temp );
213 # Recursively import Perl hash/array structure
215 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
217 my $self = shift->_get_self;
220 # struct is not a reference, so just import based on our type
222 $struct = $self->_repr( @_ );
225 return $self->_import( $struct );
230 # Rebuild entire database into new file, then move
231 # it back on top of original.
233 my $self = shift->_get_self;
235 #XXX Need to create a new test for this
236 # if ($self->_fileobj->{links} > 1) {
237 # $self->_throw_error("Cannot optimize: reference count is greater than 1");
240 my $db_temp = DBM::Deep->new(
241 file => $self->_fileobj->{file} . '.tmp',
246 $self->_copy_node( $db_temp );
250 # Attempt to copy user, group and permissions over to new file
252 my @stats = stat($self->_fh);
253 my $perms = $stats[2] & 07777;
256 chown( $uid, $gid, $self->_fileobj->{file} . '.tmp' );
257 chmod( $perms, $self->_fileobj->{file} . '.tmp' );
259 # q.v. perlport for more information on this variable
260 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
262 # Potential race condition when optmizing on Win32 with locking.
263 # The Windows filesystem requires that the filehandle be closed
264 # before it is overwritten with rename(). This could be redone
268 $self->_fileobj->close;
271 if (!rename $self->_fileobj->{file} . '.tmp', $self->_fileobj->{file}) {
272 unlink $self->_fileobj->{file} . '.tmp';
274 $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
278 $self->_fileobj->close;
279 $self->_fileobj->open;
280 $self->_engine->setup_fh( $self );
287 # Make copy of object and return
289 my $self = shift->_get_self;
291 return DBM::Deep->new(
292 type => $self->_type,
293 base_offset => $self->_base_offset,
294 fileobj => $self->_fileobj,
299 my %is_legal_filter = map {
302 store_key store_value
303 fetch_key fetch_value
308 # Setup filter function for storing or fetching the key or value
310 my $self = shift->_get_self;
314 if ( $is_legal_filter{$type} ) {
315 $self->_fileobj->{"filter_$type"} = $func;
324 my $self = shift->_get_self;
325 $self->_fileobj->begin_transaction;
330 my $self = shift->_get_self;
331 $self->_fileobj->end_transaction;
336 my $self = shift->_get_self;
337 $self->_fileobj->commit_transaction;
346 my $self = $_[0]->_get_self;
347 return $self->{engine};
351 my $self = $_[0]->_get_self;
352 return $self->{fileobj};
356 my $self = $_[0]->_get_self;
357 return $self->{type};
361 my $self = $_[0]->_get_self;
362 return $self->{base_offset};
366 my $self = $_[0]->_get_self;
367 return $self->_fileobj->{fh};
375 die "DBM::Deep: $_[1]\n";
380 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
385 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
392 #XXX This if() is redundant
393 if ( my $parent = $self->{parent} ) {
395 while ( $parent->{parent} ) {
397 $parent->_type eq TYPE_HASH
398 ? "\{$child->{parent_key}\}"
399 : "\[$child->{parent_key}\]"
403 $parent = $parent->{parent};
406 $base = "\$db->get( '$child->{parent_key}' )->" . $base;
409 $base = "\$db->get( '$child->{parent_key}' )";
417 # Store single hash key/value or array element in database.
419 my $self = shift->_get_self;
420 my ($key, $value, $orig_key) = @_;
423 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
424 $self->_throw_error( 'Cannot write to a readonly filehandle' );
427 #XXX The second condition needs to disappear
428 if ( defined $orig_key && !( $self->_type eq TYPE_ARRAY && $orig_key eq 'length') ) {
431 my $r = Scalar::Util::reftype( $value ) || '';
432 if ( $r eq 'HASH' ) {
435 elsif ( $r eq 'ARRAY' ) {
438 elsif ( defined $value ) {
445 if ( my $c = Scalar::Util::blessed( $value ) ) {
446 $rhs = "bless $rhs, '$c'";
449 my $lhs = $self->_find_parent;
451 if ( $self->_type eq TYPE_HASH ) {
452 $lhs .= "->\{$orig_key\}";
455 $lhs .= "->\[$orig_key\]";
461 $lhs = "\$db->put('$orig_key',$rhs);";
464 $self->_fileobj->audit($lhs);
468 # Request exclusive lock for writing
470 $self->lock( LOCK_EX );
472 my $md5 = $self->_engine->{digest}->($key);
474 my $tag = $self->_engine->find_blist( $self->_base_offset, $md5, { create => 1 } );
476 # User may be storing a hash, in which case we do not want it run
477 # through the filtering system
478 if ( !ref($value) && $self->_fileobj->{filter_store_value} ) {
479 $value = $self->_fileobj->{filter_store_value}->( $value );
483 # Add key/value to bucket list
485 $self->_engine->add_bucket( $tag, $md5, $key, $value, undef, $orig_key );
494 # Fetch single value or element given plain key or array index
496 my $self = shift->_get_self;
497 my ($key, $orig_key) = @_;
499 my $md5 = $self->_engine->{digest}->($key);
502 # Request shared lock for reading
504 $self->lock( LOCK_SH );
506 my $tag = $self->_engine->find_blist( $self->_base_offset, $md5 );#, { create => 1 } );
507 #XXX This needs to autovivify
514 # Get value from bucket list
516 my $result = $self->_engine->get_bucket_value( $tag, $md5, $orig_key );
520 # Filters only apply to scalar values, so the ref check is making
521 # sure the fetched bucket is a scalar, not a child hash or array.
522 return ($result && !ref($result) && $self->_fileobj->{filter_fetch_value})
523 ? $self->_fileobj->{filter_fetch_value}->($result)
529 # Delete single key/value pair or element given plain key or array index
531 my $self = shift->_get_self;
532 my ($key, $orig_key) = @_;
534 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
535 $self->_throw_error( 'Cannot write to a readonly filehandle' );
538 if ( defined $orig_key ) {
539 my $lhs = $self->_find_parent;
541 $self->_fileobj->audit( "delete $lhs;" );
544 $self->_fileobj->audit( "\$db->delete('$orig_key');" );
549 # Request exclusive lock for writing
551 $self->lock( LOCK_EX );
553 my $md5 = $self->_engine->{digest}->($key);
555 my $tag = $self->_engine->find_blist( $self->_base_offset, $md5 );
564 my $value = $self->_engine->get_bucket_value( $tag, $md5 );
566 if (defined $value && !ref($value) && $self->_fileobj->{filter_fetch_value}) {
567 $value = $self->_fileobj->{filter_fetch_value}->($value);
570 my $result = $self->_engine->delete_bucket( $tag, $md5, $orig_key );
573 # If this object is an array and the key deleted was on the end of the stack,
574 # decrement the length variable.
584 # Check if a single key or element exists given plain key or array index
586 my $self = shift->_get_self;
589 my $md5 = $self->_engine->{digest}->($key);
592 # Request shared lock for reading
594 $self->lock( LOCK_SH );
596 my $tag = $self->_engine->find_blist( $self->_base_offset, $md5 );
601 # For some reason, the built-in exists() function returns '' for false
607 # Check if bucket exists and return 1 or ''
609 my $result = $self->_engine->bucket_exists( $tag, $md5 ) || '';
618 # Clear all keys from hash, or all elements from array.
620 my $self = shift->_get_self;
622 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
623 $self->_throw_error( 'Cannot write to a readonly filehandle' );
627 my $lhs = $self->_find_parent;
629 if ( $self->_type eq TYPE_HASH ) {
630 $lhs = '%{' . $lhs . '}';
633 $lhs = '@{' . $lhs . '}';
636 $self->_fileobj->audit( "$lhs = ();" );
640 # Request exclusive lock for writing
642 $self->lock( LOCK_EX );
644 if ( $self->_type eq TYPE_HASH ) {
645 my $key = $self->first_key;
647 my $next_key = $self->next_key( $key );
648 my $md5 = $self->_engine->{digest}->($key);
649 my $tag = $self->_engine->find_blist( $self->_base_offset, $md5 );
650 $self->_engine->delete_bucket( $tag, $md5, $key );
655 my $size = $self->FETCHSIZE;
656 for my $key ( map { pack ( $self->_engine->{long_pack}, $_ ) } 0 .. $size - 1 ) {
657 my $md5 = $self->_engine->{digest}->($key);
658 my $tag = $self->_engine->find_blist( $self->_base_offset, $md5 );
659 $self->_engine->delete_bucket( $tag, $md5, $key );
661 $self->STORESIZE( 0 );
663 #XXX This needs updating to use _release_space
664 # $self->_engine->write_tag(
665 # $self->_base_offset, $self->_type,
666 # chr(0)x$self->_engine->{index_size},
675 # Public method aliases
677 sub put { (shift)->STORE( @_ ) }
678 sub store { (shift)->STORE( @_ ) }
679 sub get { (shift)->FETCH( @_ ) }
680 sub fetch { (shift)->FETCH( @_ ) }
681 sub delete { (shift)->DELETE( @_ ) }
682 sub exists { (shift)->EXISTS( @_ ) }
683 sub clear { (shift)->CLEAR( @_ ) }
690 DBM::Deep - A pure perl multi-level hash/array DBM
695 my $db = DBM::Deep->new( "foo.db" );
697 $db->{key} = 'value'; # tie() style
700 $db->put('key' => 'value'); # OO style
701 print $db->get('key');
703 # true multi-level support
704 $db->{my_complex} = [
705 'hello', { perl => 'rules' },
711 A unique flat-file database module, written in pure perl. True
712 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
713 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
714 handle millions of keys and unlimited hash levels without significant
715 slow-down. Written from the ground-up in pure perl -- this is NOT a
716 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
717 Mac OS X and Windows.
719 =head1 VERSION DIFFERENCES
721 B<NOTE>: 0.99_01 and above have significant file format differences from 0.98 and
722 before. While attempts have been made to be backwards compatible, no guarantees.
726 Hopefully you are using Perl's excellent CPAN module, which will download
727 and install the module for you. If not, get the tarball, and run these
739 Construction can be done OO-style (which is the recommended way), or using
740 Perl's tie() function. Both are examined here.
742 =head2 OO CONSTRUCTION
744 The recommended way to construct a DBM::Deep object is to use the new()
745 method, which gets you a blessed, tied hash or array reference.
747 my $db = DBM::Deep->new( "foo.db" );
749 This opens a new database handle, mapped to the file "foo.db". If this
750 file does not exist, it will automatically be created. DB files are
751 opened in "r+" (read/write) mode, and the type of object returned is a
752 hash, unless otherwise specified (see L<OPTIONS> below).
754 You can pass a number of options to the constructor to specify things like
755 locking, autoflush, etc. This is done by passing an inline hash:
757 my $db = DBM::Deep->new(
763 Notice that the filename is now specified I<inside> the hash with
764 the "file" parameter, as opposed to being the sole argument to the
765 constructor. This is required if any options are specified.
766 See L<OPTIONS> below for the complete list.
770 You can also start with an array instead of a hash. For this, you must
771 specify the C<type> parameter:
773 my $db = DBM::Deep->new(
775 type => DBM::Deep->TYPE_ARRAY
778 B<Note:> Specifing the C<type> parameter only takes effect when beginning
779 a new DB file. If you create a DBM::Deep object with an existing file, the
780 C<type> will be loaded from the file header, and an error will be thrown if
781 the wrong type is passed in.
783 =head2 TIE CONSTRUCTION
785 Alternately, you can create a DBM::Deep handle by using Perl's built-in
786 tie() function. The object returned from tie() can be used to call methods,
787 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
788 file (as expected with most tie'd objects).
791 my $db = tie %hash, "DBM::Deep", "foo.db";
794 my $db = tie @array, "DBM::Deep", "bar.db";
796 As with the OO constructor, you can replace the DB filename parameter with
797 a hash containing one or more options (see L<OPTIONS> just below for the
800 tie %hash, "DBM::Deep", {
808 There are a number of options that can be passed in when constructing your
809 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
815 Filename of the DB file to link the handle to. You can pass a full absolute
816 filesystem path, partial path, or a plain filename if the file is in the
817 current working directory. This is a required parameter (though q.v. fh).
821 If you want, you can pass in the fh instead of the file. This is most useful for doing
824 my $db = DBM::Deep->new( { fh => \*DATA } );
826 You are responsible for making sure that the fh has been opened appropriately for your
827 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
828 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
829 needs to read from the fh.
833 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
834 not need to set this. However, it's there if you want it.
836 If you pass in fh and do not set this, it will be set appropriately.
840 This parameter specifies what type of object to create, a hash or array. Use
841 one of these two constants:
845 =item * C<DBM::Deep-E<gt>TYPE_HASH>
847 =item * C<DBM::Deep-E<gt>TYPE_ARRAY>.
851 This only takes effect when beginning a new file. This is an optional
852 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
856 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
857 function to lock the database in exclusive mode for writes, and shared mode for
858 reads. Pass any true value to enable. This affects the base DB handle I<and
859 any child hashes or arrays> that use the same DB file. This is an optional
860 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
864 Specifies whether autoflush is to be enabled on the underlying filehandle.
865 This obviously slows down write operations, but is required if you may have
866 multiple processes accessing the same DB file (also consider enable I<locking>).
867 Pass any true value to enable. This is an optional parameter, and defaults to 0
872 If I<autobless> mode is enabled, DBM::Deep will preserve the class something
873 is blessed into, and restores it when fetched. This is an optional parameter, and defaults to 1 (enabled).
875 B<Note:> If you use the OO-interface, you will not be able to call any methods
876 of DBM::Deep on the blessed item. This is considered to be a feature.
880 See L</FILTERS> below.
886 With DBM::Deep you can access your databases using Perl's standard hash/array
887 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
888 treat them as such. DBM::Deep will intercept all reads/writes and direct them
889 to the right place -- the DB file. This has nothing to do with the
890 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
891 using regular hashes and arrays, rather than calling functions like C<get()>
892 and C<put()> (although those work too). It is entirely up to you how to want
893 to access your databases.
897 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
898 or even nested hashes (or arrays) using standard Perl syntax:
900 my $db = DBM::Deep->new( "foo.db" );
902 $db->{mykey} = "myvalue";
904 $db->{myhash}->{subkey} = "subvalue";
906 print $db->{myhash}->{subkey} . "\n";
908 You can even step through hash keys using the normal Perl C<keys()> function:
910 foreach my $key (keys %$db) {
911 print "$key: " . $db->{$key} . "\n";
914 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
915 pushes them onto an array, all before the loop even begins. If you have an
916 extra large hash, this may exhaust Perl's memory. Instead, consider using
917 Perl's C<each()> function, which pulls keys/values one at a time, using very
920 while (my ($key, $value) = each %$db) {
921 print "$key: $value\n";
924 Please note that when using C<each()>, you should always pass a direct
925 hash reference, not a lookup. Meaning, you should B<never> do this:
928 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
930 This causes an infinite loop, because for each iteration, Perl is calling
931 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
932 it effectively keeps returning the first key over and over again. Instead,
933 assign a temporary variable to C<$db->{foo}>, then pass that to each().
937 As with hashes, you can treat any DBM::Deep object like a normal Perl array
938 reference. This includes inserting, removing and manipulating elements,
939 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
940 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
941 or simply be a nested array reference inside a hash. Example:
943 my $db = DBM::Deep->new(
944 file => "foo-array.db",
945 type => DBM::Deep->TYPE_ARRAY
949 push @$db, "bar", "baz";
952 my $last_elem = pop @$db; # baz
953 my $first_elem = shift @$db; # bah
954 my $second_elem = $db->[1]; # bar
956 my $num_elements = scalar @$db;
960 In addition to the I<tie()> interface, you can also use a standard OO interface
961 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
962 array) has its own methods, but both types share the following common methods:
963 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
967 =item * new() / clone()
969 These are the constructor and copy-functions.
971 =item * put() / store()
973 Stores a new hash key/value pair, or sets an array element value. Takes two
974 arguments, the hash key or array index, and the new value. The value can be
975 a scalar, hash ref or array ref. Returns true on success, false on failure.
977 $db->put("foo", "bar"); # for hashes
978 $db->put(1, "bar"); # for arrays
980 =item * get() / fetch()
982 Fetches the value of a hash key or array element. Takes one argument: the hash
983 key or array index. Returns a scalar, hash ref or array ref, depending on the
986 my $value = $db->get("foo"); # for hashes
987 my $value = $db->get(1); # for arrays
991 Checks if a hash key or array index exists. Takes one argument: the hash key
992 or array index. Returns true if it exists, false if not.
994 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
995 if ($db->exists(1)) { print "yay!\n"; } # for arrays
999 Deletes one hash key/value pair or array element. Takes one argument: the hash
1000 key or array index. Returns true on success, false if not found. For arrays,
1001 the remaining elements located after the deleted element are NOT moved over.
1002 The deleted element is essentially just undefined, which is exactly how Perl's
1003 internal arrays work. Please note that the space occupied by the deleted
1004 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
1005 below for details and workarounds.
1007 $db->delete("foo"); # for hashes
1008 $db->delete(1); # for arrays
1012 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1013 value. Please note that the space occupied by the deleted keys/values or
1014 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1015 details and workarounds.
1017 $db->clear(); # hashes or arrays
1019 =item * lock() / unlock()
1025 Recover lost disk space.
1027 =item * import() / export()
1029 Data going in and out.
1035 For hashes, DBM::Deep supports all the common methods described above, and the
1036 following additional methods: C<first_key()> and C<next_key()>.
1042 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1043 fetched in an undefined order (which appears random). Takes no arguments,
1044 returns the key as a scalar value.
1046 my $key = $db->first_key();
1050 Returns the "next" key in the hash, given the previous one as the sole argument.
1051 Returns undef if there are no more keys to be fetched.
1053 $key = $db->next_key($key);
1057 Here are some examples of using hashes:
1059 my $db = DBM::Deep->new( "foo.db" );
1061 $db->put("foo", "bar");
1062 print "foo: " . $db->get("foo") . "\n";
1064 $db->put("baz", {}); # new child hash ref
1065 $db->get("baz")->put("buz", "biz");
1066 print "buz: " . $db->get("baz")->get("buz") . "\n";
1068 my $key = $db->first_key();
1070 print "$key: " . $db->get($key) . "\n";
1071 $key = $db->next_key($key);
1074 if ($db->exists("foo")) { $db->delete("foo"); }
1078 For arrays, DBM::Deep supports all the common methods described above, and the
1079 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1080 C<unshift()> and C<splice()>.
1086 Returns the number of elements in the array. Takes no arguments.
1088 my $len = $db->length();
1092 Adds one or more elements onto the end of the array. Accepts scalars, hash
1093 refs or array refs. No return value.
1095 $db->push("foo", "bar", {});
1099 Fetches the last element in the array, and deletes it. Takes no arguments.
1100 Returns undef if array is empty. Returns the element value.
1102 my $elem = $db->pop();
1106 Fetches the first element in the array, deletes it, then shifts all the
1107 remaining elements over to take up the space. Returns the element value. This
1108 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1111 my $elem = $db->shift();
1115 Inserts one or more elements onto the beginning of the array, shifting all
1116 existing elements over to make room. Accepts scalars, hash refs or array refs.
1117 No return value. This method is not recommended with large arrays -- see
1118 <LARGE ARRAYS> below for details.
1120 $db->unshift("foo", "bar", {});
1124 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1125 -f splice> for usage -- it is too complicated to document here. This method is
1126 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1130 Here are some examples of using arrays:
1132 my $db = DBM::Deep->new(
1134 type => DBM::Deep->TYPE_ARRAY
1137 $db->push("bar", "baz");
1138 $db->unshift("foo");
1141 my $len = $db->length();
1142 print "length: $len\n"; # 4
1144 for (my $k=0; $k<$len; $k++) {
1145 print "$k: " . $db->get($k) . "\n";
1148 $db->splice(1, 2, "biz", "baf");
1150 while (my $elem = shift @$db) {
1151 print "shifted: $elem\n";
1156 Enable automatic file locking by passing a true value to the C<locking>
1157 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1159 my $db = DBM::Deep->new(
1164 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1165 mode for writes, and shared mode for reads. This is required if you have
1166 multiple processes accessing the same database file, to avoid file corruption.
1167 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1168 NFS> below for more.
1170 =head2 EXPLICIT LOCKING
1172 You can explicitly lock a database, so it remains locked for multiple
1173 transactions. This is done by calling the C<lock()> method, and passing an
1174 optional lock mode argument (defaults to exclusive mode). This is particularly
1175 useful for things like counters, where the current value needs to be fetched,
1176 then incremented, then stored again.
1179 my $counter = $db->get("counter");
1181 $db->put("counter", $counter);
1190 You can pass C<lock()> an optional argument, which specifies which mode to use
1191 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1192 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1193 same as the constants defined in Perl's C<Fcntl> module.
1195 $db->lock( DBM::Deep->LOCK_SH );
1199 =head1 IMPORTING/EXPORTING
1201 You can import existing complex structures by calling the C<import()> method,
1202 and export an entire database into an in-memory structure using the C<export()>
1203 method. Both are examined here.
1207 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1208 walking the structure and adding keys/elements to the database as you go,
1209 simply pass a reference to the C<import()> method. This recursively adds
1210 everything to an existing DBM::Deep object for you. Here is an example:
1215 array1 => [ "elem0", "elem1", "elem2" ],
1217 subkey1 => "subvalue1",
1218 subkey2 => "subvalue2"
1222 my $db = DBM::Deep->new( "foo.db" );
1223 $db->import( $struct );
1225 print $db->{key1} . "\n"; # prints "value1"
1227 This recursively imports the entire C<$struct> object into C<$db>, including
1228 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1229 keys are merged with the existing ones, replacing if they already exist.
1230 The C<import()> method can be called on any database level (not just the base
1231 level), and works with both hash and array DB types.
1233 B<Note:> Make sure your existing structure has no circular references in it.
1234 These will cause an infinite loop when importing.
1238 Calling the C<export()> method on an existing DBM::Deep object will return
1239 a reference to a new in-memory copy of the database. The export is done
1240 recursively, so all nested hashes/arrays are all exported to standard Perl
1241 objects. Here is an example:
1243 my $db = DBM::Deep->new( "foo.db" );
1245 $db->{key1} = "value1";
1246 $db->{key2} = "value2";
1248 $db->{hash1}->{subkey1} = "subvalue1";
1249 $db->{hash1}->{subkey2} = "subvalue2";
1251 my $struct = $db->export();
1253 print $struct->{key1} . "\n"; # prints "value1"
1255 This makes a complete copy of the database in memory, and returns a reference
1256 to it. The C<export()> method can be called on any database level (not just
1257 the base level), and works with both hash and array DB types. Be careful of
1258 large databases -- you can store a lot more data in a DBM::Deep object than an
1259 in-memory Perl structure.
1261 B<Note:> Make sure your database has no circular references in it.
1262 These will cause an infinite loop when exporting.
1266 DBM::Deep has a number of hooks where you can specify your own Perl function
1267 to perform filtering on incoming or outgoing data. This is a perfect
1268 way to extend the engine, and implement things like real-time compression or
1269 encryption. Filtering applies to the base DB level, and all child hashes /
1270 arrays. Filter hooks can be specified when your DBM::Deep object is first
1271 constructed, or by calling the C<set_filter()> method at any time. There are
1272 four available filter hooks, described below:
1276 =item * filter_store_key
1278 This filter is called whenever a hash key is stored. It
1279 is passed the incoming key, and expected to return a transformed key.
1281 =item * filter_store_value
1283 This filter is called whenever a hash key or array element is stored. It
1284 is passed the incoming value, and expected to return a transformed value.
1286 =item * filter_fetch_key
1288 This filter is called whenever a hash key is fetched (i.e. via
1289 C<first_key()> or C<next_key()>). It is passed the transformed key,
1290 and expected to return the plain key.
1292 =item * filter_fetch_value
1294 This filter is called whenever a hash key or array element is fetched.
1295 It is passed the transformed value, and expected to return the plain value.
1299 Here are the two ways to setup a filter hook:
1301 my $db = DBM::Deep->new(
1303 filter_store_value => \&my_filter_store,
1304 filter_fetch_value => \&my_filter_fetch
1309 $db->set_filter( "filter_store_value", \&my_filter_store );
1310 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1312 Your filter function will be called only when dealing with SCALAR keys or
1313 values. When nested hashes and arrays are being stored/fetched, filtering
1314 is bypassed. Filters are called as static functions, passed a single SCALAR
1315 argument, and expected to return a single SCALAR value. If you want to
1316 remove a filter, set the function reference to C<undef>:
1318 $db->set_filter( "filter_store_value", undef );
1320 =head2 REAL-TIME ENCRYPTION EXAMPLE
1322 Here is a working example that uses the I<Crypt::Blowfish> module to
1323 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1324 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1325 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1328 use Crypt::Blowfish;
1331 my $cipher = Crypt::CBC->new({
1332 'key' => 'my secret key',
1333 'cipher' => 'Blowfish',
1335 'regenerate_key' => 0,
1336 'padding' => 'space',
1340 my $db = DBM::Deep->new(
1341 file => "foo-encrypt.db",
1342 filter_store_key => \&my_encrypt,
1343 filter_store_value => \&my_encrypt,
1344 filter_fetch_key => \&my_decrypt,
1345 filter_fetch_value => \&my_decrypt,
1348 $db->{key1} = "value1";
1349 $db->{key2} = "value2";
1350 print "key1: " . $db->{key1} . "\n";
1351 print "key2: " . $db->{key2} . "\n";
1357 return $cipher->encrypt( $_[0] );
1360 return $cipher->decrypt( $_[0] );
1363 =head2 REAL-TIME COMPRESSION EXAMPLE
1365 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1366 compression / decompression of keys & values with DBM::Deep Filters.
1367 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1368 more on I<Compress::Zlib>.
1373 my $db = DBM::Deep->new(
1374 file => "foo-compress.db",
1375 filter_store_key => \&my_compress,
1376 filter_store_value => \&my_compress,
1377 filter_fetch_key => \&my_decompress,
1378 filter_fetch_value => \&my_decompress,
1381 $db->{key1} = "value1";
1382 $db->{key2} = "value2";
1383 print "key1: " . $db->{key1} . "\n";
1384 print "key2: " . $db->{key2} . "\n";
1390 return Compress::Zlib::memGzip( $_[0] ) ;
1393 return Compress::Zlib::memGunzip( $_[0] ) ;
1396 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1397 actually numerical index numbers, and are not filtered.
1399 =head1 ERROR HANDLING
1401 Most DBM::Deep methods return a true value for success, and call die() on
1402 failure. You can wrap calls in an eval block to catch the die.
1404 my $db = DBM::Deep->new( "foo.db" ); # create hash
1405 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1407 print $@; # prints error message
1409 =head1 LARGEFILE SUPPORT
1411 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1412 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1413 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1414 by specifying the 'pack_size' parameter when constructing the file.
1417 filename => $filename,
1418 pack_size => 'large',
1421 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1422 instead of 32-bit longs. After setting these values your DB files have a
1423 theoretical maximum size of 16 XB (exabytes).
1425 You can also use C<pack_size =E<gt> 'small'> in order to use 16-bit file
1428 B<Note:> Changing these values will B<NOT> work for existing database files.
1429 Only change this for new files. Once the value has been set, it is stored in
1430 the file's header and cannot be changed for the life of the file. These
1431 parameters are per-file, meaning you can access 32-bit and 64-bit files, as
1434 B<Note:> We have not personally tested files larger than 2 GB -- all my
1435 systems have only a 32-bit Perl. However, I have received user reports that
1436 this does indeed work!
1438 =head1 LOW-LEVEL ACCESS
1440 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1441 you can call the C<_fh()> method, which returns the handle:
1443 my $fh = $db->_fh();
1445 This method can be called on the root level of the datbase, or any child
1446 hashes or arrays. All levels share a I<root> structure, which contains things
1447 like the filehandle, a reference counter, and all the options specified
1448 when you created the object. You can get access to this file object by
1449 calling the C<_fileobj()> method.
1451 my $file_obj = $db->_fileobj();
1453 This is useful for changing options after the object has already been created,
1454 such as enabling/disabling locking. You can also store your own temporary user
1455 data in this structure (be wary of name collision), which is then accessible from
1456 any child hash or array.
1458 =head1 CUSTOM DIGEST ALGORITHM
1460 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1461 keys. However you can override this, and use another algorithm (such as SHA-256)
1462 or even write your own. But please note that DBM::Deep currently expects zero
1463 collisions, so your algorithm has to be I<perfect>, so to speak. Collision
1464 detection may be introduced in a later version.
1466 You can specify a custom digest algorithm by passing it into the parameter
1467 list for new(), passing a reference to a subroutine as the 'digest' parameter,
1468 and the length of the algorithm's hashes (in bytes) as the 'hash_size'
1469 parameter. Here is a working example that uses a 256-bit hash from the
1470 I<Digest::SHA256> module. Please see
1471 L<http://search.cpan.org/search?module=Digest::SHA256> for more information.
1476 my $context = Digest::SHA256::new(256);
1478 my $db = DBM::Deep->new(
1479 filename => "foo-sha.db",
1480 digest => \&my_digest,
1484 $db->{key1} = "value1";
1485 $db->{key2} = "value2";
1486 print "key1: " . $db->{key1} . "\n";
1487 print "key2: " . $db->{key2} . "\n";
1493 return substr( $context->hash($_[0]), 0, 32 );
1496 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1497 of bytes you specify in the hash_size parameter (in this case 32).
1499 B<Note:> If you do choose to use a custom digest algorithm, you must set it
1500 every time you access this file. Otherwise, the default (MD5) will be used.
1502 =head1 CIRCULAR REFERENCES
1504 DBM::Deep has B<experimental> support for circular references. Meaning you
1505 can have a nested hash key or array element that points to a parent object.
1506 This relationship is stored in the DB file, and is preserved between sessions.
1509 my $db = DBM::Deep->new( "foo.db" );
1512 $db->{circle} = $db; # ref to self
1514 print $db->{foo} . "\n"; # prints "bar"
1515 print $db->{circle}->{foo} . "\n"; # prints "bar" again
1517 B<Note>: Passing the object to a function that recursively walks the
1518 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1519 C<export()> methods) will result in an infinite loop. This will be fixed in
1522 =head1 CAVEATS / ISSUES / BUGS
1524 This section describes all the known issues with DBM::Deep. It you have found
1525 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1527 =head2 UNUSED SPACE RECOVERY
1529 One major caveat with DBM::Deep is that space occupied by existing keys and
1530 values is not recovered when they are deleted. Meaning if you keep deleting
1531 and adding new keys, your file will continuously grow. I am working on this,
1532 but in the meantime you can call the built-in C<optimize()> method from time to
1533 time (perhaps in a crontab or something) to recover all your unused space.
1535 $db->optimize(); # returns true on success
1537 This rebuilds the ENTIRE database into a new file, then moves it on top of
1538 the original. The new file will have no unused space, thus it will take up as
1539 little disk space as possible. Please note that this operation can take
1540 a long time for large files, and you need enough disk space to temporarily hold
1541 2 copies of your DB file. The temporary file is created in the same directory
1542 as the original, named with a ".tmp" extension, and is deleted when the
1543 operation completes. Oh, and if locking is enabled, the DB is automatically
1544 locked for the entire duration of the copy.
1546 B<WARNING:> Only call optimize() on the top-level node of the database, and
1547 make sure there are no child references lying around. DBM::Deep keeps a reference
1548 counter, and if it is greater than 1, optimize() will abort and return undef.
1552 (The reasons given assume a high level of Perl understanding, specifically of
1553 references. You can safely skip this section.)
1555 Currently, the only references supported are HASH and ARRAY. The other reference
1556 types (SCALAR, CODE, GLOB, and REF) cannot be supported for various reasons.
1562 These are things like filehandles and other sockets. They can't be supported
1563 because it's completely unclear how DBM::Deep should serialize them.
1565 =item * SCALAR / REF
1567 The discussion here refers to the following type of example:
1574 # In some other process ...
1576 my $val = ${ $db->{key1} };
1578 is( $val, 50, "What actually gets stored in the DB file?" );
1580 The problem is one of synchronization. When the variable being referred to
1581 changes value, the reference isn't notified. This means that the new value won't
1582 be stored in the datafile for other processes to read. There is no TIEREF.
1584 It is theoretically possible to store references to values already within a
1585 DBM::Deep object because everything already is synchronized, but the change to
1586 the internals would be quite large. Specifically, DBM::Deep would have to tie
1587 every single value that is stored. This would bloat the RAM footprint of
1588 DBM::Deep at least twofold (if not more) and be a significant performance drain,
1589 all to support a feature that has never been requested.
1593 L<http://search.cpan.org/search?module=Data::Dump::Streamer> provides a
1594 mechanism for serializing coderefs, including saving off all closure state.
1595 However, just as for SCALAR and REF, that closure state may change without
1596 notifying the DBM::Deep object storing the reference.
1600 =head2 FILE CORRUPTION
1602 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1603 for a 32-bit signature when opened, but other corruption in files can cause
1604 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1605 stuck in an infinite loop depending on the level of corruption. File write
1606 operations are not checked for failure (for speed), so if you happen to run
1607 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1608 be addressed in a later version of DBM::Deep.
1612 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1613 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1614 about setting up your NFS server with a locking daemon, then using lockf() to
1615 lock your files, but your mileage may vary there as well. From what I
1616 understand, there is no real way to do it. However, if you need access to the
1617 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1618 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1620 =head2 COPYING OBJECTS
1622 Beware of copying tied objects in Perl. Very strange things can happen.
1623 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1624 returns a new, blessed, tied hash or array to the same level in the DB.
1626 my $copy = $db->clone();
1628 B<Note>: Since clone() here is cloning the object, not the database location, any
1629 modifications to either $db or $copy will be visible in both.
1633 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1634 These functions cause every element in the array to move, which can be murder
1635 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1636 a different location. This will be addressed in the forthcoming version 1.00.
1638 =head2 WRITEONLY FILES
1640 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1641 writeonly mode. STORE will verify that the filehandle is writable. However, there
1642 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1643 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1647 This section discusses DBM::Deep's speed and memory usage.
1651 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1652 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1653 multi-level hash/array support, and cross-platform FTPable files. Even so,
1654 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1655 with huge databases. Here is some test data:
1657 Adding 1,000,000 keys to new DB file...
1659 At 100 keys, avg. speed is 2,703 keys/sec
1660 At 200 keys, avg. speed is 2,642 keys/sec
1661 At 300 keys, avg. speed is 2,598 keys/sec
1662 At 400 keys, avg. speed is 2,578 keys/sec
1663 At 500 keys, avg. speed is 2,722 keys/sec
1664 At 600 keys, avg. speed is 2,628 keys/sec
1665 At 700 keys, avg. speed is 2,700 keys/sec
1666 At 800 keys, avg. speed is 2,607 keys/sec
1667 At 900 keys, avg. speed is 2,190 keys/sec
1668 At 1,000 keys, avg. speed is 2,570 keys/sec
1669 At 2,000 keys, avg. speed is 2,417 keys/sec
1670 At 3,000 keys, avg. speed is 1,982 keys/sec
1671 At 4,000 keys, avg. speed is 1,568 keys/sec
1672 At 5,000 keys, avg. speed is 1,533 keys/sec
1673 At 6,000 keys, avg. speed is 1,787 keys/sec
1674 At 7,000 keys, avg. speed is 1,977 keys/sec
1675 At 8,000 keys, avg. speed is 2,028 keys/sec
1676 At 9,000 keys, avg. speed is 2,077 keys/sec
1677 At 10,000 keys, avg. speed is 2,031 keys/sec
1678 At 20,000 keys, avg. speed is 1,970 keys/sec
1679 At 30,000 keys, avg. speed is 2,050 keys/sec
1680 At 40,000 keys, avg. speed is 2,073 keys/sec
1681 At 50,000 keys, avg. speed is 1,973 keys/sec
1682 At 60,000 keys, avg. speed is 1,914 keys/sec
1683 At 70,000 keys, avg. speed is 2,091 keys/sec
1684 At 80,000 keys, avg. speed is 2,103 keys/sec
1685 At 90,000 keys, avg. speed is 1,886 keys/sec
1686 At 100,000 keys, avg. speed is 1,970 keys/sec
1687 At 200,000 keys, avg. speed is 2,053 keys/sec
1688 At 300,000 keys, avg. speed is 1,697 keys/sec
1689 At 400,000 keys, avg. speed is 1,838 keys/sec
1690 At 500,000 keys, avg. speed is 1,941 keys/sec
1691 At 600,000 keys, avg. speed is 1,930 keys/sec
1692 At 700,000 keys, avg. speed is 1,735 keys/sec
1693 At 800,000 keys, avg. speed is 1,795 keys/sec
1694 At 900,000 keys, avg. speed is 1,221 keys/sec
1695 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1697 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1698 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1699 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1700 Run time was 12 min 3 sec.
1704 One of the great things about DBM::Deep is that it uses very little memory.
1705 Even with huge databases (1,000,000+ keys) you will not see much increased
1706 memory on your process. DBM::Deep relies solely on the filesystem for storing
1707 and fetching data. Here is output from I</usr/bin/top> before even opening a
1710 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1711 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1713 Basically the process is taking 2,716K of memory. And here is the same
1714 process after storing and fetching 1,000,000 keys:
1716 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1717 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1719 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1720 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1722 =head1 DB FILE FORMAT
1724 In case you were interested in the underlying DB file format, it is documented
1725 here in this section. You don't need to know this to use the module, it's just
1726 included for reference.
1730 DBM::Deep files always start with a 32-bit signature to identify the file type.
1731 This is at offset 0. The signature is "DPDB" in network byte order. This is
1732 checked for when the file is opened and an error will be thrown if it's not found.
1736 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1737 has a standard header containing the type of data, the length of data, and then
1738 the data itself. The type is a single character (1 byte), the length is a
1739 32-bit unsigned long in network byte order, and the data is, well, the data.
1740 Here is how it unfolds:
1744 Immediately after the 32-bit file signature is the I<Master Index> record.
1745 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1746 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1747 depending on how the DBM::Deep object was constructed.
1749 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1750 number). The first 8-bit char of the MD5 signature is the offset into the
1751 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1752 index element is a file offset of the next tag for the key/element in question,
1753 which is usually a I<Bucket List> tag (see below).
1755 The next tag I<could> be another index, depending on how many keys/elements
1756 exist. See L<RE-INDEXING> below for details.
1760 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1761 file offsets to where the actual data is stored. It starts with a standard
1762 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1763 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1764 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1765 When the list fills up, a I<Re-Index> operation is performed (See
1766 L<RE-INDEXING> below).
1770 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1771 index/value pair (in array mode). It starts with a standard tag header with
1772 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1773 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1774 header. The size reported in the tag header is only for the value, but then,
1775 just after the value is another size (32-bit unsigned long) and then the plain
1776 key itself. Since the value is likely to be fetched more often than the plain
1777 key, I figured it would be I<slightly> faster to store the value first.
1779 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1780 record for the nested structure, where the process begins all over again.
1784 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1785 exhausted. Then, when another key/element comes in, the list is converted to a
1786 new index record. However, this index will look at the next char in the MD5
1787 hash, and arrange new Bucket List pointers accordingly. This process is called
1788 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1789 17 (16 + new one) keys/elements are removed from the old Bucket List and
1790 inserted into the new index. Several new Bucket Lists are created in the
1791 process, as a new MD5 char from the key is being examined (it is unlikely that
1792 the keys will all share the same next char of their MD5s).
1794 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1795 when the Bucket Lists will turn into indexes, but the first round tends to
1796 happen right around 4,000 keys. You will see a I<slight> decrease in
1797 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1798 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1799 right around 900,000 keys. This process can continue nearly indefinitely --
1800 right up until the point the I<MD5> signatures start colliding with each other,
1801 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1802 getting struck by lightning while you are walking to cash in your tickets.
1803 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1804 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1805 this is 340 unodecillion, but don't quote me).
1809 When a new key/element is stored, the key (or index number) is first run through
1810 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1811 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1812 for the first char of the signature (in this case I<b0>). If it does not exist,
1813 a new I<Bucket List> is created for our key (and the next 15 future keys that
1814 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1815 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1816 this point, unless we are replacing an existing I<Bucket>), where the actual
1817 data will be stored.
1821 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1822 (or index number), then walking along the indexes. If there are enough
1823 keys/elements in this DB level, there might be nested indexes, each linked to
1824 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1825 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1826 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1827 plain key are stored.
1829 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1830 methods. In this process the indexes are walked systematically, and each key
1831 fetched in increasing MD5 order (which is why it appears random). Once the
1832 I<Bucket> is found, the value is skipped and the plain key returned instead.
1833 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1834 alphabetically sorted. This only happens on an index-level -- as soon as the
1835 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1836 so it's pretty much undefined how the keys will come out -- just like Perl's
1839 =head1 CODE COVERAGE
1841 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1842 B<Devel::Cover> report on this module's test suite.
1844 ----------------------------------- ------ ------ ------ ------ ------ ------
1845 File stmt bran cond sub time total
1846 ----------------------------------- ------ ------ ------ ------ ------ ------
1847 blib/lib/DBM/Deep.pm 94.9 80.6 73.0 100.0 37.9 90.4
1848 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 18.2 98.1
1849 blib/lib/DBM/Deep/Engine.pm 98.9 87.3 80.0 100.0 34.2 95.2
1850 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 9.7 97.3
1851 Total 97.9 85.9 79.7 100.0 100.0 94.3
1852 ----------------------------------- ------ ------ ------ ------ ------ ------
1854 =head1 MORE INFORMATION
1856 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1857 or send email to L<DBM-Deep@googlegroups.com>.
1861 Joseph Huckaby, L<jhuckaby@cpan.org>
1863 Rob Kinyon, L<rkinyon@cpan.org>
1865 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1869 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1870 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1874 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1875 This is free software, you may use it and distribute it under the
1876 same terms as Perl itself.