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 use Fcntl qw( :DEFAULT :flock :seek );
41 use DBM::Deep::Engine;
44 use vars qw( $VERSION );
45 $VERSION = q(0.99_01);
48 # Setup constants for users to pass to new()
50 sub TYPE_HASH () { DBM::Deep::Engine->SIG_HASH }
51 sub TYPE_ARRAY () { DBM::Deep::Engine->SIG_ARRAY }
59 $proto->_throw_error( "Odd number of parameters to " . (caller(1))[2] );
64 unless ( eval { local $SIG{'__DIE__'}; %{$_[0]} || 1 } ) {
65 $proto->_throw_error( "Not a hashref in args to " . (caller(1))[2] );
70 $args = { file => shift };
78 # Class constructor method for Perl OO interface.
79 # Calls tie() and returns blessed reference to tied hash or array,
80 # providing a hybrid OO/tie interface.
83 my $args = $class->_get_args( @_ );
86 # Check if we want a tied hash or array.
89 if (defined($args->{type}) && $args->{type} eq TYPE_ARRAY) {
90 $class = 'DBM::Deep::Array';
91 require DBM::Deep::Array;
92 tie @$self, $class, %$args;
95 $class = 'DBM::Deep::Hash';
96 require DBM::Deep::Hash;
97 tie %$self, $class, %$args;
100 return bless $self, $class;
103 # This initializer is called from the various TIE* methods. new() calls tie(),
104 # which allows for a single point of entry.
109 $args->{fileobj} = DBM::Deep::File->new( $args )
110 unless exists $args->{fileobj};
112 # locking implicitly enables autoflush
113 if ($args->{locking}) { $args->{autoflush} = 1; }
115 # These are the defaults to be optionally overridden below
118 base_offset => undef,
125 $self->{engine} = DBM::Deep::Engine->new( { %{$args}, obj => $self } );
127 # Grab the parameters we want to use
128 foreach my $param ( keys %$self ) {
129 next unless exists $args->{$param};
130 $self->{$param} = $args->{$param};
133 $self->{engine}->setup_fh( $self );
135 $self->{fileobj}->set_db( $self );
142 require DBM::Deep::Hash;
143 return DBM::Deep::Hash->TIEHASH( @_ );
148 require DBM::Deep::Array;
149 return DBM::Deep::Array->TIEARRAY( @_ );
153 my $self = shift->_get_self;
154 return $self->_fileobj->lock( $self, @_ );
158 my $self = shift->_get_self;
159 return $self->_fileobj->unlock( $self, @_ );
163 my $self = shift->_get_self;
164 my ($spot, $value) = @_;
169 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
170 ${$spot} = $value->_repr;
171 $value->_copy_node( ${$spot} );
174 my $r = Scalar::Util::reftype( $value );
175 my $c = Scalar::Util::blessed( $value );
176 if ( $r eq 'ARRAY' ) {
177 ${$spot} = [ @{$value} ];
180 ${$spot} = { %{$value} };
182 ${$spot} = bless ${$spot}, $c
190 die "Must be implemented in a child class\n";
194 die "Must be implemented in a child class\n";
199 # Recursively export into standard Perl hashes and arrays.
201 my $self = shift->_get_self;
203 my $temp = $self->_repr;
206 $self->_copy_node( $temp );
214 # Recursively import Perl hash/array structure
216 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
218 my $self = shift->_get_self;
221 # struct is not a reference, so just import based on our type
223 $struct = $self->_repr( @_ );
226 return $self->_import( $struct );
231 # Rebuild entire database into new file, then move
232 # it back on top of original.
234 my $self = shift->_get_self;
236 #XXX Need to create a new test for this
237 # if ($self->_fileobj->{links} > 1) {
238 # $self->_throw_error("Cannot optimize: reference count is greater than 1");
241 my $db_temp = DBM::Deep->new(
242 file => $self->_fileobj->{file} . '.tmp',
247 $self->_copy_node( $db_temp );
251 # Attempt to copy user, group and permissions over to new file
253 my @stats = stat($self->_fh);
254 my $perms = $stats[2] & 07777;
257 chown( $uid, $gid, $self->_fileobj->{file} . '.tmp' );
258 chmod( $perms, $self->_fileobj->{file} . '.tmp' );
260 # q.v. perlport for more information on this variable
261 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
263 # Potential race condition when optmizing on Win32 with locking.
264 # The Windows filesystem requires that the filehandle be closed
265 # before it is overwritten with rename(). This could be redone
269 $self->_fileobj->close;
272 if (!rename $self->_fileobj->{file} . '.tmp', $self->_fileobj->{file}) {
273 unlink $self->_fileobj->{file} . '.tmp';
275 $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
279 $self->_fileobj->close;
280 $self->_fileobj->open;
281 $self->{engine}->setup_fh( $self );
288 # Make copy of object and return
290 my $self = shift->_get_self;
292 return DBM::Deep->new(
293 type => $self->_type,
294 base_offset => $self->_base_offset,
295 fileobj => $self->_fileobj,
300 my %is_legal_filter = map {
303 store_key store_value
304 fetch_key fetch_value
309 # Setup filter function for storing or fetching the key or value
311 my $self = shift->_get_self;
315 if ( $is_legal_filter{$type} ) {
316 $self->_fileobj->{"filter_$type"} = $func;
325 my $self = shift->_get_self;
326 $self->_fileobj->begin_transaction;
331 my $self = shift->_get_self;
332 $self->_fileobj->end_transaction;
337 my $self = shift->_get_self;
338 $self->_fileobj->commit_transaction;
347 my $self = $_[0]->_get_self;
348 return $self->{fileobj};
352 my $self = $_[0]->_get_self;
353 return $self->{type};
357 my $self = $_[0]->_get_self;
358 return $self->{base_offset};
362 my $self = $_[0]->_get_self;
363 return $self->_fileobj->{fh};
371 die "DBM::Deep: $_[1]\n";
376 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
381 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
388 if ( my $parent = $self->{parent} ) {
390 while ( $parent->{parent} ) {
392 $parent->_type eq TYPE_HASH
393 ? "\{$child->{parent_key}\}"
394 : "\[$child->{parent_key}\]"
395 # "->get('$child->{parent_key}')"
399 $parent = $parent->{parent};
400 # last unless $parent;
403 $base = "\$db->get( '$child->{parent_key}' )->" . $base;
406 $base = "\$db->get( '$child->{parent_key}' )";
409 # return '$db->' . $base;
410 # return '$db' . $base;
416 # Store single hash key/value or array element in database.
418 my $self = shift->_get_self;
419 my ($key, $value, $orig_key) = @_;
422 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
423 $self->_throw_error( 'Cannot write to a readonly filehandle' );
426 if ( defined $orig_key ) {
429 my $r = Scalar::Util::reftype( $value ) || '';
430 if ( $r eq 'HASH' ) {
433 elsif ( $r eq 'ARRAY' ) {
436 elsif ( defined $value ) {
443 if ( my $c = Scalar::Util::blessed( $value ) ) {
444 $rhs = "bless $rhs, '$c'";
447 my $lhs = $self->_find_parent;
449 if ( $self->_type eq TYPE_HASH ) {
450 $lhs .= "->\{$orig_key\}";
453 $lhs .= "->\[$orig_key\]";
459 $lhs = "\$db->put('$orig_key',$rhs);";
462 # $self->_fileobj->audit( "$lhs = $rhs;" );
463 # $self->_fileobj->audit( "$lhs $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_bucket_list( $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 my $result = $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_bucket_list( $self->_base_offset, $md5 );
513 # Get value from bucket list
515 my $result = $self->{engine}->get_bucket_value( $tag, $md5, $orig_key );
519 # Filters only apply to scalar values, so the ref check is making
520 # sure the fetched bucket is a scalar, not a child hash or array.
521 return ($result && !ref($result) && $self->_fileobj->{filter_fetch_value})
522 ? $self->_fileobj->{filter_fetch_value}->($result)
528 # Delete single key/value pair or element given plain key or array index
530 my $self = shift->_get_self;
531 my ($key, $orig_key) = @_;
533 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
534 $self->_throw_error( 'Cannot write to a readonly filehandle' );
537 if ( defined $orig_key ) {
538 my $lhs = $self->_find_parent;
539 # if ( $self->_type eq TYPE_HASH ) {
540 # $lhs .= "\{$orig_key\}";
543 # $lhs .= "\[$orig_key]\]";
546 # $self->_fileobj->audit( "delete $lhs;" );
547 # $self->_fileobj->audit( "$lhs->delete('$orig_key');" );
549 $self->_fileobj->audit( "delete $lhs;" );
552 $self->_fileobj->audit( "\$db->delete('$orig_key');" );
557 # Request exclusive lock for writing
559 $self->lock( LOCK_EX );
561 my $md5 = $self->{engine}{digest}->($key);
563 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
572 my $value = $self->{engine}->get_bucket_value( $tag, $md5 );
574 if (defined $value && !ref($value) && $self->_fileobj->{filter_fetch_value}) {
575 $value = $self->_fileobj->{filter_fetch_value}->($value);
578 my $result = $self->{engine}->delete_bucket( $tag, $md5, $orig_key );
581 # If this object is an array and the key deleted was on the end of the stack,
582 # decrement the length variable.
592 # Check if a single key or element exists given plain key or array index
594 my $self = shift->_get_self;
597 my $md5 = $self->{engine}{digest}->($key);
600 # Request shared lock for reading
602 $self->lock( LOCK_SH );
604 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
609 # For some reason, the built-in exists() function returns '' for false
615 # Check if bucket exists and return 1 or ''
617 my $result = $self->{engine}->bucket_exists( $tag, $md5 ) || '';
626 # Clear all keys from hash, or all elements from array.
628 my $self = shift->_get_self;
630 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
631 $self->_throw_error( 'Cannot write to a readonly filehandle' );
635 my $lhs = $self->_find_parent;
637 if ( $self->_type eq TYPE_HASH ) {
638 $lhs = '%{' . $lhs . '}';
641 $lhs = '@{' . $lhs . '}';
644 $self->_fileobj->audit( "$lhs = ();" );
648 # Request exclusive lock for writing
650 $self->lock( LOCK_EX );
652 #XXX This needs updating to use _release_space
653 $self->{engine}->write_tag(
654 $self->_base_offset, $self->_type,
655 chr(0)x$self->{engine}{index_size},
664 # Public method aliases
666 sub put { (shift)->STORE( @_ ) }
667 sub store { (shift)->STORE( @_ ) }
668 sub get { (shift)->FETCH( @_ ) }
669 sub fetch { (shift)->FETCH( @_ ) }
670 sub delete { (shift)->DELETE( @_ ) }
671 sub exists { (shift)->EXISTS( @_ ) }
672 sub clear { (shift)->CLEAR( @_ ) }
679 DBM::Deep - A pure perl multi-level hash/array DBM
684 my $db = DBM::Deep->new( "foo.db" );
686 $db->{key} = 'value'; # tie() style
689 $db->put('key' => 'value'); # OO style
690 print $db->get('key');
692 # true multi-level support
693 $db->{my_complex} = [
694 'hello', { perl => 'rules' },
700 A unique flat-file database module, written in pure perl. True
701 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
702 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
703 handle millions of keys and unlimited hash levels without significant
704 slow-down. Written from the ground-up in pure perl -- this is NOT a
705 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
706 Mac OS X and Windows.
708 =head1 VERSION DIFFERENCES
710 B<NOTE>: 0.99_01 and above have significant file format differences from 0.98 and
711 before. While attempts have been made to be backwards compatible, no guarantees.
715 Hopefully you are using Perl's excellent CPAN module, which will download
716 and install the module for you. If not, get the tarball, and run these
728 Construction can be done OO-style (which is the recommended way), or using
729 Perl's tie() function. Both are examined here.
731 =head2 OO CONSTRUCTION
733 The recommended way to construct a DBM::Deep object is to use the new()
734 method, which gets you a blessed, tied hash or array reference.
736 my $db = DBM::Deep->new( "foo.db" );
738 This opens a new database handle, mapped to the file "foo.db". If this
739 file does not exist, it will automatically be created. DB files are
740 opened in "r+" (read/write) mode, and the type of object returned is a
741 hash, unless otherwise specified (see L<OPTIONS> below).
743 You can pass a number of options to the constructor to specify things like
744 locking, autoflush, etc. This is done by passing an inline hash:
746 my $db = DBM::Deep->new(
752 Notice that the filename is now specified I<inside> the hash with
753 the "file" parameter, as opposed to being the sole argument to the
754 constructor. This is required if any options are specified.
755 See L<OPTIONS> below for the complete list.
759 You can also start with an array instead of a hash. For this, you must
760 specify the C<type> parameter:
762 my $db = DBM::Deep->new(
764 type => DBM::Deep->TYPE_ARRAY
767 B<Note:> Specifing the C<type> parameter only takes effect when beginning
768 a new DB file. If you create a DBM::Deep object with an existing file, the
769 C<type> will be loaded from the file header, and an error will be thrown if
770 the wrong type is passed in.
772 =head2 TIE CONSTRUCTION
774 Alternately, you can create a DBM::Deep handle by using Perl's built-in
775 tie() function. The object returned from tie() can be used to call methods,
776 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
777 file (as expected with most tie'd objects).
780 my $db = tie %hash, "DBM::Deep", "foo.db";
783 my $db = tie @array, "DBM::Deep", "bar.db";
785 As with the OO constructor, you can replace the DB filename parameter with
786 a hash containing one or more options (see L<OPTIONS> just below for the
789 tie %hash, "DBM::Deep", {
797 There are a number of options that can be passed in when constructing your
798 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
804 Filename of the DB file to link the handle to. You can pass a full absolute
805 filesystem path, partial path, or a plain filename if the file is in the
806 current working directory. This is a required parameter (though q.v. fh).
810 If you want, you can pass in the fh instead of the file. This is most useful for doing
813 my $db = DBM::Deep->new( { fh => \*DATA } );
815 You are responsible for making sure that the fh has been opened appropriately for your
816 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
817 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
818 needs to read from the fh.
822 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
823 not need to set this. However, it's there if you want it.
825 If you pass in fh and do not set this, it will be set appropriately.
829 This parameter specifies what type of object to create, a hash or array. Use
830 one of these two constants:
834 =item * C<DBM::Deep-E<gt>TYPE_HASH>
836 =item * C<DBM::Deep-E<gt>TYPE_ARRAY>.
840 This only takes effect when beginning a new file. This is an optional
841 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
845 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
846 function to lock the database in exclusive mode for writes, and shared mode for
847 reads. Pass any true value to enable. This affects the base DB handle I<and
848 any child hashes or arrays> that use the same DB file. This is an optional
849 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
853 Specifies whether autoflush is to be enabled on the underlying filehandle.
854 This obviously slows down write operations, but is required if you may have
855 multiple processes accessing the same DB file (also consider enable I<locking>).
856 Pass any true value to enable. This is an optional parameter, and defaults to 0
861 If I<autobless> mode is enabled, DBM::Deep will preserve the class something
862 is blessed into, and restores it when fetched. This is an optional parameter, and defaults to 1 (enabled).
864 B<Note:> If you use the OO-interface, you will not be able to call any methods
865 of DBM::Deep on the blessed item. This is considered to be a feature.
869 See L</FILTERS> below.
875 With DBM::Deep you can access your databases using Perl's standard hash/array
876 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
877 treat them as such. DBM::Deep will intercept all reads/writes and direct them
878 to the right place -- the DB file. This has nothing to do with the
879 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
880 using regular hashes and arrays, rather than calling functions like C<get()>
881 and C<put()> (although those work too). It is entirely up to you how to want
882 to access your databases.
886 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
887 or even nested hashes (or arrays) using standard Perl syntax:
889 my $db = DBM::Deep->new( "foo.db" );
891 $db->{mykey} = "myvalue";
893 $db->{myhash}->{subkey} = "subvalue";
895 print $db->{myhash}->{subkey} . "\n";
897 You can even step through hash keys using the normal Perl C<keys()> function:
899 foreach my $key (keys %$db) {
900 print "$key: " . $db->{$key} . "\n";
903 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
904 pushes them onto an array, all before the loop even begins. If you have an
905 extra large hash, this may exhaust Perl's memory. Instead, consider using
906 Perl's C<each()> function, which pulls keys/values one at a time, using very
909 while (my ($key, $value) = each %$db) {
910 print "$key: $value\n";
913 Please note that when using C<each()>, you should always pass a direct
914 hash reference, not a lookup. Meaning, you should B<never> do this:
917 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
919 This causes an infinite loop, because for each iteration, Perl is calling
920 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
921 it effectively keeps returning the first key over and over again. Instead,
922 assign a temporary variable to C<$db->{foo}>, then pass that to each().
926 As with hashes, you can treat any DBM::Deep object like a normal Perl array
927 reference. This includes inserting, removing and manipulating elements,
928 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
929 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
930 or simply be a nested array reference inside a hash. Example:
932 my $db = DBM::Deep->new(
933 file => "foo-array.db",
934 type => DBM::Deep->TYPE_ARRAY
938 push @$db, "bar", "baz";
941 my $last_elem = pop @$db; # baz
942 my $first_elem = shift @$db; # bah
943 my $second_elem = $db->[1]; # bar
945 my $num_elements = scalar @$db;
949 In addition to the I<tie()> interface, you can also use a standard OO interface
950 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
951 array) has its own methods, but both types share the following common methods:
952 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
956 =item * new() / clone()
958 These are the constructor and copy-functions.
960 =item * put() / store()
962 Stores a new hash key/value pair, or sets an array element value. Takes two
963 arguments, the hash key or array index, and the new value. The value can be
964 a scalar, hash ref or array ref. Returns true on success, false on failure.
966 $db->put("foo", "bar"); # for hashes
967 $db->put(1, "bar"); # for arrays
969 =item * get() / fetch()
971 Fetches the value of a hash key or array element. Takes one argument: the hash
972 key or array index. Returns a scalar, hash ref or array ref, depending on the
975 my $value = $db->get("foo"); # for hashes
976 my $value = $db->get(1); # for arrays
980 Checks if a hash key or array index exists. Takes one argument: the hash key
981 or array index. Returns true if it exists, false if not.
983 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
984 if ($db->exists(1)) { print "yay!\n"; } # for arrays
988 Deletes one hash key/value pair or array element. Takes one argument: the hash
989 key or array index. Returns true on success, false if not found. For arrays,
990 the remaining elements located after the deleted element are NOT moved over.
991 The deleted element is essentially just undefined, which is exactly how Perl's
992 internal arrays work. Please note that the space occupied by the deleted
993 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
994 below for details and workarounds.
996 $db->delete("foo"); # for hashes
997 $db->delete(1); # for arrays
1001 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1002 value. Please note that the space occupied by the deleted keys/values or
1003 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1004 details and workarounds.
1006 $db->clear(); # hashes or arrays
1008 =item * lock() / unlock()
1014 Recover lost disk space.
1016 =item * import() / export()
1018 Data going in and out.
1024 For hashes, DBM::Deep supports all the common methods described above, and the
1025 following additional methods: C<first_key()> and C<next_key()>.
1031 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1032 fetched in an undefined order (which appears random). Takes no arguments,
1033 returns the key as a scalar value.
1035 my $key = $db->first_key();
1039 Returns the "next" key in the hash, given the previous one as the sole argument.
1040 Returns undef if there are no more keys to be fetched.
1042 $key = $db->next_key($key);
1046 Here are some examples of using hashes:
1048 my $db = DBM::Deep->new( "foo.db" );
1050 $db->put("foo", "bar");
1051 print "foo: " . $db->get("foo") . "\n";
1053 $db->put("baz", {}); # new child hash ref
1054 $db->get("baz")->put("buz", "biz");
1055 print "buz: " . $db->get("baz")->get("buz") . "\n";
1057 my $key = $db->first_key();
1059 print "$key: " . $db->get($key) . "\n";
1060 $key = $db->next_key($key);
1063 if ($db->exists("foo")) { $db->delete("foo"); }
1067 For arrays, DBM::Deep supports all the common methods described above, and the
1068 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1069 C<unshift()> and C<splice()>.
1075 Returns the number of elements in the array. Takes no arguments.
1077 my $len = $db->length();
1081 Adds one or more elements onto the end of the array. Accepts scalars, hash
1082 refs or array refs. No return value.
1084 $db->push("foo", "bar", {});
1088 Fetches the last element in the array, and deletes it. Takes no arguments.
1089 Returns undef if array is empty. Returns the element value.
1091 my $elem = $db->pop();
1095 Fetches the first element in the array, deletes it, then shifts all the
1096 remaining elements over to take up the space. Returns the element value. This
1097 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1100 my $elem = $db->shift();
1104 Inserts one or more elements onto the beginning of the array, shifting all
1105 existing elements over to make room. Accepts scalars, hash refs or array refs.
1106 No return value. This method is not recommended with large arrays -- see
1107 <LARGE ARRAYS> below for details.
1109 $db->unshift("foo", "bar", {});
1113 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1114 -f splice> for usage -- it is too complicated to document here. This method is
1115 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1119 Here are some examples of using arrays:
1121 my $db = DBM::Deep->new(
1123 type => DBM::Deep->TYPE_ARRAY
1126 $db->push("bar", "baz");
1127 $db->unshift("foo");
1130 my $len = $db->length();
1131 print "length: $len\n"; # 4
1133 for (my $k=0; $k<$len; $k++) {
1134 print "$k: " . $db->get($k) . "\n";
1137 $db->splice(1, 2, "biz", "baf");
1139 while (my $elem = shift @$db) {
1140 print "shifted: $elem\n";
1145 Enable automatic file locking by passing a true value to the C<locking>
1146 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1148 my $db = DBM::Deep->new(
1153 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1154 mode for writes, and shared mode for reads. This is required if you have
1155 multiple processes accessing the same database file, to avoid file corruption.
1156 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1157 NFS> below for more.
1159 =head2 EXPLICIT LOCKING
1161 You can explicitly lock a database, so it remains locked for multiple
1162 transactions. This is done by calling the C<lock()> method, and passing an
1163 optional lock mode argument (defaults to exclusive mode). This is particularly
1164 useful for things like counters, where the current value needs to be fetched,
1165 then incremented, then stored again.
1168 my $counter = $db->get("counter");
1170 $db->put("counter", $counter);
1179 You can pass C<lock()> an optional argument, which specifies which mode to use
1180 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1181 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1182 same as the constants defined in Perl's C<Fcntl> module.
1184 $db->lock( DBM::Deep->LOCK_SH );
1188 =head1 IMPORTING/EXPORTING
1190 You can import existing complex structures by calling the C<import()> method,
1191 and export an entire database into an in-memory structure using the C<export()>
1192 method. Both are examined here.
1196 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1197 walking the structure and adding keys/elements to the database as you go,
1198 simply pass a reference to the C<import()> method. This recursively adds
1199 everything to an existing DBM::Deep object for you. Here is an example:
1204 array1 => [ "elem0", "elem1", "elem2" ],
1206 subkey1 => "subvalue1",
1207 subkey2 => "subvalue2"
1211 my $db = DBM::Deep->new( "foo.db" );
1212 $db->import( $struct );
1214 print $db->{key1} . "\n"; # prints "value1"
1216 This recursively imports the entire C<$struct> object into C<$db>, including
1217 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1218 keys are merged with the existing ones, replacing if they already exist.
1219 The C<import()> method can be called on any database level (not just the base
1220 level), and works with both hash and array DB types.
1222 B<Note:> Make sure your existing structure has no circular references in it.
1223 These will cause an infinite loop when importing.
1227 Calling the C<export()> method on an existing DBM::Deep object will return
1228 a reference to a new in-memory copy of the database. The export is done
1229 recursively, so all nested hashes/arrays are all exported to standard Perl
1230 objects. Here is an example:
1232 my $db = DBM::Deep->new( "foo.db" );
1234 $db->{key1} = "value1";
1235 $db->{key2} = "value2";
1237 $db->{hash1}->{subkey1} = "subvalue1";
1238 $db->{hash1}->{subkey2} = "subvalue2";
1240 my $struct = $db->export();
1242 print $struct->{key1} . "\n"; # prints "value1"
1244 This makes a complete copy of the database in memory, and returns a reference
1245 to it. The C<export()> method can be called on any database level (not just
1246 the base level), and works with both hash and array DB types. Be careful of
1247 large databases -- you can store a lot more data in a DBM::Deep object than an
1248 in-memory Perl structure.
1250 B<Note:> Make sure your database has no circular references in it.
1251 These will cause an infinite loop when exporting.
1255 DBM::Deep has a number of hooks where you can specify your own Perl function
1256 to perform filtering on incoming or outgoing data. This is a perfect
1257 way to extend the engine, and implement things like real-time compression or
1258 encryption. Filtering applies to the base DB level, and all child hashes /
1259 arrays. Filter hooks can be specified when your DBM::Deep object is first
1260 constructed, or by calling the C<set_filter()> method at any time. There are
1261 four available filter hooks, described below:
1265 =item * filter_store_key
1267 This filter is called whenever a hash key is stored. It
1268 is passed the incoming key, and expected to return a transformed key.
1270 =item * filter_store_value
1272 This filter is called whenever a hash key or array element is stored. It
1273 is passed the incoming value, and expected to return a transformed value.
1275 =item * filter_fetch_key
1277 This filter is called whenever a hash key is fetched (i.e. via
1278 C<first_key()> or C<next_key()>). It is passed the transformed key,
1279 and expected to return the plain key.
1281 =item * filter_fetch_value
1283 This filter is called whenever a hash key or array element is fetched.
1284 It is passed the transformed value, and expected to return the plain value.
1288 Here are the two ways to setup a filter hook:
1290 my $db = DBM::Deep->new(
1292 filter_store_value => \&my_filter_store,
1293 filter_fetch_value => \&my_filter_fetch
1298 $db->set_filter( "filter_store_value", \&my_filter_store );
1299 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1301 Your filter function will be called only when dealing with SCALAR keys or
1302 values. When nested hashes and arrays are being stored/fetched, filtering
1303 is bypassed. Filters are called as static functions, passed a single SCALAR
1304 argument, and expected to return a single SCALAR value. If you want to
1305 remove a filter, set the function reference to C<undef>:
1307 $db->set_filter( "filter_store_value", undef );
1309 =head2 REAL-TIME ENCRYPTION EXAMPLE
1311 Here is a working example that uses the I<Crypt::Blowfish> module to
1312 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1313 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1314 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1317 use Crypt::Blowfish;
1320 my $cipher = Crypt::CBC->new({
1321 'key' => 'my secret key',
1322 'cipher' => 'Blowfish',
1324 'regenerate_key' => 0,
1325 'padding' => 'space',
1329 my $db = DBM::Deep->new(
1330 file => "foo-encrypt.db",
1331 filter_store_key => \&my_encrypt,
1332 filter_store_value => \&my_encrypt,
1333 filter_fetch_key => \&my_decrypt,
1334 filter_fetch_value => \&my_decrypt,
1337 $db->{key1} = "value1";
1338 $db->{key2} = "value2";
1339 print "key1: " . $db->{key1} . "\n";
1340 print "key2: " . $db->{key2} . "\n";
1346 return $cipher->encrypt( $_[0] );
1349 return $cipher->decrypt( $_[0] );
1352 =head2 REAL-TIME COMPRESSION EXAMPLE
1354 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1355 compression / decompression of keys & values with DBM::Deep Filters.
1356 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1357 more on I<Compress::Zlib>.
1362 my $db = DBM::Deep->new(
1363 file => "foo-compress.db",
1364 filter_store_key => \&my_compress,
1365 filter_store_value => \&my_compress,
1366 filter_fetch_key => \&my_decompress,
1367 filter_fetch_value => \&my_decompress,
1370 $db->{key1} = "value1";
1371 $db->{key2} = "value2";
1372 print "key1: " . $db->{key1} . "\n";
1373 print "key2: " . $db->{key2} . "\n";
1379 return Compress::Zlib::memGzip( $_[0] ) ;
1382 return Compress::Zlib::memGunzip( $_[0] ) ;
1385 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1386 actually numerical index numbers, and are not filtered.
1388 =head1 ERROR HANDLING
1390 Most DBM::Deep methods return a true value for success, and call die() on
1391 failure. You can wrap calls in an eval block to catch the die.
1393 my $db = DBM::Deep->new( "foo.db" ); # create hash
1394 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1396 print $@; # prints error message
1398 =head1 LARGEFILE SUPPORT
1400 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1401 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1402 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1403 by specifying the 'pack_size' parameter when constructing the file.
1406 filename => $filename,
1407 pack_size => 'large',
1410 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1411 instead of 32-bit longs. After setting these values your DB files have a
1412 theoretical maximum size of 16 XB (exabytes).
1414 You can also use C<pack_size =E<gt> 'small'> in order to use 16-bit file
1417 B<Note:> Changing these values will B<NOT> work for existing database files.
1418 Only change this for new files. Once the value has been set, it is stored in
1419 the file's header and cannot be changed for the life of the file. These
1420 parameters are per-file, meaning you can access 32-bit and 64-bit files, as
1423 B<Note:> We have not personally tested files larger than 2 GB -- all my
1424 systems have only a 32-bit Perl. However, I have received user reports that
1425 this does indeed work!
1427 =head1 LOW-LEVEL ACCESS
1429 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1430 you can call the C<_fh()> method, which returns the handle:
1432 my $fh = $db->_fh();
1434 This method can be called on the root level of the datbase, or any child
1435 hashes or arrays. All levels share a I<root> structure, which contains things
1436 like the filehandle, a reference counter, and all the options specified
1437 when you created the object. You can get access to this file object by
1438 calling the C<_fileobj()> method.
1440 my $file_obj = $db->_fileobj();
1442 This is useful for changing options after the object has already been created,
1443 such as enabling/disabling locking. You can also store your own temporary user
1444 data in this structure (be wary of name collision), which is then accessible from
1445 any child hash or array.
1447 =head1 CUSTOM DIGEST ALGORITHM
1449 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1450 keys. However you can override this, and use another algorithm (such as SHA-256)
1451 or even write your own. But please note that DBM::Deep currently expects zero
1452 collisions, so your algorithm has to be I<perfect>, so to speak. Collision
1453 detection may be introduced in a later version.
1455 You can specify a custom digest algorithm by passing it into the parameter
1456 list for new(), passing a reference to a subroutine as the 'digest' parameter,
1457 and the length of the algorithm's hashes (in bytes) as the 'hash_size'
1458 parameter. Here is a working example that uses a 256-bit hash from the
1459 I<Digest::SHA256> module. Please see
1460 L<http://search.cpan.org/search?module=Digest::SHA256> for more information.
1465 my $context = Digest::SHA256::new(256);
1467 my $db = DBM::Deep->new(
1468 filename => "foo-sha.db",
1469 digest => \&my_digest,
1473 $db->{key1} = "value1";
1474 $db->{key2} = "value2";
1475 print "key1: " . $db->{key1} . "\n";
1476 print "key2: " . $db->{key2} . "\n";
1482 return substr( $context->hash($_[0]), 0, 32 );
1485 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1486 of bytes you specify in the hash_size parameter (in this case 32).
1488 B<Note:> If you do choose to use a custom digest algorithm, you must set it
1489 every time you access this file. Otherwise, the default (MD5) will be used.
1491 =head1 CIRCULAR REFERENCES
1493 DBM::Deep has B<experimental> support for circular references. Meaning you
1494 can have a nested hash key or array element that points to a parent object.
1495 This relationship is stored in the DB file, and is preserved between sessions.
1498 my $db = DBM::Deep->new( "foo.db" );
1501 $db->{circle} = $db; # ref to self
1503 print $db->{foo} . "\n"; # prints "bar"
1504 print $db->{circle}->{foo} . "\n"; # prints "bar" again
1506 B<Note>: Passing the object to a function that recursively walks the
1507 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1508 C<export()> methods) will result in an infinite loop. This will be fixed in
1511 =head1 CAVEATS / ISSUES / BUGS
1513 This section describes all the known issues with DBM::Deep. It you have found
1514 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1516 =head2 UNUSED SPACE RECOVERY
1518 One major caveat with DBM::Deep is that space occupied by existing keys and
1519 values is not recovered when they are deleted. Meaning if you keep deleting
1520 and adding new keys, your file will continuously grow. I am working on this,
1521 but in the meantime you can call the built-in C<optimize()> method from time to
1522 time (perhaps in a crontab or something) to recover all your unused space.
1524 $db->optimize(); # returns true on success
1526 This rebuilds the ENTIRE database into a new file, then moves it on top of
1527 the original. The new file will have no unused space, thus it will take up as
1528 little disk space as possible. Please note that this operation can take
1529 a long time for large files, and you need enough disk space to temporarily hold
1530 2 copies of your DB file. The temporary file is created in the same directory
1531 as the original, named with a ".tmp" extension, and is deleted when the
1532 operation completes. Oh, and if locking is enabled, the DB is automatically
1533 locked for the entire duration of the copy.
1535 B<WARNING:> Only call optimize() on the top-level node of the database, and
1536 make sure there are no child references lying around. DBM::Deep keeps a reference
1537 counter, and if it is greater than 1, optimize() will abort and return undef.
1541 (The reasons given assume a high level of Perl understanding, specifically of
1542 references. You can safely skip this section.)
1544 Currently, the only references supported are HASH and ARRAY. The other reference
1545 types (SCALAR, CODE, GLOB, and REF) cannot be supported for various reasons.
1551 These are things like filehandles and other sockets. They can't be supported
1552 because it's completely unclear how DBM::Deep should serialize them.
1554 =item * SCALAR / REF
1556 The discussion here refers to the following type of example:
1563 # In some other process ...
1565 my $val = ${ $db->{key1} };
1567 is( $val, 50, "What actually gets stored in the DB file?" );
1569 The problem is one of synchronization. When the variable being referred to
1570 changes value, the reference isn't notified. This means that the new value won't
1571 be stored in the datafile for other processes to read. There is no TIEREF.
1573 It is theoretically possible to store references to values already within a
1574 DBM::Deep object because everything already is synchronized, but the change to
1575 the internals would be quite large. Specifically, DBM::Deep would have to tie
1576 every single value that is stored. This would bloat the RAM footprint of
1577 DBM::Deep at least twofold (if not more) and be a significant performance drain,
1578 all to support a feature that has never been requested.
1582 L<http://search.cpan.org/search?module=Data::Dump::Streamer> provides a
1583 mechanism for serializing coderefs, including saving off all closure state.
1584 However, just as for SCALAR and REF, that closure state may change without
1585 notifying the DBM::Deep object storing the reference.
1589 =head2 FILE CORRUPTION
1591 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1592 for a 32-bit signature when opened, but other corruption in files can cause
1593 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1594 stuck in an infinite loop depending on the level of corruption. File write
1595 operations are not checked for failure (for speed), so if you happen to run
1596 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1597 be addressed in a later version of DBM::Deep.
1601 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1602 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1603 about setting up your NFS server with a locking daemon, then using lockf() to
1604 lock your files, but your mileage may vary there as well. From what I
1605 understand, there is no real way to do it. However, if you need access to the
1606 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1607 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1609 =head2 COPYING OBJECTS
1611 Beware of copying tied objects in Perl. Very strange things can happen.
1612 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1613 returns a new, blessed, tied hash or array to the same level in the DB.
1615 my $copy = $db->clone();
1617 B<Note>: Since clone() here is cloning the object, not the database location, any
1618 modifications to either $db or $copy will be visible in both.
1622 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1623 These functions cause every element in the array to move, which can be murder
1624 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1625 a different location. This will be addressed in the forthcoming version 1.00.
1627 =head2 WRITEONLY FILES
1629 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1630 writeonly mode. STORE will verify that the filehandle is writable. However, there
1631 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1632 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1636 This section discusses DBM::Deep's speed and memory usage.
1640 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1641 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1642 multi-level hash/array support, and cross-platform FTPable files. Even so,
1643 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1644 with huge databases. Here is some test data:
1646 Adding 1,000,000 keys to new DB file...
1648 At 100 keys, avg. speed is 2,703 keys/sec
1649 At 200 keys, avg. speed is 2,642 keys/sec
1650 At 300 keys, avg. speed is 2,598 keys/sec
1651 At 400 keys, avg. speed is 2,578 keys/sec
1652 At 500 keys, avg. speed is 2,722 keys/sec
1653 At 600 keys, avg. speed is 2,628 keys/sec
1654 At 700 keys, avg. speed is 2,700 keys/sec
1655 At 800 keys, avg. speed is 2,607 keys/sec
1656 At 900 keys, avg. speed is 2,190 keys/sec
1657 At 1,000 keys, avg. speed is 2,570 keys/sec
1658 At 2,000 keys, avg. speed is 2,417 keys/sec
1659 At 3,000 keys, avg. speed is 1,982 keys/sec
1660 At 4,000 keys, avg. speed is 1,568 keys/sec
1661 At 5,000 keys, avg. speed is 1,533 keys/sec
1662 At 6,000 keys, avg. speed is 1,787 keys/sec
1663 At 7,000 keys, avg. speed is 1,977 keys/sec
1664 At 8,000 keys, avg. speed is 2,028 keys/sec
1665 At 9,000 keys, avg. speed is 2,077 keys/sec
1666 At 10,000 keys, avg. speed is 2,031 keys/sec
1667 At 20,000 keys, avg. speed is 1,970 keys/sec
1668 At 30,000 keys, avg. speed is 2,050 keys/sec
1669 At 40,000 keys, avg. speed is 2,073 keys/sec
1670 At 50,000 keys, avg. speed is 1,973 keys/sec
1671 At 60,000 keys, avg. speed is 1,914 keys/sec
1672 At 70,000 keys, avg. speed is 2,091 keys/sec
1673 At 80,000 keys, avg. speed is 2,103 keys/sec
1674 At 90,000 keys, avg. speed is 1,886 keys/sec
1675 At 100,000 keys, avg. speed is 1,970 keys/sec
1676 At 200,000 keys, avg. speed is 2,053 keys/sec
1677 At 300,000 keys, avg. speed is 1,697 keys/sec
1678 At 400,000 keys, avg. speed is 1,838 keys/sec
1679 At 500,000 keys, avg. speed is 1,941 keys/sec
1680 At 600,000 keys, avg. speed is 1,930 keys/sec
1681 At 700,000 keys, avg. speed is 1,735 keys/sec
1682 At 800,000 keys, avg. speed is 1,795 keys/sec
1683 At 900,000 keys, avg. speed is 1,221 keys/sec
1684 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1686 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1687 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1688 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1689 Run time was 12 min 3 sec.
1693 One of the great things about DBM::Deep is that it uses very little memory.
1694 Even with huge databases (1,000,000+ keys) you will not see much increased
1695 memory on your process. DBM::Deep relies solely on the filesystem for storing
1696 and fetching data. Here is output from I</usr/bin/top> before even opening a
1699 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1700 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1702 Basically the process is taking 2,716K of memory. And here is the same
1703 process after storing and fetching 1,000,000 keys:
1705 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1706 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1708 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1709 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1711 =head1 DB FILE FORMAT
1713 In case you were interested in the underlying DB file format, it is documented
1714 here in this section. You don't need to know this to use the module, it's just
1715 included for reference.
1719 DBM::Deep files always start with a 32-bit signature to identify the file type.
1720 This is at offset 0. The signature is "DPDB" in network byte order. This is
1721 checked for when the file is opened and an error will be thrown if it's not found.
1725 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1726 has a standard header containing the type of data, the length of data, and then
1727 the data itself. The type is a single character (1 byte), the length is a
1728 32-bit unsigned long in network byte order, and the data is, well, the data.
1729 Here is how it unfolds:
1733 Immediately after the 32-bit file signature is the I<Master Index> record.
1734 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1735 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1736 depending on how the DBM::Deep object was constructed.
1738 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1739 number). The first 8-bit char of the MD5 signature is the offset into the
1740 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1741 index element is a file offset of the next tag for the key/element in question,
1742 which is usually a I<Bucket List> tag (see below).
1744 The next tag I<could> be another index, depending on how many keys/elements
1745 exist. See L<RE-INDEXING> below for details.
1749 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1750 file offsets to where the actual data is stored. It starts with a standard
1751 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1752 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1753 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1754 When the list fills up, a I<Re-Index> operation is performed (See
1755 L<RE-INDEXING> below).
1759 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1760 index/value pair (in array mode). It starts with a standard tag header with
1761 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1762 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1763 header. The size reported in the tag header is only for the value, but then,
1764 just after the value is another size (32-bit unsigned long) and then the plain
1765 key itself. Since the value is likely to be fetched more often than the plain
1766 key, I figured it would be I<slightly> faster to store the value first.
1768 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1769 record for the nested structure, where the process begins all over again.
1773 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1774 exhausted. Then, when another key/element comes in, the list is converted to a
1775 new index record. However, this index will look at the next char in the MD5
1776 hash, and arrange new Bucket List pointers accordingly. This process is called
1777 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1778 17 (16 + new one) keys/elements are removed from the old Bucket List and
1779 inserted into the new index. Several new Bucket Lists are created in the
1780 process, as a new MD5 char from the key is being examined (it is unlikely that
1781 the keys will all share the same next char of their MD5s).
1783 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1784 when the Bucket Lists will turn into indexes, but the first round tends to
1785 happen right around 4,000 keys. You will see a I<slight> decrease in
1786 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1787 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1788 right around 900,000 keys. This process can continue nearly indefinitely --
1789 right up until the point the I<MD5> signatures start colliding with each other,
1790 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1791 getting struck by lightning while you are walking to cash in your tickets.
1792 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1793 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1794 this is 340 unodecillion, but don't quote me).
1798 When a new key/element is stored, the key (or index number) is first run through
1799 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1800 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1801 for the first char of the signature (in this case I<b0>). If it does not exist,
1802 a new I<Bucket List> is created for our key (and the next 15 future keys that
1803 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1804 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1805 this point, unless we are replacing an existing I<Bucket>), where the actual
1806 data will be stored.
1810 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1811 (or index number), then walking along the indexes. If there are enough
1812 keys/elements in this DB level, there might be nested indexes, each linked to
1813 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1814 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1815 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1816 plain key are stored.
1818 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1819 methods. In this process the indexes are walked systematically, and each key
1820 fetched in increasing MD5 order (which is why it appears random). Once the
1821 I<Bucket> is found, the value is skipped and the plain key returned instead.
1822 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1823 alphabetically sorted. This only happens on an index-level -- as soon as the
1824 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1825 so it's pretty much undefined how the keys will come out -- just like Perl's
1828 =head1 CODE COVERAGE
1830 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1831 B<Devel::Cover> report on this module's test suite.
1833 ----------------------------------- ------ ------ ------ ------ ------ ------
1834 File stmt bran cond sub time total
1835 ----------------------------------- ------ ------ ------ ------ ------ ------
1836 blib/lib/DBM/Deep.pm 94.9 80.6 73.0 100.0 37.9 90.4
1837 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 18.2 98.1
1838 blib/lib/DBM/Deep/Engine.pm 98.9 87.3 80.0 100.0 34.2 95.2
1839 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 9.7 97.3
1840 Total 97.9 85.9 79.7 100.0 100.0 94.3
1841 ----------------------------------- ------ ------ ------ ------ ------ ------
1843 =head1 MORE INFORMATION
1845 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1846 or send email to L<DBM-Deep@googlegroups.com>.
1850 Joseph Huckaby, L<jhuckaby@cpan.org>
1852 Rob Kinyon, L<rkinyon@cpan.org>
1854 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1858 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1859 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1863 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1864 This is free software, you may use it and distribute it under the
1865 same terms as Perl itself.