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 #XXX This if() is redundant
389 if ( my $parent = $self->{parent} ) {
391 while ( $parent->{parent} ) {
393 $parent->_type eq TYPE_HASH
394 ? "\{$child->{parent_key}\}"
395 : "\[$child->{parent_key}\]"
399 $parent = $parent->{parent};
402 $base = "\$db->get( '$child->{parent_key}' )->" . $base;
405 $base = "\$db->get( '$child->{parent_key}' )";
413 # Store single hash key/value or array element in database.
415 my $self = shift->_get_self;
416 my ($key, $value, $orig_key) = @_;
419 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
420 $self->_throw_error( 'Cannot write to a readonly filehandle' );
423 if ( defined $orig_key ) {
426 my $r = Scalar::Util::reftype( $value ) || '';
427 if ( $r eq 'HASH' ) {
430 elsif ( $r eq 'ARRAY' ) {
433 elsif ( defined $value ) {
440 if ( my $c = Scalar::Util::blessed( $value ) ) {
441 $rhs = "bless $rhs, '$c'";
444 my $lhs = $self->_find_parent;
446 if ( $self->_type eq TYPE_HASH ) {
447 $lhs .= "->\{$orig_key\}";
450 $lhs .= "->\[$orig_key\]";
456 $lhs = "\$db->put('$orig_key',$rhs);";
459 $self->_fileobj->audit($lhs);
463 # Request exclusive lock for writing
465 $self->lock( LOCK_EX );
467 my $md5 = $self->{engine}{digest}->($key);
469 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5, { create => 1 } );
471 # User may be storing a hash, in which case we do not want it run
472 # through the filtering system
473 if ( !ref($value) && $self->_fileobj->{filter_store_value} ) {
474 $value = $self->_fileobj->{filter_store_value}->( $value );
478 # Add key/value to bucket list
480 my $result = $self->{engine}->add_bucket( $tag, $md5, $key, $value, undef, $orig_key );
489 # Fetch single value or element given plain key or array index
491 my $self = shift->_get_self;
492 my ($key, $orig_key) = @_;
494 my $md5 = $self->{engine}{digest}->($key);
497 # Request shared lock for reading
499 $self->lock( LOCK_SH );
501 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );#, { create => 1 } );
502 #XXX This needs to autovivify
509 # Get value from bucket list
511 my $result = $self->{engine}->get_bucket_value( $tag, $md5, $orig_key );
515 # Filters only apply to scalar values, so the ref check is making
516 # sure the fetched bucket is a scalar, not a child hash or array.
517 return ($result && !ref($result) && $self->_fileobj->{filter_fetch_value})
518 ? $self->_fileobj->{filter_fetch_value}->($result)
524 # Delete single key/value pair or element given plain key or array index
526 my $self = shift->_get_self;
527 my ($key, $orig_key) = @_;
529 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
530 $self->_throw_error( 'Cannot write to a readonly filehandle' );
533 if ( defined $orig_key ) {
534 my $lhs = $self->_find_parent;
536 $self->_fileobj->audit( "delete $lhs;" );
539 $self->_fileobj->audit( "\$db->delete('$orig_key');" );
544 # Request exclusive lock for writing
546 $self->lock( LOCK_EX );
548 my $md5 = $self->{engine}{digest}->($key);
550 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
559 my $value = $self->{engine}->get_bucket_value( $tag, $md5 );
561 if (defined $value && !ref($value) && $self->_fileobj->{filter_fetch_value}) {
562 $value = $self->_fileobj->{filter_fetch_value}->($value);
565 my $result = $self->{engine}->delete_bucket( $tag, $md5, $orig_key );
568 # If this object is an array and the key deleted was on the end of the stack,
569 # decrement the length variable.
579 # Check if a single key or element exists given plain key or array index
581 my $self = shift->_get_self;
584 my $md5 = $self->{engine}{digest}->($key);
587 # Request shared lock for reading
589 $self->lock( LOCK_SH );
591 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
596 # For some reason, the built-in exists() function returns '' for false
602 # Check if bucket exists and return 1 or ''
604 my $result = $self->{engine}->bucket_exists( $tag, $md5 ) || '';
613 # Clear all keys from hash, or all elements from array.
615 my $self = shift->_get_self;
617 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
618 $self->_throw_error( 'Cannot write to a readonly filehandle' );
622 my $lhs = $self->_find_parent;
624 if ( $self->_type eq TYPE_HASH ) {
625 $lhs = '%{' . $lhs . '}';
628 $lhs = '@{' . $lhs . '}';
631 $self->_fileobj->audit( "$lhs = ();" );
635 # Request exclusive lock for writing
637 $self->lock( LOCK_EX );
639 #XXX This needs updating to use _release_space
640 $self->{engine}->write_tag(
641 $self->_base_offset, $self->_type,
642 chr(0)x$self->{engine}{index_size},
651 # Public method aliases
653 sub put { (shift)->STORE( @_ ) }
654 sub store { (shift)->STORE( @_ ) }
655 sub get { (shift)->FETCH( @_ ) }
656 sub fetch { (shift)->FETCH( @_ ) }
657 sub delete { (shift)->DELETE( @_ ) }
658 sub exists { (shift)->EXISTS( @_ ) }
659 sub clear { (shift)->CLEAR( @_ ) }
666 DBM::Deep - A pure perl multi-level hash/array DBM
671 my $db = DBM::Deep->new( "foo.db" );
673 $db->{key} = 'value'; # tie() style
676 $db->put('key' => 'value'); # OO style
677 print $db->get('key');
679 # true multi-level support
680 $db->{my_complex} = [
681 'hello', { perl => 'rules' },
687 A unique flat-file database module, written in pure perl. True
688 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
689 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
690 handle millions of keys and unlimited hash levels without significant
691 slow-down. Written from the ground-up in pure perl -- this is NOT a
692 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
693 Mac OS X and Windows.
695 =head1 VERSION DIFFERENCES
697 B<NOTE>: 0.99_01 and above have significant file format differences from 0.98 and
698 before. While attempts have been made to be backwards compatible, no guarantees.
702 Hopefully you are using Perl's excellent CPAN module, which will download
703 and install the module for you. If not, get the tarball, and run these
715 Construction can be done OO-style (which is the recommended way), or using
716 Perl's tie() function. Both are examined here.
718 =head2 OO CONSTRUCTION
720 The recommended way to construct a DBM::Deep object is to use the new()
721 method, which gets you a blessed, tied hash or array reference.
723 my $db = DBM::Deep->new( "foo.db" );
725 This opens a new database handle, mapped to the file "foo.db". If this
726 file does not exist, it will automatically be created. DB files are
727 opened in "r+" (read/write) mode, and the type of object returned is a
728 hash, unless otherwise specified (see L<OPTIONS> below).
730 You can pass a number of options to the constructor to specify things like
731 locking, autoflush, etc. This is done by passing an inline hash:
733 my $db = DBM::Deep->new(
739 Notice that the filename is now specified I<inside> the hash with
740 the "file" parameter, as opposed to being the sole argument to the
741 constructor. This is required if any options are specified.
742 See L<OPTIONS> below for the complete list.
746 You can also start with an array instead of a hash. For this, you must
747 specify the C<type> parameter:
749 my $db = DBM::Deep->new(
751 type => DBM::Deep->TYPE_ARRAY
754 B<Note:> Specifing the C<type> parameter only takes effect when beginning
755 a new DB file. If you create a DBM::Deep object with an existing file, the
756 C<type> will be loaded from the file header, and an error will be thrown if
757 the wrong type is passed in.
759 =head2 TIE CONSTRUCTION
761 Alternately, you can create a DBM::Deep handle by using Perl's built-in
762 tie() function. The object returned from tie() can be used to call methods,
763 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
764 file (as expected with most tie'd objects).
767 my $db = tie %hash, "DBM::Deep", "foo.db";
770 my $db = tie @array, "DBM::Deep", "bar.db";
772 As with the OO constructor, you can replace the DB filename parameter with
773 a hash containing one or more options (see L<OPTIONS> just below for the
776 tie %hash, "DBM::Deep", {
784 There are a number of options that can be passed in when constructing your
785 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
791 Filename of the DB file to link the handle to. You can pass a full absolute
792 filesystem path, partial path, or a plain filename if the file is in the
793 current working directory. This is a required parameter (though q.v. fh).
797 If you want, you can pass in the fh instead of the file. This is most useful for doing
800 my $db = DBM::Deep->new( { fh => \*DATA } );
802 You are responsible for making sure that the fh has been opened appropriately for your
803 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
804 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
805 needs to read from the fh.
809 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
810 not need to set this. However, it's there if you want it.
812 If you pass in fh and do not set this, it will be set appropriately.
816 This parameter specifies what type of object to create, a hash or array. Use
817 one of these two constants:
821 =item * C<DBM::Deep-E<gt>TYPE_HASH>
823 =item * C<DBM::Deep-E<gt>TYPE_ARRAY>.
827 This only takes effect when beginning a new file. This is an optional
828 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
832 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
833 function to lock the database in exclusive mode for writes, and shared mode for
834 reads. Pass any true value to enable. This affects the base DB handle I<and
835 any child hashes or arrays> that use the same DB file. This is an optional
836 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
840 Specifies whether autoflush is to be enabled on the underlying filehandle.
841 This obviously slows down write operations, but is required if you may have
842 multiple processes accessing the same DB file (also consider enable I<locking>).
843 Pass any true value to enable. This is an optional parameter, and defaults to 0
848 If I<autobless> mode is enabled, DBM::Deep will preserve the class something
849 is blessed into, and restores it when fetched. This is an optional parameter, and defaults to 1 (enabled).
851 B<Note:> If you use the OO-interface, you will not be able to call any methods
852 of DBM::Deep on the blessed item. This is considered to be a feature.
856 See L</FILTERS> below.
862 With DBM::Deep you can access your databases using Perl's standard hash/array
863 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
864 treat them as such. DBM::Deep will intercept all reads/writes and direct them
865 to the right place -- the DB file. This has nothing to do with the
866 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
867 using regular hashes and arrays, rather than calling functions like C<get()>
868 and C<put()> (although those work too). It is entirely up to you how to want
869 to access your databases.
873 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
874 or even nested hashes (or arrays) using standard Perl syntax:
876 my $db = DBM::Deep->new( "foo.db" );
878 $db->{mykey} = "myvalue";
880 $db->{myhash}->{subkey} = "subvalue";
882 print $db->{myhash}->{subkey} . "\n";
884 You can even step through hash keys using the normal Perl C<keys()> function:
886 foreach my $key (keys %$db) {
887 print "$key: " . $db->{$key} . "\n";
890 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
891 pushes them onto an array, all before the loop even begins. If you have an
892 extra large hash, this may exhaust Perl's memory. Instead, consider using
893 Perl's C<each()> function, which pulls keys/values one at a time, using very
896 while (my ($key, $value) = each %$db) {
897 print "$key: $value\n";
900 Please note that when using C<each()>, you should always pass a direct
901 hash reference, not a lookup. Meaning, you should B<never> do this:
904 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
906 This causes an infinite loop, because for each iteration, Perl is calling
907 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
908 it effectively keeps returning the first key over and over again. Instead,
909 assign a temporary variable to C<$db->{foo}>, then pass that to each().
913 As with hashes, you can treat any DBM::Deep object like a normal Perl array
914 reference. This includes inserting, removing and manipulating elements,
915 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
916 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
917 or simply be a nested array reference inside a hash. Example:
919 my $db = DBM::Deep->new(
920 file => "foo-array.db",
921 type => DBM::Deep->TYPE_ARRAY
925 push @$db, "bar", "baz";
928 my $last_elem = pop @$db; # baz
929 my $first_elem = shift @$db; # bah
930 my $second_elem = $db->[1]; # bar
932 my $num_elements = scalar @$db;
936 In addition to the I<tie()> interface, you can also use a standard OO interface
937 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
938 array) has its own methods, but both types share the following common methods:
939 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
943 =item * new() / clone()
945 These are the constructor and copy-functions.
947 =item * put() / store()
949 Stores a new hash key/value pair, or sets an array element value. Takes two
950 arguments, the hash key or array index, and the new value. The value can be
951 a scalar, hash ref or array ref. Returns true on success, false on failure.
953 $db->put("foo", "bar"); # for hashes
954 $db->put(1, "bar"); # for arrays
956 =item * get() / fetch()
958 Fetches the value of a hash key or array element. Takes one argument: the hash
959 key or array index. Returns a scalar, hash ref or array ref, depending on the
962 my $value = $db->get("foo"); # for hashes
963 my $value = $db->get(1); # for arrays
967 Checks if a hash key or array index exists. Takes one argument: the hash key
968 or array index. Returns true if it exists, false if not.
970 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
971 if ($db->exists(1)) { print "yay!\n"; } # for arrays
975 Deletes one hash key/value pair or array element. Takes one argument: the hash
976 key or array index. Returns true on success, false if not found. For arrays,
977 the remaining elements located after the deleted element are NOT moved over.
978 The deleted element is essentially just undefined, which is exactly how Perl's
979 internal arrays work. Please note that the space occupied by the deleted
980 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
981 below for details and workarounds.
983 $db->delete("foo"); # for hashes
984 $db->delete(1); # for arrays
988 Deletes B<all> hash keys or array elements. Takes no arguments. No return
989 value. Please note that the space occupied by the deleted keys/values or
990 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
991 details and workarounds.
993 $db->clear(); # hashes or arrays
995 =item * lock() / unlock()
1001 Recover lost disk space.
1003 =item * import() / export()
1005 Data going in and out.
1011 For hashes, DBM::Deep supports all the common methods described above, and the
1012 following additional methods: C<first_key()> and C<next_key()>.
1018 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1019 fetched in an undefined order (which appears random). Takes no arguments,
1020 returns the key as a scalar value.
1022 my $key = $db->first_key();
1026 Returns the "next" key in the hash, given the previous one as the sole argument.
1027 Returns undef if there are no more keys to be fetched.
1029 $key = $db->next_key($key);
1033 Here are some examples of using hashes:
1035 my $db = DBM::Deep->new( "foo.db" );
1037 $db->put("foo", "bar");
1038 print "foo: " . $db->get("foo") . "\n";
1040 $db->put("baz", {}); # new child hash ref
1041 $db->get("baz")->put("buz", "biz");
1042 print "buz: " . $db->get("baz")->get("buz") . "\n";
1044 my $key = $db->first_key();
1046 print "$key: " . $db->get($key) . "\n";
1047 $key = $db->next_key($key);
1050 if ($db->exists("foo")) { $db->delete("foo"); }
1054 For arrays, DBM::Deep supports all the common methods described above, and the
1055 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1056 C<unshift()> and C<splice()>.
1062 Returns the number of elements in the array. Takes no arguments.
1064 my $len = $db->length();
1068 Adds one or more elements onto the end of the array. Accepts scalars, hash
1069 refs or array refs. No return value.
1071 $db->push("foo", "bar", {});
1075 Fetches the last element in the array, and deletes it. Takes no arguments.
1076 Returns undef if array is empty. Returns the element value.
1078 my $elem = $db->pop();
1082 Fetches the first element in the array, deletes it, then shifts all the
1083 remaining elements over to take up the space. Returns the element value. This
1084 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1087 my $elem = $db->shift();
1091 Inserts one or more elements onto the beginning of the array, shifting all
1092 existing elements over to make room. Accepts scalars, hash refs or array refs.
1093 No return value. This method is not recommended with large arrays -- see
1094 <LARGE ARRAYS> below for details.
1096 $db->unshift("foo", "bar", {});
1100 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1101 -f splice> for usage -- it is too complicated to document here. This method is
1102 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1106 Here are some examples of using arrays:
1108 my $db = DBM::Deep->new(
1110 type => DBM::Deep->TYPE_ARRAY
1113 $db->push("bar", "baz");
1114 $db->unshift("foo");
1117 my $len = $db->length();
1118 print "length: $len\n"; # 4
1120 for (my $k=0; $k<$len; $k++) {
1121 print "$k: " . $db->get($k) . "\n";
1124 $db->splice(1, 2, "biz", "baf");
1126 while (my $elem = shift @$db) {
1127 print "shifted: $elem\n";
1132 Enable automatic file locking by passing a true value to the C<locking>
1133 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1135 my $db = DBM::Deep->new(
1140 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1141 mode for writes, and shared mode for reads. This is required if you have
1142 multiple processes accessing the same database file, to avoid file corruption.
1143 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1144 NFS> below for more.
1146 =head2 EXPLICIT LOCKING
1148 You can explicitly lock a database, so it remains locked for multiple
1149 transactions. This is done by calling the C<lock()> method, and passing an
1150 optional lock mode argument (defaults to exclusive mode). This is particularly
1151 useful for things like counters, where the current value needs to be fetched,
1152 then incremented, then stored again.
1155 my $counter = $db->get("counter");
1157 $db->put("counter", $counter);
1166 You can pass C<lock()> an optional argument, which specifies which mode to use
1167 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1168 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1169 same as the constants defined in Perl's C<Fcntl> module.
1171 $db->lock( DBM::Deep->LOCK_SH );
1175 =head1 IMPORTING/EXPORTING
1177 You can import existing complex structures by calling the C<import()> method,
1178 and export an entire database into an in-memory structure using the C<export()>
1179 method. Both are examined here.
1183 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1184 walking the structure and adding keys/elements to the database as you go,
1185 simply pass a reference to the C<import()> method. This recursively adds
1186 everything to an existing DBM::Deep object for you. Here is an example:
1191 array1 => [ "elem0", "elem1", "elem2" ],
1193 subkey1 => "subvalue1",
1194 subkey2 => "subvalue2"
1198 my $db = DBM::Deep->new( "foo.db" );
1199 $db->import( $struct );
1201 print $db->{key1} . "\n"; # prints "value1"
1203 This recursively imports the entire C<$struct> object into C<$db>, including
1204 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1205 keys are merged with the existing ones, replacing if they already exist.
1206 The C<import()> method can be called on any database level (not just the base
1207 level), and works with both hash and array DB types.
1209 B<Note:> Make sure your existing structure has no circular references in it.
1210 These will cause an infinite loop when importing.
1214 Calling the C<export()> method on an existing DBM::Deep object will return
1215 a reference to a new in-memory copy of the database. The export is done
1216 recursively, so all nested hashes/arrays are all exported to standard Perl
1217 objects. Here is an example:
1219 my $db = DBM::Deep->new( "foo.db" );
1221 $db->{key1} = "value1";
1222 $db->{key2} = "value2";
1224 $db->{hash1}->{subkey1} = "subvalue1";
1225 $db->{hash1}->{subkey2} = "subvalue2";
1227 my $struct = $db->export();
1229 print $struct->{key1} . "\n"; # prints "value1"
1231 This makes a complete copy of the database in memory, and returns a reference
1232 to it. The C<export()> method can be called on any database level (not just
1233 the base level), and works with both hash and array DB types. Be careful of
1234 large databases -- you can store a lot more data in a DBM::Deep object than an
1235 in-memory Perl structure.
1237 B<Note:> Make sure your database has no circular references in it.
1238 These will cause an infinite loop when exporting.
1242 DBM::Deep has a number of hooks where you can specify your own Perl function
1243 to perform filtering on incoming or outgoing data. This is a perfect
1244 way to extend the engine, and implement things like real-time compression or
1245 encryption. Filtering applies to the base DB level, and all child hashes /
1246 arrays. Filter hooks can be specified when your DBM::Deep object is first
1247 constructed, or by calling the C<set_filter()> method at any time. There are
1248 four available filter hooks, described below:
1252 =item * filter_store_key
1254 This filter is called whenever a hash key is stored. It
1255 is passed the incoming key, and expected to return a transformed key.
1257 =item * filter_store_value
1259 This filter is called whenever a hash key or array element is stored. It
1260 is passed the incoming value, and expected to return a transformed value.
1262 =item * filter_fetch_key
1264 This filter is called whenever a hash key is fetched (i.e. via
1265 C<first_key()> or C<next_key()>). It is passed the transformed key,
1266 and expected to return the plain key.
1268 =item * filter_fetch_value
1270 This filter is called whenever a hash key or array element is fetched.
1271 It is passed the transformed value, and expected to return the plain value.
1275 Here are the two ways to setup a filter hook:
1277 my $db = DBM::Deep->new(
1279 filter_store_value => \&my_filter_store,
1280 filter_fetch_value => \&my_filter_fetch
1285 $db->set_filter( "filter_store_value", \&my_filter_store );
1286 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1288 Your filter function will be called only when dealing with SCALAR keys or
1289 values. When nested hashes and arrays are being stored/fetched, filtering
1290 is bypassed. Filters are called as static functions, passed a single SCALAR
1291 argument, and expected to return a single SCALAR value. If you want to
1292 remove a filter, set the function reference to C<undef>:
1294 $db->set_filter( "filter_store_value", undef );
1296 =head2 REAL-TIME ENCRYPTION EXAMPLE
1298 Here is a working example that uses the I<Crypt::Blowfish> module to
1299 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1300 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1301 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1304 use Crypt::Blowfish;
1307 my $cipher = Crypt::CBC->new({
1308 'key' => 'my secret key',
1309 'cipher' => 'Blowfish',
1311 'regenerate_key' => 0,
1312 'padding' => 'space',
1316 my $db = DBM::Deep->new(
1317 file => "foo-encrypt.db",
1318 filter_store_key => \&my_encrypt,
1319 filter_store_value => \&my_encrypt,
1320 filter_fetch_key => \&my_decrypt,
1321 filter_fetch_value => \&my_decrypt,
1324 $db->{key1} = "value1";
1325 $db->{key2} = "value2";
1326 print "key1: " . $db->{key1} . "\n";
1327 print "key2: " . $db->{key2} . "\n";
1333 return $cipher->encrypt( $_[0] );
1336 return $cipher->decrypt( $_[0] );
1339 =head2 REAL-TIME COMPRESSION EXAMPLE
1341 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1342 compression / decompression of keys & values with DBM::Deep Filters.
1343 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1344 more on I<Compress::Zlib>.
1349 my $db = DBM::Deep->new(
1350 file => "foo-compress.db",
1351 filter_store_key => \&my_compress,
1352 filter_store_value => \&my_compress,
1353 filter_fetch_key => \&my_decompress,
1354 filter_fetch_value => \&my_decompress,
1357 $db->{key1} = "value1";
1358 $db->{key2} = "value2";
1359 print "key1: " . $db->{key1} . "\n";
1360 print "key2: " . $db->{key2} . "\n";
1366 return Compress::Zlib::memGzip( $_[0] ) ;
1369 return Compress::Zlib::memGunzip( $_[0] ) ;
1372 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1373 actually numerical index numbers, and are not filtered.
1375 =head1 ERROR HANDLING
1377 Most DBM::Deep methods return a true value for success, and call die() on
1378 failure. You can wrap calls in an eval block to catch the die.
1380 my $db = DBM::Deep->new( "foo.db" ); # create hash
1381 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1383 print $@; # prints error message
1385 =head1 LARGEFILE SUPPORT
1387 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1388 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1389 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1390 by specifying the 'pack_size' parameter when constructing the file.
1393 filename => $filename,
1394 pack_size => 'large',
1397 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1398 instead of 32-bit longs. After setting these values your DB files have a
1399 theoretical maximum size of 16 XB (exabytes).
1401 You can also use C<pack_size =E<gt> 'small'> in order to use 16-bit file
1404 B<Note:> Changing these values will B<NOT> work for existing database files.
1405 Only change this for new files. Once the value has been set, it is stored in
1406 the file's header and cannot be changed for the life of the file. These
1407 parameters are per-file, meaning you can access 32-bit and 64-bit files, as
1410 B<Note:> We have not personally tested files larger than 2 GB -- all my
1411 systems have only a 32-bit Perl. However, I have received user reports that
1412 this does indeed work!
1414 =head1 LOW-LEVEL ACCESS
1416 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1417 you can call the C<_fh()> method, which returns the handle:
1419 my $fh = $db->_fh();
1421 This method can be called on the root level of the datbase, or any child
1422 hashes or arrays. All levels share a I<root> structure, which contains things
1423 like the filehandle, a reference counter, and all the options specified
1424 when you created the object. You can get access to this file object by
1425 calling the C<_fileobj()> method.
1427 my $file_obj = $db->_fileobj();
1429 This is useful for changing options after the object has already been created,
1430 such as enabling/disabling locking. You can also store your own temporary user
1431 data in this structure (be wary of name collision), which is then accessible from
1432 any child hash or array.
1434 =head1 CUSTOM DIGEST ALGORITHM
1436 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1437 keys. However you can override this, and use another algorithm (such as SHA-256)
1438 or even write your own. But please note that DBM::Deep currently expects zero
1439 collisions, so your algorithm has to be I<perfect>, so to speak. Collision
1440 detection may be introduced in a later version.
1442 You can specify a custom digest algorithm by passing it into the parameter
1443 list for new(), passing a reference to a subroutine as the 'digest' parameter,
1444 and the length of the algorithm's hashes (in bytes) as the 'hash_size'
1445 parameter. Here is a working example that uses a 256-bit hash from the
1446 I<Digest::SHA256> module. Please see
1447 L<http://search.cpan.org/search?module=Digest::SHA256> for more information.
1452 my $context = Digest::SHA256::new(256);
1454 my $db = DBM::Deep->new(
1455 filename => "foo-sha.db",
1456 digest => \&my_digest,
1460 $db->{key1} = "value1";
1461 $db->{key2} = "value2";
1462 print "key1: " . $db->{key1} . "\n";
1463 print "key2: " . $db->{key2} . "\n";
1469 return substr( $context->hash($_[0]), 0, 32 );
1472 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1473 of bytes you specify in the hash_size parameter (in this case 32).
1475 B<Note:> If you do choose to use a custom digest algorithm, you must set it
1476 every time you access this file. Otherwise, the default (MD5) will be used.
1478 =head1 CIRCULAR REFERENCES
1480 DBM::Deep has B<experimental> support for circular references. Meaning you
1481 can have a nested hash key or array element that points to a parent object.
1482 This relationship is stored in the DB file, and is preserved between sessions.
1485 my $db = DBM::Deep->new( "foo.db" );
1488 $db->{circle} = $db; # ref to self
1490 print $db->{foo} . "\n"; # prints "bar"
1491 print $db->{circle}->{foo} . "\n"; # prints "bar" again
1493 B<Note>: Passing the object to a function that recursively walks the
1494 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1495 C<export()> methods) will result in an infinite loop. This will be fixed in
1498 =head1 CAVEATS / ISSUES / BUGS
1500 This section describes all the known issues with DBM::Deep. It you have found
1501 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1503 =head2 UNUSED SPACE RECOVERY
1505 One major caveat with DBM::Deep is that space occupied by existing keys and
1506 values is not recovered when they are deleted. Meaning if you keep deleting
1507 and adding new keys, your file will continuously grow. I am working on this,
1508 but in the meantime you can call the built-in C<optimize()> method from time to
1509 time (perhaps in a crontab or something) to recover all your unused space.
1511 $db->optimize(); # returns true on success
1513 This rebuilds the ENTIRE database into a new file, then moves it on top of
1514 the original. The new file will have no unused space, thus it will take up as
1515 little disk space as possible. Please note that this operation can take
1516 a long time for large files, and you need enough disk space to temporarily hold
1517 2 copies of your DB file. The temporary file is created in the same directory
1518 as the original, named with a ".tmp" extension, and is deleted when the
1519 operation completes. Oh, and if locking is enabled, the DB is automatically
1520 locked for the entire duration of the copy.
1522 B<WARNING:> Only call optimize() on the top-level node of the database, and
1523 make sure there are no child references lying around. DBM::Deep keeps a reference
1524 counter, and if it is greater than 1, optimize() will abort and return undef.
1528 (The reasons given assume a high level of Perl understanding, specifically of
1529 references. You can safely skip this section.)
1531 Currently, the only references supported are HASH and ARRAY. The other reference
1532 types (SCALAR, CODE, GLOB, and REF) cannot be supported for various reasons.
1538 These are things like filehandles and other sockets. They can't be supported
1539 because it's completely unclear how DBM::Deep should serialize them.
1541 =item * SCALAR / REF
1543 The discussion here refers to the following type of example:
1550 # In some other process ...
1552 my $val = ${ $db->{key1} };
1554 is( $val, 50, "What actually gets stored in the DB file?" );
1556 The problem is one of synchronization. When the variable being referred to
1557 changes value, the reference isn't notified. This means that the new value won't
1558 be stored in the datafile for other processes to read. There is no TIEREF.
1560 It is theoretically possible to store references to values already within a
1561 DBM::Deep object because everything already is synchronized, but the change to
1562 the internals would be quite large. Specifically, DBM::Deep would have to tie
1563 every single value that is stored. This would bloat the RAM footprint of
1564 DBM::Deep at least twofold (if not more) and be a significant performance drain,
1565 all to support a feature that has never been requested.
1569 L<http://search.cpan.org/search?module=Data::Dump::Streamer> provides a
1570 mechanism for serializing coderefs, including saving off all closure state.
1571 However, just as for SCALAR and REF, that closure state may change without
1572 notifying the DBM::Deep object storing the reference.
1576 =head2 FILE CORRUPTION
1578 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1579 for a 32-bit signature when opened, but other corruption in files can cause
1580 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1581 stuck in an infinite loop depending on the level of corruption. File write
1582 operations are not checked for failure (for speed), so if you happen to run
1583 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1584 be addressed in a later version of DBM::Deep.
1588 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1589 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1590 about setting up your NFS server with a locking daemon, then using lockf() to
1591 lock your files, but your mileage may vary there as well. From what I
1592 understand, there is no real way to do it. However, if you need access to the
1593 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1594 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1596 =head2 COPYING OBJECTS
1598 Beware of copying tied objects in Perl. Very strange things can happen.
1599 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1600 returns a new, blessed, tied hash or array to the same level in the DB.
1602 my $copy = $db->clone();
1604 B<Note>: Since clone() here is cloning the object, not the database location, any
1605 modifications to either $db or $copy will be visible in both.
1609 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1610 These functions cause every element in the array to move, which can be murder
1611 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1612 a different location. This will be addressed in the forthcoming version 1.00.
1614 =head2 WRITEONLY FILES
1616 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1617 writeonly mode. STORE will verify that the filehandle is writable. However, there
1618 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1619 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1623 This section discusses DBM::Deep's speed and memory usage.
1627 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1628 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1629 multi-level hash/array support, and cross-platform FTPable files. Even so,
1630 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1631 with huge databases. Here is some test data:
1633 Adding 1,000,000 keys to new DB file...
1635 At 100 keys, avg. speed is 2,703 keys/sec
1636 At 200 keys, avg. speed is 2,642 keys/sec
1637 At 300 keys, avg. speed is 2,598 keys/sec
1638 At 400 keys, avg. speed is 2,578 keys/sec
1639 At 500 keys, avg. speed is 2,722 keys/sec
1640 At 600 keys, avg. speed is 2,628 keys/sec
1641 At 700 keys, avg. speed is 2,700 keys/sec
1642 At 800 keys, avg. speed is 2,607 keys/sec
1643 At 900 keys, avg. speed is 2,190 keys/sec
1644 At 1,000 keys, avg. speed is 2,570 keys/sec
1645 At 2,000 keys, avg. speed is 2,417 keys/sec
1646 At 3,000 keys, avg. speed is 1,982 keys/sec
1647 At 4,000 keys, avg. speed is 1,568 keys/sec
1648 At 5,000 keys, avg. speed is 1,533 keys/sec
1649 At 6,000 keys, avg. speed is 1,787 keys/sec
1650 At 7,000 keys, avg. speed is 1,977 keys/sec
1651 At 8,000 keys, avg. speed is 2,028 keys/sec
1652 At 9,000 keys, avg. speed is 2,077 keys/sec
1653 At 10,000 keys, avg. speed is 2,031 keys/sec
1654 At 20,000 keys, avg. speed is 1,970 keys/sec
1655 At 30,000 keys, avg. speed is 2,050 keys/sec
1656 At 40,000 keys, avg. speed is 2,073 keys/sec
1657 At 50,000 keys, avg. speed is 1,973 keys/sec
1658 At 60,000 keys, avg. speed is 1,914 keys/sec
1659 At 70,000 keys, avg. speed is 2,091 keys/sec
1660 At 80,000 keys, avg. speed is 2,103 keys/sec
1661 At 90,000 keys, avg. speed is 1,886 keys/sec
1662 At 100,000 keys, avg. speed is 1,970 keys/sec
1663 At 200,000 keys, avg. speed is 2,053 keys/sec
1664 At 300,000 keys, avg. speed is 1,697 keys/sec
1665 At 400,000 keys, avg. speed is 1,838 keys/sec
1666 At 500,000 keys, avg. speed is 1,941 keys/sec
1667 At 600,000 keys, avg. speed is 1,930 keys/sec
1668 At 700,000 keys, avg. speed is 1,735 keys/sec
1669 At 800,000 keys, avg. speed is 1,795 keys/sec
1670 At 900,000 keys, avg. speed is 1,221 keys/sec
1671 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1673 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1674 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1675 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1676 Run time was 12 min 3 sec.
1680 One of the great things about DBM::Deep is that it uses very little memory.
1681 Even with huge databases (1,000,000+ keys) you will not see much increased
1682 memory on your process. DBM::Deep relies solely on the filesystem for storing
1683 and fetching data. Here is output from I</usr/bin/top> before even opening a
1686 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1687 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1689 Basically the process is taking 2,716K of memory. And here is the same
1690 process after storing and fetching 1,000,000 keys:
1692 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1693 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1695 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1696 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1698 =head1 DB FILE FORMAT
1700 In case you were interested in the underlying DB file format, it is documented
1701 here in this section. You don't need to know this to use the module, it's just
1702 included for reference.
1706 DBM::Deep files always start with a 32-bit signature to identify the file type.
1707 This is at offset 0. The signature is "DPDB" in network byte order. This is
1708 checked for when the file is opened and an error will be thrown if it's not found.
1712 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1713 has a standard header containing the type of data, the length of data, and then
1714 the data itself. The type is a single character (1 byte), the length is a
1715 32-bit unsigned long in network byte order, and the data is, well, the data.
1716 Here is how it unfolds:
1720 Immediately after the 32-bit file signature is the I<Master Index> record.
1721 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1722 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1723 depending on how the DBM::Deep object was constructed.
1725 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1726 number). The first 8-bit char of the MD5 signature is the offset into the
1727 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1728 index element is a file offset of the next tag for the key/element in question,
1729 which is usually a I<Bucket List> tag (see below).
1731 The next tag I<could> be another index, depending on how many keys/elements
1732 exist. See L<RE-INDEXING> below for details.
1736 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1737 file offsets to where the actual data is stored. It starts with a standard
1738 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1739 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1740 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1741 When the list fills up, a I<Re-Index> operation is performed (See
1742 L<RE-INDEXING> below).
1746 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1747 index/value pair (in array mode). It starts with a standard tag header with
1748 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1749 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1750 header. The size reported in the tag header is only for the value, but then,
1751 just after the value is another size (32-bit unsigned long) and then the plain
1752 key itself. Since the value is likely to be fetched more often than the plain
1753 key, I figured it would be I<slightly> faster to store the value first.
1755 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1756 record for the nested structure, where the process begins all over again.
1760 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1761 exhausted. Then, when another key/element comes in, the list is converted to a
1762 new index record. However, this index will look at the next char in the MD5
1763 hash, and arrange new Bucket List pointers accordingly. This process is called
1764 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1765 17 (16 + new one) keys/elements are removed from the old Bucket List and
1766 inserted into the new index. Several new Bucket Lists are created in the
1767 process, as a new MD5 char from the key is being examined (it is unlikely that
1768 the keys will all share the same next char of their MD5s).
1770 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1771 when the Bucket Lists will turn into indexes, but the first round tends to
1772 happen right around 4,000 keys. You will see a I<slight> decrease in
1773 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1774 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1775 right around 900,000 keys. This process can continue nearly indefinitely --
1776 right up until the point the I<MD5> signatures start colliding with each other,
1777 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1778 getting struck by lightning while you are walking to cash in your tickets.
1779 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1780 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1781 this is 340 unodecillion, but don't quote me).
1785 When a new key/element is stored, the key (or index number) is first run through
1786 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1787 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1788 for the first char of the signature (in this case I<b0>). If it does not exist,
1789 a new I<Bucket List> is created for our key (and the next 15 future keys that
1790 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1791 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1792 this point, unless we are replacing an existing I<Bucket>), where the actual
1793 data will be stored.
1797 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1798 (or index number), then walking along the indexes. If there are enough
1799 keys/elements in this DB level, there might be nested indexes, each linked to
1800 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1801 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1802 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1803 plain key are stored.
1805 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1806 methods. In this process the indexes are walked systematically, and each key
1807 fetched in increasing MD5 order (which is why it appears random). Once the
1808 I<Bucket> is found, the value is skipped and the plain key returned instead.
1809 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1810 alphabetically sorted. This only happens on an index-level -- as soon as the
1811 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1812 so it's pretty much undefined how the keys will come out -- just like Perl's
1815 =head1 CODE COVERAGE
1817 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1818 B<Devel::Cover> report on this module's test suite.
1820 ----------------------------------- ------ ------ ------ ------ ------ ------
1821 File stmt bran cond sub time total
1822 ----------------------------------- ------ ------ ------ ------ ------ ------
1823 blib/lib/DBM/Deep.pm 94.9 80.6 73.0 100.0 37.9 90.4
1824 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 18.2 98.1
1825 blib/lib/DBM/Deep/Engine.pm 98.9 87.3 80.0 100.0 34.2 95.2
1826 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 9.7 97.3
1827 Total 97.9 85.9 79.7 100.0 100.0 94.3
1828 ----------------------------------- ------ ------ ------ ------ ------ ------
1830 =head1 MORE INFORMATION
1832 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1833 or send email to L<DBM-Deep@googlegroups.com>.
1837 Joseph Huckaby, L<jhuckaby@cpan.org>
1839 Rob Kinyon, L<rkinyon@cpan.org>
1841 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1845 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1846 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1850 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1851 This is free software, you may use it and distribute it under the
1852 same terms as Perl itself.