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.
34 use Fcntl qw( :DEFAULT :flock :seek );
38 use DBM::Deep::Engine;
40 use vars qw( $VERSION );
41 $VERSION = q(0.99_01);
44 # Setup constants for users to pass to new()
46 sub TYPE_HASH () { DBM::Deep::Engine->SIG_HASH }
47 sub TYPE_ARRAY () { DBM::Deep::Engine->SIG_ARRAY }
55 $proto->_throw_error( "Odd number of parameters to " . (caller(1))[2] );
60 unless ( eval { local $SIG{'__DIE__'}; %{$_[0]} || 1 } ) {
61 $proto->_throw_error( "Not a hashref in args to " . (caller(1))[2] );
66 $args = { file => shift };
74 # Class constructor method for Perl OO interface.
75 # Calls tie() and returns blessed reference to tied hash or array,
76 # providing a hybrid OO/tie interface.
79 my $args = $class->_get_args( @_ );
82 # Check if we want a tied hash or array.
85 if (defined($args->{type}) && $args->{type} eq TYPE_ARRAY) {
86 $class = 'DBM::Deep::Array';
87 require DBM::Deep::Array;
88 tie @$self, $class, %$args;
91 $class = 'DBM::Deep::Hash';
92 require DBM::Deep::Hash;
93 tie %$self, $class, %$args;
96 return bless $self, $class;
99 # This initializer is called from the various TIE* methods. new() calls tie(),
100 # which allows for a single point of entry.
105 # These are the defaults to be optionally overridden below
108 engine => DBM::Deep::Engine->new( $args ),
109 base_offset => undef,
112 # Grab the parameters we want to use
113 foreach my $param ( keys %$self ) {
114 next unless exists $args->{$param};
115 $self->{$param} = $args->{$param};
118 # locking implicitly enables autoflush
119 if ($args->{locking}) { $args->{autoflush} = 1; }
121 $self->{root} = exists $args->{root}
123 : DBM::Deep::_::Root->new( $args );
125 $self->{engine}->setup_fh( $self );
132 require DBM::Deep::Hash;
133 return DBM::Deep::Hash->TIEHASH( @_ );
138 require DBM::Deep::Array;
139 return DBM::Deep::Array->TIEARRAY( @_ );
142 #XXX Unneeded now ...
148 # If db locking is set, flock() the db file. If called multiple
149 # times before unlock(), then the same number of unlocks() must
150 # be called before the lock is released.
152 my $self = shift->_get_self;
154 $type = LOCK_EX unless defined $type;
156 if (!defined($self->_fh)) { return; }
158 if ($self->_root->{locking}) {
159 if (!$self->_root->{locked}) {
160 flock($self->_fh, $type);
162 # refresh end counter in case file has changed size
163 my @stats = stat($self->_fh);
164 $self->_root->{end} = $stats[7];
166 # double-check file inode, in case another process
167 # has optimize()d our file while we were waiting.
168 if ($stats[1] != $self->_root->{inode}) {
169 $self->{engine}->close_fh( $self );
170 $self->{engine}->setup_fh( $self );
171 flock($self->_fh, $type); # re-lock
173 # This may not be necessary after re-opening
174 $self->_root->{end} = (stat($self->_fh))[7]; # re-end
177 $self->_root->{locked}++;
187 # If db locking is set, unlock the db file. See note in lock()
188 # regarding calling lock() multiple times.
190 my $self = shift->_get_self;
192 if (!defined($self->_fh)) { return; }
194 if ($self->_root->{locking} && $self->_root->{locked} > 0) {
195 $self->_root->{locked}--;
196 if (!$self->_root->{locked}) { flock($self->_fh, LOCK_UN); }
205 my $self = shift->_get_self;
206 my ($spot, $value) = @_;
211 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
212 ${$spot} = $value->_repr;
213 $value->_copy_node( ${$spot} );
216 my $r = Scalar::Util::reftype( $value );
217 my $c = Scalar::Util::blessed( $value );
218 if ( $r eq 'ARRAY' ) {
219 ${$spot} = [ @{$value} ];
222 ${$spot} = { %{$value} };
224 ${$spot} = bless ${$spot}, $c
232 die "Must be implemented in a child class\n";
236 die "Must be implemented in a child class\n";
241 # Recursively export into standard Perl hashes and arrays.
243 my $self = shift->_get_self;
245 my $temp = $self->_repr;
248 $self->_copy_node( $temp );
256 # Recursively import Perl hash/array structure
258 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
260 my $self = shift->_get_self;
263 # struct is not a reference, so just import based on our type
265 $struct = $self->_repr( @_ );
268 return $self->_import( $struct );
273 # Rebuild entire database into new file, then move
274 # it back on top of original.
276 my $self = shift->_get_self;
278 #XXX Need to create a new test for this
279 # if ($self->_root->{links} > 1) {
280 # $self->_throw_error("Cannot optimize: reference count is greater than 1");
283 my $db_temp = DBM::Deep->new(
284 file => $self->_root->{file} . '.tmp',
289 $self->_copy_node( $db_temp );
293 # Attempt to copy user, group and permissions over to new file
295 my @stats = stat($self->_fh);
296 my $perms = $stats[2] & 07777;
299 chown( $uid, $gid, $self->_root->{file} . '.tmp' );
300 chmod( $perms, $self->_root->{file} . '.tmp' );
302 # q.v. perlport for more information on this variable
303 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
305 # Potential race condition when optmizing on Win32 with locking.
306 # The Windows filesystem requires that the filehandle be closed
307 # before it is overwritten with rename(). This could be redone
311 $self->{engine}->close_fh( $self );
314 if (!rename $self->_root->{file} . '.tmp', $self->_root->{file}) {
315 unlink $self->_root->{file} . '.tmp';
317 $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
321 $self->{engine}->close_fh( $self );
322 $self->{engine}->setup_fh( $self );
329 # Make copy of object and return
331 my $self = shift->_get_self;
333 return DBM::Deep->new(
334 type => $self->_type,
335 base_offset => $self->_base_offset,
341 my %is_legal_filter = map {
344 store_key store_value
345 fetch_key fetch_value
350 # Setup filter function for storing or fetching the key or value
352 my $self = shift->_get_self;
356 if ( $is_legal_filter{$type} ) {
357 $self->_root->{"filter_$type"} = $func;
371 # Get access to the root structure
373 my $self = $_[0]->_get_self;
374 return $self->{root};
379 # Get type of current node (TYPE_HASH or TYPE_ARRAY)
381 my $self = $_[0]->_get_self;
382 return $self->{type};
387 # Get base_offset of current node (TYPE_HASH or TYPE_ARRAY)
389 my $self = $_[0]->_get_self;
390 return $self->{base_offset};
395 # Get access to the raw fh
397 my $self = $_[0]->_get_self;
398 return $self->_root->{fh};
406 die "DBM::Deep: $_[1]\n";
411 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
416 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
421 # Store single hash key/value or array element in database.
423 my $self = shift->_get_self;
424 my ($key, $value) = @_;
426 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
427 $self->_throw_error( 'Cannot write to a readonly filehandle' );
431 # Request exclusive lock for writing
433 $self->lock( LOCK_EX );
435 my $md5 = $self->{engine}{digest}->($key);
437 my $tag = $self->{engine}->find_bucket_list( $self, $md5, { create => 1 } );
439 # User may be storing a hash, in which case we do not want it run
440 # through the filtering system
441 if ( !ref($value) && $self->_root->{filter_store_value} ) {
442 $value = $self->_root->{filter_store_value}->( $value );
446 # Add key/value to bucket list
448 my $result = $self->{engine}->add_bucket( $self, $tag, $md5, $key, $value );
457 # Fetch single value or element given plain key or array index
459 my $self = shift->_get_self;
462 my $md5 = $self->{engine}{digest}->($key);
465 # Request shared lock for reading
467 $self->lock( LOCK_SH );
469 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
476 # Get value from bucket list
478 my $result = $self->{engine}->get_bucket_value( $self, $tag, $md5 );
482 # Filters only apply to scalar values, so the ref check is making
483 # sure the fetched bucket is a scalar, not a child hash or array.
484 return ($result && !ref($result) && $self->_root->{filter_fetch_value})
485 ? $self->_root->{filter_fetch_value}->($result)
491 # Delete single key/value pair or element given plain key or array index
493 my $self = $_[0]->_get_self;
496 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
497 $self->_throw_error( 'Cannot write to a readonly filehandle' );
501 # Request exclusive lock for writing
503 $self->lock( LOCK_EX );
505 my $md5 = $self->{engine}{digest}->($key);
507 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
516 my $value = $self->{engine}->get_bucket_value($self, $tag, $md5 );
518 if (defined $value && !ref($value) && $self->_root->{filter_fetch_value}) {
519 $value = $self->_root->{filter_fetch_value}->($value);
522 my $result = $self->{engine}->delete_bucket( $self, $tag, $md5 );
525 # If this object is an array and the key deleted was on the end of the stack,
526 # decrement the length variable.
536 # Check if a single key or element exists given plain key or array index
538 my $self = $_[0]->_get_self;
541 my $md5 = $self->{engine}{digest}->($key);
544 # Request shared lock for reading
546 $self->lock( LOCK_SH );
548 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
553 # For some reason, the built-in exists() function returns '' for false
559 # Check if bucket exists and return 1 or ''
561 my $result = $self->{engine}->bucket_exists( $self, $tag, $md5 ) || '';
570 # Clear all keys from hash, or all elements from array.
572 my $self = $_[0]->_get_self;
574 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
575 $self->_throw_error( 'Cannot write to a readonly filehandle' );
579 # Request exclusive lock for writing
581 $self->lock( LOCK_EX );
585 seek($fh, $self->_base_offset + $self->_root->{file_offset}, SEEK_SET);
591 #XXX This needs updating to use _release_space
592 $self->{engine}->write_tag(
593 $self, $self->_base_offset, $self->_type,
594 chr(0)x$self->{engine}{index_size},
603 # Public method aliases
605 sub put { (shift)->STORE( @_ ) }
606 sub store { (shift)->STORE( @_ ) }
607 sub get { (shift)->FETCH( @_ ) }
608 sub fetch { (shift)->FETCH( @_ ) }
609 sub delete { (shift)->DELETE( @_ ) }
610 sub exists { (shift)->EXISTS( @_ ) }
611 sub clear { (shift)->CLEAR( @_ ) }
613 package DBM::Deep::_::Root;
628 filter_store_key => undef,
629 filter_store_value => undef,
630 filter_fetch_key => undef,
631 filter_fetch_value => undef,
634 # Grab the parameters we want to use
635 foreach my $param ( keys %$self ) {
636 next unless exists $args->{$param};
637 $self->{$param} = $args->{$param};
640 if ( $self->{fh} && !$self->{file_offset} ) {
641 $self->{file_offset} = tell( $self->{fh} );
651 close $self->{fh} if $self->{fh};
661 DBM::Deep - A pure perl multi-level hash/array DBM
666 my $db = DBM::Deep->new( "foo.db" );
668 $db->{key} = 'value'; # tie() style
671 $db->put('key' => 'value'); # OO style
672 print $db->get('key');
674 # true multi-level support
675 $db->{my_complex} = [
676 'hello', { perl => 'rules' },
682 A unique flat-file database module, written in pure perl. True
683 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
684 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
685 handle millions of keys and unlimited hash levels without significant
686 slow-down. Written from the ground-up in pure perl -- this is NOT a
687 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
688 Mac OS X and Windows.
690 =head1 VERSION DIFFERENCES
692 B<NOTE>: 0.99_01 and above have significant file format differences from 0.98 and
693 before. While attempts have been made to be backwards compatible, no guarantees.
697 Hopefully you are using Perl's excellent CPAN module, which will download
698 and install the module for you. If not, get the tarball, and run these
710 Construction can be done OO-style (which is the recommended way), or using
711 Perl's tie() function. Both are examined here.
713 =head2 OO CONSTRUCTION
715 The recommended way to construct a DBM::Deep object is to use the new()
716 method, which gets you a blessed, tied hash or array reference.
718 my $db = DBM::Deep->new( "foo.db" );
720 This opens a new database handle, mapped to the file "foo.db". If this
721 file does not exist, it will automatically be created. DB files are
722 opened in "r+" (read/write) mode, and the type of object returned is a
723 hash, unless otherwise specified (see L<OPTIONS> below).
725 You can pass a number of options to the constructor to specify things like
726 locking, autoflush, etc. This is done by passing an inline hash:
728 my $db = DBM::Deep->new(
734 Notice that the filename is now specified I<inside> the hash with
735 the "file" parameter, as opposed to being the sole argument to the
736 constructor. This is required if any options are specified.
737 See L<OPTIONS> below for the complete list.
741 You can also start with an array instead of a hash. For this, you must
742 specify the C<type> parameter:
744 my $db = DBM::Deep->new(
746 type => DBM::Deep->TYPE_ARRAY
749 B<Note:> Specifing the C<type> parameter only takes effect when beginning
750 a new DB file. If you create a DBM::Deep object with an existing file, the
751 C<type> will be loaded from the file header, and an error will be thrown if
752 the wrong type is passed in.
754 =head2 TIE CONSTRUCTION
756 Alternately, you can create a DBM::Deep handle by using Perl's built-in
757 tie() function. The object returned from tie() can be used to call methods,
758 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
759 file (as expected with most tie'd objects).
762 my $db = tie %hash, "DBM::Deep", "foo.db";
765 my $db = tie @array, "DBM::Deep", "bar.db";
767 As with the OO constructor, you can replace the DB filename parameter with
768 a hash containing one or more options (see L<OPTIONS> just below for the
771 tie %hash, "DBM::Deep", {
779 There are a number of options that can be passed in when constructing your
780 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
786 Filename of the DB file to link the handle to. You can pass a full absolute
787 filesystem path, partial path, or a plain filename if the file is in the
788 current working directory. This is a required parameter (though q.v. fh).
792 If you want, you can pass in the fh instead of the file. This is most useful for doing
795 my $db = DBM::Deep->new( { fh => \*DATA } );
797 You are responsible for making sure that the fh has been opened appropriately for your
798 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
799 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
800 needs to read from the fh.
804 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
805 not need to set this. However, it's there if you want it.
807 If you pass in fh and do not set this, it will be set appropriately.
811 This parameter specifies what type of object to create, a hash or array. Use
812 one of these two constants: C<DBM::Deep-E<gt>TYPE_HASH> or C<DBM::Deep-E<gt>TYPE_ARRAY>.
813 This only takes effect when beginning a new file. This is an optional
814 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
818 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
819 function to lock the database in exclusive mode for writes, and shared mode for
820 reads. Pass any true value to enable. This affects the base DB handle I<and
821 any child hashes or arrays> that use the same DB file. This is an optional
822 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
826 Specifies whether autoflush is to be enabled on the underlying filehandle.
827 This obviously slows down write operations, but is required if you may have
828 multiple processes accessing the same DB file (also consider enable I<locking>).
829 Pass any true value to enable. This is an optional parameter, and defaults to 0
834 If I<autobless> mode is enabled, DBM::Deep will preserve blessed hashes, and
835 restore them when fetched. This is an B<experimental> feature, and does have
836 side-effects. Basically, when hashes are re-blessed into their original
837 classes, they are no longer blessed into the DBM::Deep class! So you won't be
838 able to call any DBM::Deep methods on them. You have been warned.
839 This is an optional parameter, and defaults to 0 (disabled).
843 See L<FILTERS> below.
849 With DBM::Deep you can access your databases using Perl's standard hash/array
850 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
851 treat them as such. DBM::Deep will intercept all reads/writes and direct them
852 to the right place -- the DB file. This has nothing to do with the
853 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
854 using regular hashes and arrays, rather than calling functions like C<get()>
855 and C<put()> (although those work too). It is entirely up to you how to want
856 to access your databases.
860 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
861 or even nested hashes (or arrays) using standard Perl syntax:
863 my $db = DBM::Deep->new( "foo.db" );
865 $db->{mykey} = "myvalue";
867 $db->{myhash}->{subkey} = "subvalue";
869 print $db->{myhash}->{subkey} . "\n";
871 You can even step through hash keys using the normal Perl C<keys()> function:
873 foreach my $key (keys %$db) {
874 print "$key: " . $db->{$key} . "\n";
877 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
878 pushes them onto an array, all before the loop even begins. If you have an
879 extra large hash, this may exhaust Perl's memory. Instead, consider using
880 Perl's C<each()> function, which pulls keys/values one at a time, using very
883 while (my ($key, $value) = each %$db) {
884 print "$key: $value\n";
887 Please note that when using C<each()>, you should always pass a direct
888 hash reference, not a lookup. Meaning, you should B<never> do this:
891 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
893 This causes an infinite loop, because for each iteration, Perl is calling
894 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
895 it effectively keeps returning the first key over and over again. Instead,
896 assign a temporary variable to C<$db->{foo}>, then pass that to each().
900 As with hashes, you can treat any DBM::Deep object like a normal Perl array
901 reference. This includes inserting, removing and manipulating elements,
902 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
903 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
904 or simply be a nested array reference inside a hash. Example:
906 my $db = DBM::Deep->new(
907 file => "foo-array.db",
908 type => DBM::Deep->TYPE_ARRAY
912 push @$db, "bar", "baz";
915 my $last_elem = pop @$db; # baz
916 my $first_elem = shift @$db; # bah
917 my $second_elem = $db->[1]; # bar
919 my $num_elements = scalar @$db;
923 In addition to the I<tie()> interface, you can also use a standard OO interface
924 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
925 array) has its own methods, but both types share the following common methods:
926 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
930 =item * new() / clone()
932 These are the constructor and copy-functions.
934 =item * put() / store()
936 Stores a new hash key/value pair, or sets an array element value. Takes two
937 arguments, the hash key or array index, and the new value. The value can be
938 a scalar, hash ref or array ref. Returns true on success, false on failure.
940 $db->put("foo", "bar"); # for hashes
941 $db->put(1, "bar"); # for arrays
943 =item * get() / fetch()
945 Fetches the value of a hash key or array element. Takes one argument: the hash
946 key or array index. Returns a scalar, hash ref or array ref, depending on the
949 my $value = $db->get("foo"); # for hashes
950 my $value = $db->get(1); # for arrays
954 Checks if a hash key or array index exists. Takes one argument: the hash key
955 or array index. Returns true if it exists, false if not.
957 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
958 if ($db->exists(1)) { print "yay!\n"; } # for arrays
962 Deletes one hash key/value pair or array element. Takes one argument: the hash
963 key or array index. Returns true on success, false if not found. For arrays,
964 the remaining elements located after the deleted element are NOT moved over.
965 The deleted element is essentially just undefined, which is exactly how Perl's
966 internal arrays work. Please note that the space occupied by the deleted
967 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
968 below for details and workarounds.
970 $db->delete("foo"); # for hashes
971 $db->delete(1); # for arrays
975 Deletes B<all> hash keys or array elements. Takes no arguments. No return
976 value. Please note that the space occupied by the deleted keys/values or
977 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
978 details and workarounds.
980 $db->clear(); # hashes or arrays
982 =item * lock() / unlock()
988 Recover lost disk space.
990 =item * import() / export()
992 Data going in and out.
998 For hashes, DBM::Deep supports all the common methods described above, and the
999 following additional methods: C<first_key()> and C<next_key()>.
1005 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1006 fetched in an undefined order (which appears random). Takes no arguments,
1007 returns the key as a scalar value.
1009 my $key = $db->first_key();
1013 Returns the "next" key in the hash, given the previous one as the sole argument.
1014 Returns undef if there are no more keys to be fetched.
1016 $key = $db->next_key($key);
1020 Here are some examples of using hashes:
1022 my $db = DBM::Deep->new( "foo.db" );
1024 $db->put("foo", "bar");
1025 print "foo: " . $db->get("foo") . "\n";
1027 $db->put("baz", {}); # new child hash ref
1028 $db->get("baz")->put("buz", "biz");
1029 print "buz: " . $db->get("baz")->get("buz") . "\n";
1031 my $key = $db->first_key();
1033 print "$key: " . $db->get($key) . "\n";
1034 $key = $db->next_key($key);
1037 if ($db->exists("foo")) { $db->delete("foo"); }
1041 For arrays, DBM::Deep supports all the common methods described above, and the
1042 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1043 C<unshift()> and C<splice()>.
1049 Returns the number of elements in the array. Takes no arguments.
1051 my $len = $db->length();
1055 Adds one or more elements onto the end of the array. Accepts scalars, hash
1056 refs or array refs. No return value.
1058 $db->push("foo", "bar", {});
1062 Fetches the last element in the array, and deletes it. Takes no arguments.
1063 Returns undef if array is empty. Returns the element value.
1065 my $elem = $db->pop();
1069 Fetches the first element in the array, deletes it, then shifts all the
1070 remaining elements over to take up the space. Returns the element value. This
1071 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1074 my $elem = $db->shift();
1078 Inserts one or more elements onto the beginning of the array, shifting all
1079 existing elements over to make room. Accepts scalars, hash refs or array refs.
1080 No return value. This method is not recommended with large arrays -- see
1081 <LARGE ARRAYS> below for details.
1083 $db->unshift("foo", "bar", {});
1087 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1088 -f splice> for usage -- it is too complicated to document here. This method is
1089 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1093 Here are some examples of using arrays:
1095 my $db = DBM::Deep->new(
1097 type => DBM::Deep->TYPE_ARRAY
1100 $db->push("bar", "baz");
1101 $db->unshift("foo");
1104 my $len = $db->length();
1105 print "length: $len\n"; # 4
1107 for (my $k=0; $k<$len; $k++) {
1108 print "$k: " . $db->get($k) . "\n";
1111 $db->splice(1, 2, "biz", "baf");
1113 while (my $elem = shift @$db) {
1114 print "shifted: $elem\n";
1119 Enable automatic file locking by passing a true value to the C<locking>
1120 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1122 my $db = DBM::Deep->new(
1127 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1128 mode for writes, and shared mode for reads. This is required if you have
1129 multiple processes accessing the same database file, to avoid file corruption.
1130 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1131 NFS> below for more.
1133 =head2 EXPLICIT LOCKING
1135 You can explicitly lock a database, so it remains locked for multiple
1136 transactions. This is done by calling the C<lock()> method, and passing an
1137 optional lock mode argument (defaults to exclusive mode). This is particularly
1138 useful for things like counters, where the current value needs to be fetched,
1139 then incremented, then stored again.
1142 my $counter = $db->get("counter");
1144 $db->put("counter", $counter);
1153 You can pass C<lock()> an optional argument, which specifies which mode to use
1154 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1155 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1156 same as the constants defined in Perl's C<Fcntl> module.
1158 $db->lock( DBM::Deep->LOCK_SH );
1162 =head1 IMPORTING/EXPORTING
1164 You can import existing complex structures by calling the C<import()> method,
1165 and export an entire database into an in-memory structure using the C<export()>
1166 method. Both are examined here.
1170 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1171 walking the structure and adding keys/elements to the database as you go,
1172 simply pass a reference to the C<import()> method. This recursively adds
1173 everything to an existing DBM::Deep object for you. Here is an example:
1178 array1 => [ "elem0", "elem1", "elem2" ],
1180 subkey1 => "subvalue1",
1181 subkey2 => "subvalue2"
1185 my $db = DBM::Deep->new( "foo.db" );
1186 $db->import( $struct );
1188 print $db->{key1} . "\n"; # prints "value1"
1190 This recursively imports the entire C<$struct> object into C<$db>, including
1191 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1192 keys are merged with the existing ones, replacing if they already exist.
1193 The C<import()> method can be called on any database level (not just the base
1194 level), and works with both hash and array DB types.
1196 B<Note:> Make sure your existing structure has no circular references in it.
1197 These will cause an infinite loop when importing.
1201 Calling the C<export()> method on an existing DBM::Deep object will return
1202 a reference to a new in-memory copy of the database. The export is done
1203 recursively, so all nested hashes/arrays are all exported to standard Perl
1204 objects. Here is an example:
1206 my $db = DBM::Deep->new( "foo.db" );
1208 $db->{key1} = "value1";
1209 $db->{key2} = "value2";
1211 $db->{hash1}->{subkey1} = "subvalue1";
1212 $db->{hash1}->{subkey2} = "subvalue2";
1214 my $struct = $db->export();
1216 print $struct->{key1} . "\n"; # prints "value1"
1218 This makes a complete copy of the database in memory, and returns a reference
1219 to it. The C<export()> method can be called on any database level (not just
1220 the base level), and works with both hash and array DB types. Be careful of
1221 large databases -- you can store a lot more data in a DBM::Deep object than an
1222 in-memory Perl structure.
1224 B<Note:> Make sure your database has no circular references in it.
1225 These will cause an infinite loop when exporting.
1229 DBM::Deep has a number of hooks where you can specify your own Perl function
1230 to perform filtering on incoming or outgoing data. This is a perfect
1231 way to extend the engine, and implement things like real-time compression or
1232 encryption. Filtering applies to the base DB level, and all child hashes /
1233 arrays. Filter hooks can be specified when your DBM::Deep object is first
1234 constructed, or by calling the C<set_filter()> method at any time. There are
1235 four available filter hooks, described below:
1239 =item * filter_store_key
1241 This filter is called whenever a hash key is stored. It
1242 is passed the incoming key, and expected to return a transformed key.
1244 =item * filter_store_value
1246 This filter is called whenever a hash key or array element is stored. It
1247 is passed the incoming value, and expected to return a transformed value.
1249 =item * filter_fetch_key
1251 This filter is called whenever a hash key is fetched (i.e. via
1252 C<first_key()> or C<next_key()>). It is passed the transformed key,
1253 and expected to return the plain key.
1255 =item * filter_fetch_value
1257 This filter is called whenever a hash key or array element is fetched.
1258 It is passed the transformed value, and expected to return the plain value.
1262 Here are the two ways to setup a filter hook:
1264 my $db = DBM::Deep->new(
1266 filter_store_value => \&my_filter_store,
1267 filter_fetch_value => \&my_filter_fetch
1272 $db->set_filter( "filter_store_value", \&my_filter_store );
1273 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1275 Your filter function will be called only when dealing with SCALAR keys or
1276 values. When nested hashes and arrays are being stored/fetched, filtering
1277 is bypassed. Filters are called as static functions, passed a single SCALAR
1278 argument, and expected to return a single SCALAR value. If you want to
1279 remove a filter, set the function reference to C<undef>:
1281 $db->set_filter( "filter_store_value", undef );
1283 =head2 REAL-TIME ENCRYPTION EXAMPLE
1285 Here is a working example that uses the I<Crypt::Blowfish> module to
1286 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1287 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1288 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1291 use Crypt::Blowfish;
1294 my $cipher = Crypt::CBC->new({
1295 'key' => 'my secret key',
1296 'cipher' => 'Blowfish',
1298 'regenerate_key' => 0,
1299 'padding' => 'space',
1303 my $db = DBM::Deep->new(
1304 file => "foo-encrypt.db",
1305 filter_store_key => \&my_encrypt,
1306 filter_store_value => \&my_encrypt,
1307 filter_fetch_key => \&my_decrypt,
1308 filter_fetch_value => \&my_decrypt,
1311 $db->{key1} = "value1";
1312 $db->{key2} = "value2";
1313 print "key1: " . $db->{key1} . "\n";
1314 print "key2: " . $db->{key2} . "\n";
1320 return $cipher->encrypt( $_[0] );
1323 return $cipher->decrypt( $_[0] );
1326 =head2 REAL-TIME COMPRESSION EXAMPLE
1328 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1329 compression / decompression of keys & values with DBM::Deep Filters.
1330 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1331 more on I<Compress::Zlib>.
1336 my $db = DBM::Deep->new(
1337 file => "foo-compress.db",
1338 filter_store_key => \&my_compress,
1339 filter_store_value => \&my_compress,
1340 filter_fetch_key => \&my_decompress,
1341 filter_fetch_value => \&my_decompress,
1344 $db->{key1} = "value1";
1345 $db->{key2} = "value2";
1346 print "key1: " . $db->{key1} . "\n";
1347 print "key2: " . $db->{key2} . "\n";
1353 return Compress::Zlib::memGzip( $_[0] ) ;
1356 return Compress::Zlib::memGunzip( $_[0] ) ;
1359 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1360 actually numerical index numbers, and are not filtered.
1362 =head1 ERROR HANDLING
1364 Most DBM::Deep methods return a true value for success, and call die() on
1365 failure. You can wrap calls in an eval block to catch the die.
1367 my $db = DBM::Deep->new( "foo.db" ); # create hash
1368 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1370 print $@; # prints error message
1372 =head1 LARGEFILE SUPPORT
1374 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1375 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1376 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1377 by specifying the 'pack_size' parameter when constructing the file.
1380 filename => $filename,
1381 pack_size => 'large',
1384 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1385 instead of 32-bit longs. After setting these values your DB files have a
1386 theoretical maximum size of 16 XB (exabytes).
1388 You can also use C<pack_size =E<gt> 'small'> in order to use 16-bit file
1391 B<Note:> Changing these values will B<NOT> work for existing database files.
1392 Only change this for new files. Once the value has been set, it is stored in
1393 the file's header and cannot be changed for the life of the file. These
1394 parameters are per-file, meaning you can access 32-bit and 64-bit files, as
1397 B<Note:> We have not personally tested files larger than 2 GB -- all my
1398 systems have only a 32-bit Perl. However, I have received user reports that
1399 this does indeed work!
1401 =head1 LOW-LEVEL ACCESS
1403 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1404 you can call the C<_fh()> method, which returns the handle:
1406 my $fh = $db->_fh();
1408 This method can be called on the root level of the datbase, or any child
1409 hashes or arrays. All levels share a I<root> structure, which contains things
1410 like the filehandle, a reference counter, and all the options specified
1411 when you created the object. You can get access to this root structure by
1412 calling the C<root()> method.
1414 my $root = $db->_root();
1416 This is useful for changing options after the object has already been created,
1417 such as enabling/disabling locking. You can also store your own temporary user
1418 data in this structure (be wary of name collision), which is then accessible from
1419 any child hash or array.
1421 =head1 CUSTOM DIGEST ALGORITHM
1423 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1424 keys. However you can override this, and use another algorithm (such as SHA-256)
1425 or even write your own. But please note that DBM::Deep currently expects zero
1426 collisions, so your algorithm has to be I<perfect>, so to speak. Collision
1427 detection may be introduced in a later version.
1429 You can specify a custom digest algorithm by passing it into the parameter
1430 list for new(), passing a reference to a subroutine as the 'digest' parameter,
1431 and the length of the algorithm's hashes (in bytes) as the 'hash_size'
1432 parameter. Here is a working example that uses a 256-bit hash from the
1433 I<Digest::SHA256> module. Please see
1434 L<http://search.cpan.org/search?module=Digest::SHA256> for more information.
1439 my $context = Digest::SHA256::new(256);
1441 my $db = DBM::Deep->new(
1442 filename => "foo-sha.db",
1443 digest => \&my_digest,
1447 $db->{key1} = "value1";
1448 $db->{key2} = "value2";
1449 print "key1: " . $db->{key1} . "\n";
1450 print "key2: " . $db->{key2} . "\n";
1456 return substr( $context->hash($_[0]), 0, 32 );
1459 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1460 of bytes you specify in the hash_size parameter (in this case 32).
1462 B<Note:> If you do choose to use a custom digest algorithm, you must set it
1463 every time you access this file. Otherwise, the default (MD5) will be used.
1465 =head1 CIRCULAR REFERENCES
1467 DBM::Deep has B<experimental> support for circular references. Meaning you
1468 can have a nested hash key or array element that points to a parent object.
1469 This relationship is stored in the DB file, and is preserved between sessions.
1472 my $db = DBM::Deep->new( "foo.db" );
1475 $db->{circle} = $db; # ref to self
1477 print $db->{foo} . "\n"; # prints "bar"
1478 print $db->{circle}->{foo} . "\n"; # prints "bar" again
1480 B<Note>: Passing the object to a function that recursively walks the
1481 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1482 C<export()> methods) will result in an infinite loop. This will be fixed in
1485 =head1 CAVEATS / ISSUES / BUGS
1487 This section describes all the known issues with DBM::Deep. It you have found
1488 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1490 =head2 UNUSED SPACE RECOVERY
1492 One major caveat with DBM::Deep is that space occupied by existing keys and
1493 values is not recovered when they are deleted. Meaning if you keep deleting
1494 and adding new keys, your file will continuously grow. I am working on this,
1495 but in the meantime you can call the built-in C<optimize()> method from time to
1496 time (perhaps in a crontab or something) to recover all your unused space.
1498 $db->optimize(); # returns true on success
1500 This rebuilds the ENTIRE database into a new file, then moves it on top of
1501 the original. The new file will have no unused space, thus it will take up as
1502 little disk space as possible. Please note that this operation can take
1503 a long time for large files, and you need enough disk space to temporarily hold
1504 2 copies of your DB file. The temporary file is created in the same directory
1505 as the original, named with a ".tmp" extension, and is deleted when the
1506 operation completes. Oh, and if locking is enabled, the DB is automatically
1507 locked for the entire duration of the copy.
1509 B<WARNING:> Only call optimize() on the top-level node of the database, and
1510 make sure there are no child references lying around. DBM::Deep keeps a reference
1511 counter, and if it is greater than 1, optimize() will abort and return undef.
1515 (The reasons given assume a high level of Perl understanding, specifically of
1516 references. You can safely skip this section.)
1518 Currently, the only references supported are HASH and ARRAY. The other reference
1519 types (SCALAR, CODE, GLOB, and REF) cannot be supported for various reasons.
1525 These are things like filehandles and other sockets. They can't be supported
1526 because it's completely unclear how DBM::Deep should serialize them.
1528 =item * SCALAR / REF
1530 The discussion here refers to the following type of example:
1537 # In some other process ...
1539 my $val = ${ $db->{key1} };
1541 is( $val, 50, "What actually gets stored in the DB file?" );
1543 The problem is one of synchronization. When the variable being referred to
1544 changes value, the reference isn't notified. This means that the new value won't
1545 be stored in the datafile for other processes to read. There is no TIEREF.
1547 It is theoretically possible to store references to values already within a
1548 DBM::Deep object because everything already is synchronized, but the change to
1549 the internals would be quite large. Specifically, DBM::Deep would have to tie
1550 every single value that is stored. This would bloat the RAM footprint of
1551 DBM::Deep at least twofold (if not more) and be a significant performance drain,
1552 all to support a feature that has never been requested.
1556 L<http://search.cpan.org/search?module=Data::Dump::Streamer> provides a
1557 mechanism for serializing coderefs, including saving off all closure state.
1558 However, just as for SCALAR and REF, that closure state may change without
1559 notifying the DBM::Deep object storing the reference.
1563 =head2 FILE CORRUPTION
1565 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1566 for a 32-bit signature when opened, but other corruption in files can cause
1567 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1568 stuck in an infinite loop depending on the level of corruption. File write
1569 operations are not checked for failure (for speed), so if you happen to run
1570 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1571 be addressed in a later version of DBM::Deep.
1575 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1576 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1577 about setting up your NFS server with a locking daemon, then using lockf() to
1578 lock your files, but your mileage may vary there as well. From what I
1579 understand, there is no real way to do it. However, if you need access to the
1580 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1581 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1583 =head2 COPYING OBJECTS
1585 Beware of copying tied objects in Perl. Very strange things can happen.
1586 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1587 returns a new, blessed, tied hash or array to the same level in the DB.
1589 my $copy = $db->clone();
1591 B<Note>: Since clone() here is cloning the object, not the database location, any
1592 modifications to either $db or $copy will be visible in both.
1596 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1597 These functions cause every element in the array to move, which can be murder
1598 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1599 a different location. This will be addressed in the forthcoming version 1.00.
1601 =head2 WRITEONLY FILES
1603 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1604 writeonly mode. STORE will verify that the filehandle is writable. However, there
1605 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1606 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1610 This section discusses DBM::Deep's speed and memory usage.
1614 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1615 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1616 multi-level hash/array support, and cross-platform FTPable files. Even so,
1617 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1618 with huge databases. Here is some test data:
1620 Adding 1,000,000 keys to new DB file...
1622 At 100 keys, avg. speed is 2,703 keys/sec
1623 At 200 keys, avg. speed is 2,642 keys/sec
1624 At 300 keys, avg. speed is 2,598 keys/sec
1625 At 400 keys, avg. speed is 2,578 keys/sec
1626 At 500 keys, avg. speed is 2,722 keys/sec
1627 At 600 keys, avg. speed is 2,628 keys/sec
1628 At 700 keys, avg. speed is 2,700 keys/sec
1629 At 800 keys, avg. speed is 2,607 keys/sec
1630 At 900 keys, avg. speed is 2,190 keys/sec
1631 At 1,000 keys, avg. speed is 2,570 keys/sec
1632 At 2,000 keys, avg. speed is 2,417 keys/sec
1633 At 3,000 keys, avg. speed is 1,982 keys/sec
1634 At 4,000 keys, avg. speed is 1,568 keys/sec
1635 At 5,000 keys, avg. speed is 1,533 keys/sec
1636 At 6,000 keys, avg. speed is 1,787 keys/sec
1637 At 7,000 keys, avg. speed is 1,977 keys/sec
1638 At 8,000 keys, avg. speed is 2,028 keys/sec
1639 At 9,000 keys, avg. speed is 2,077 keys/sec
1640 At 10,000 keys, avg. speed is 2,031 keys/sec
1641 At 20,000 keys, avg. speed is 1,970 keys/sec
1642 At 30,000 keys, avg. speed is 2,050 keys/sec
1643 At 40,000 keys, avg. speed is 2,073 keys/sec
1644 At 50,000 keys, avg. speed is 1,973 keys/sec
1645 At 60,000 keys, avg. speed is 1,914 keys/sec
1646 At 70,000 keys, avg. speed is 2,091 keys/sec
1647 At 80,000 keys, avg. speed is 2,103 keys/sec
1648 At 90,000 keys, avg. speed is 1,886 keys/sec
1649 At 100,000 keys, avg. speed is 1,970 keys/sec
1650 At 200,000 keys, avg. speed is 2,053 keys/sec
1651 At 300,000 keys, avg. speed is 1,697 keys/sec
1652 At 400,000 keys, avg. speed is 1,838 keys/sec
1653 At 500,000 keys, avg. speed is 1,941 keys/sec
1654 At 600,000 keys, avg. speed is 1,930 keys/sec
1655 At 700,000 keys, avg. speed is 1,735 keys/sec
1656 At 800,000 keys, avg. speed is 1,795 keys/sec
1657 At 900,000 keys, avg. speed is 1,221 keys/sec
1658 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1660 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1661 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1662 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1663 Run time was 12 min 3 sec.
1667 One of the great things about DBM::Deep is that it uses very little memory.
1668 Even with huge databases (1,000,000+ keys) you will not see much increased
1669 memory on your process. DBM::Deep relies solely on the filesystem for storing
1670 and fetching data. Here is output from I</usr/bin/top> before even opening a
1673 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1674 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1676 Basically the process is taking 2,716K of memory. And here is the same
1677 process after storing and fetching 1,000,000 keys:
1679 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1680 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1682 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1683 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1685 =head1 DB FILE FORMAT
1687 In case you were interested in the underlying DB file format, it is documented
1688 here in this section. You don't need to know this to use the module, it's just
1689 included for reference.
1693 DBM::Deep files always start with a 32-bit signature to identify the file type.
1694 This is at offset 0. The signature is "DPDB" in network byte order. This is
1695 checked for when the file is opened and an error will be thrown if it's not found.
1699 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1700 has a standard header containing the type of data, the length of data, and then
1701 the data itself. The type is a single character (1 byte), the length is a
1702 32-bit unsigned long in network byte order, and the data is, well, the data.
1703 Here is how it unfolds:
1707 Immediately after the 32-bit file signature is the I<Master Index> record.
1708 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1709 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1710 depending on how the DBM::Deep object was constructed.
1712 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1713 number). The first 8-bit char of the MD5 signature is the offset into the
1714 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1715 index element is a file offset of the next tag for the key/element in question,
1716 which is usually a I<Bucket List> tag (see below).
1718 The next tag I<could> be another index, depending on how many keys/elements
1719 exist. See L<RE-INDEXING> below for details.
1723 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1724 file offsets to where the actual data is stored. It starts with a standard
1725 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1726 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1727 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1728 When the list fills up, a I<Re-Index> operation is performed (See
1729 L<RE-INDEXING> below).
1733 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1734 index/value pair (in array mode). It starts with a standard tag header with
1735 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1736 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1737 header. The size reported in the tag header is only for the value, but then,
1738 just after the value is another size (32-bit unsigned long) and then the plain
1739 key itself. Since the value is likely to be fetched more often than the plain
1740 key, I figured it would be I<slightly> faster to store the value first.
1742 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1743 record for the nested structure, where the process begins all over again.
1747 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1748 exhausted. Then, when another key/element comes in, the list is converted to a
1749 new index record. However, this index will look at the next char in the MD5
1750 hash, and arrange new Bucket List pointers accordingly. This process is called
1751 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1752 17 (16 + new one) keys/elements are removed from the old Bucket List and
1753 inserted into the new index. Several new Bucket Lists are created in the
1754 process, as a new MD5 char from the key is being examined (it is unlikely that
1755 the keys will all share the same next char of their MD5s).
1757 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1758 when the Bucket Lists will turn into indexes, but the first round tends to
1759 happen right around 4,000 keys. You will see a I<slight> decrease in
1760 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1761 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1762 right around 900,000 keys. This process can continue nearly indefinitely --
1763 right up until the point the I<MD5> signatures start colliding with each other,
1764 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1765 getting struck by lightning while you are walking to cash in your tickets.
1766 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1767 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1768 this is 340 unodecillion, but don't quote me).
1772 When a new key/element is stored, the key (or index number) is first run through
1773 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1774 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1775 for the first char of the signature (in this case I<b0>). If it does not exist,
1776 a new I<Bucket List> is created for our key (and the next 15 future keys that
1777 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1778 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1779 this point, unless we are replacing an existing I<Bucket>), where the actual
1780 data will be stored.
1784 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1785 (or index number), then walking along the indexes. If there are enough
1786 keys/elements in this DB level, there might be nested indexes, each linked to
1787 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1788 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1789 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1790 plain key are stored.
1792 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1793 methods. In this process the indexes are walked systematically, and each key
1794 fetched in increasing MD5 order (which is why it appears random). Once the
1795 I<Bucket> is found, the value is skipped and the plain key returned instead.
1796 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1797 alphabetically sorted. This only happens on an index-level -- as soon as the
1798 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1799 so it's pretty much undefined how the keys will come out -- just like Perl's
1802 =head1 CODE COVERAGE
1804 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1805 B<Devel::Cover> report on this module's test suite.
1807 ----------------------------------- ------ ------ ------ ------ ------ ------
1808 File stmt bran cond sub time total
1809 ----------------------------------- ------ ------ ------ ------ ------ ------
1810 blib/lib/DBM/Deep.pm 94.9 80.6 73.0 100.0 37.9 90.4
1811 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 18.2 98.1
1812 blib/lib/DBM/Deep/Engine.pm 98.9 87.3 80.0 100.0 34.2 95.2
1813 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 9.7 97.3
1814 Total 97.9 85.9 79.7 100.0 100.0 94.3
1815 ----------------------------------- ------ ------ ------ ------ ------ ------
1817 =head1 MORE INFORMATION
1819 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1820 or send email to L<DBM-Deep@googlegroups.com>.
1824 Joseph Huckaby, L<jhuckaby@cpan.org>
1826 Rob Kinyon, L<rkinyon@cpan.org>
1828 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1832 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1833 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1837 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1838 This is free software, you may use it and distribute it under the
1839 same terms as Perl itself.