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 # At this point, we need to replay the actions taken
339 $self->_fileobj->end_transaction;
348 my $self = $_[0]->_get_self;
349 return $self->{fileobj};
353 my $self = $_[0]->_get_self;
354 return $self->{type};
358 my $self = $_[0]->_get_self;
359 return $self->{base_offset};
363 my $self = $_[0]->_get_self;
364 return $self->_fileobj->{fh};
372 die "DBM::Deep: $_[1]\n";
377 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
382 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
387 if ( $self->{parent} ) {
388 my $base = $self->{parent}->_find_parent();
389 if ( $self->{parent}->_type eq TYPE_HASH ) {
390 return $base . "\{$self->{parent_key}\}";
392 return $base . "\[$self->{parent_key}\]";
399 # Store single hash key/value or array element in database.
401 my $self = shift->_get_self;
402 my ($key, $value, $orig_key) = @_;
404 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
405 $self->_throw_error( 'Cannot write to a readonly filehandle' );
408 if ( my $afh = $self->_fileobj->{audit_fh} ) {
409 if ( defined $orig_key ) {
410 my $lhs = $self->_find_parent;
411 if ( $self->_type eq TYPE_HASH ) {
412 $lhs .= "\{$orig_key\}";
415 $lhs .= "\[$orig_key\]";
420 my $r = Scalar::Util::reftype( $value ) || '';
421 if ( $r eq 'HASH' ) {
424 elsif ( $r eq 'ARRAY' ) {
431 if ( my $c = Scalar::Util::blessed( $value ) ) {
432 $rhs = "bless $rhs, '$c'";
435 flock( $afh, LOCK_EX );
436 print( $afh "$lhs = $rhs; # STORE " . localtime(time) . "\n" );
437 flock( $afh, LOCK_UN );
442 # Request exclusive lock for writing
444 $self->lock( LOCK_EX );
446 my $md5 = $self->{engine}{digest}->($key);
448 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5, { create => 1 } );
450 # User may be storing a hash, in which case we do not want it run
451 # through the filtering system
452 if ( !ref($value) && $self->_fileobj->{filter_store_value} ) {
453 $value = $self->_fileobj->{filter_store_value}->( $value );
457 # Add key/value to bucket list
459 my $result = $self->{engine}->add_bucket( $tag, $md5, $key, $value, undef, $orig_key );
468 # Fetch single value or element given plain key or array index
470 my $self = shift->_get_self;
471 my ($key, $orig_key) = @_;
473 my $md5 = $self->{engine}{digest}->($key);
476 # Request shared lock for reading
478 $self->lock( LOCK_SH );
480 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
487 # Get value from bucket list
489 my $result = $self->{engine}->get_bucket_value( $tag, $md5, $orig_key );
493 # Filters only apply to scalar values, so the ref check is making
494 # sure the fetched bucket is a scalar, not a child hash or array.
495 return ($result && !ref($result) && $self->_fileobj->{filter_fetch_value})
496 ? $self->_fileobj->{filter_fetch_value}->($result)
502 # Delete single key/value pair or element given plain key or array index
504 my $self = shift->_get_self;
505 my ($key, $orig_key) = @_;
507 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
508 $self->_throw_error( 'Cannot write to a readonly filehandle' );
511 if ( my $afh = $self->_fileobj->{audit_fh} ) {
512 if ( defined $orig_key ) {
513 my $lhs = $self->_find_parent;
514 if ( $self->_type eq TYPE_HASH ) {
515 $lhs .= "\{$orig_key\}";
518 $lhs .= "\[$orig_key]\]";
521 flock( $afh, LOCK_EX );
522 print( $afh "delete $lhs; # " . localtime(time) . "\n" );
523 flock( $afh, LOCK_UN );
528 # Request exclusive lock for writing
530 $self->lock( LOCK_EX );
532 my $md5 = $self->{engine}{digest}->($key);
534 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
543 my $value = $self->{engine}->get_bucket_value( $tag, $md5 );
545 if (defined $value && !ref($value) && $self->_fileobj->{filter_fetch_value}) {
546 $value = $self->_fileobj->{filter_fetch_value}->($value);
549 my $result = $self->{engine}->delete_bucket( $tag, $md5, $orig_key );
552 # If this object is an array and the key deleted was on the end of the stack,
553 # decrement the length variable.
563 # Check if a single key or element exists given plain key or array index
565 my $self = shift->_get_self;
568 my $md5 = $self->{engine}{digest}->($key);
571 # Request shared lock for reading
573 $self->lock( LOCK_SH );
575 my $tag = $self->{engine}->find_bucket_list( $self->_base_offset, $md5 );
580 # For some reason, the built-in exists() function returns '' for false
586 # Check if bucket exists and return 1 or ''
588 my $result = $self->{engine}->bucket_exists( $tag, $md5 ) || '';
597 # Clear all keys from hash, or all elements from array.
599 my $self = shift->_get_self;
601 if ( $^O ne 'MSWin32' && !_is_writable( $self->_fh ) ) {
602 $self->_throw_error( 'Cannot write to a readonly filehandle' );
605 if ( my $afh = $self->_fileobj->{audit_fh} ) {
606 my $lhs = $self->_find_parent;
609 if ( $self->_type eq TYPE_HASH ) {
610 $lhs = '%{' . $lhs . '}';
613 $lhs = '@{' . $lhs . '}';
616 flock( $afh, LOCK_EX );
617 print( $afh "$lhs = $rhs; # " . localtime(time) . "\n" );
618 flock( $afh, LOCK_UN );
622 # Request exclusive lock for writing
624 $self->lock( LOCK_EX );
628 seek($fh, $self->_base_offset + $self->_fileobj->{file_offset}, SEEK_SET);
634 #XXX This needs updating to use _release_space
635 $self->{engine}->write_tag(
636 $self->_base_offset, $self->_type,
637 chr(0)x$self->{engine}{index_size},
646 # Public method aliases
648 sub put { (shift)->STORE( @_ ) }
649 sub store { (shift)->STORE( @_ ) }
650 sub get { (shift)->FETCH( @_ ) }
651 sub fetch { (shift)->FETCH( @_ ) }
652 sub delete { (shift)->DELETE( @_ ) }
653 sub exists { (shift)->EXISTS( @_ ) }
654 sub clear { (shift)->CLEAR( @_ ) }
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:
816 =item * C<DBM::Deep-E<gt>TYPE_HASH>
818 =item * C<DBM::Deep-E<gt>TYPE_ARRAY>.
822 This only takes effect when beginning a new file. This is an optional
823 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
827 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
828 function to lock the database in exclusive mode for writes, and shared mode for
829 reads. Pass any true value to enable. This affects the base DB handle I<and
830 any child hashes or arrays> that use the same DB file. This is an optional
831 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
835 Specifies whether autoflush is to be enabled on the underlying filehandle.
836 This obviously slows down write operations, but is required if you may have
837 multiple processes accessing the same DB file (also consider enable I<locking>).
838 Pass any true value to enable. This is an optional parameter, and defaults to 0
843 If I<autobless> mode is enabled, DBM::Deep will preserve the class something
844 is blessed into, and restores it when fetched. This is an optional parameter, and defaults to 1 (enabled).
846 B<Note:> If you use the OO-interface, you will not be able to call any methods
847 of DBM::Deep on the blessed item. This is considered to be a feature.
851 See L</FILTERS> below.
857 With DBM::Deep you can access your databases using Perl's standard hash/array
858 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
859 treat them as such. DBM::Deep will intercept all reads/writes and direct them
860 to the right place -- the DB file. This has nothing to do with the
861 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
862 using regular hashes and arrays, rather than calling functions like C<get()>
863 and C<put()> (although those work too). It is entirely up to you how to want
864 to access your databases.
868 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
869 or even nested hashes (or arrays) using standard Perl syntax:
871 my $db = DBM::Deep->new( "foo.db" );
873 $db->{mykey} = "myvalue";
875 $db->{myhash}->{subkey} = "subvalue";
877 print $db->{myhash}->{subkey} . "\n";
879 You can even step through hash keys using the normal Perl C<keys()> function:
881 foreach my $key (keys %$db) {
882 print "$key: " . $db->{$key} . "\n";
885 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
886 pushes them onto an array, all before the loop even begins. If you have an
887 extra large hash, this may exhaust Perl's memory. Instead, consider using
888 Perl's C<each()> function, which pulls keys/values one at a time, using very
891 while (my ($key, $value) = each %$db) {
892 print "$key: $value\n";
895 Please note that when using C<each()>, you should always pass a direct
896 hash reference, not a lookup. Meaning, you should B<never> do this:
899 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
901 This causes an infinite loop, because for each iteration, Perl is calling
902 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
903 it effectively keeps returning the first key over and over again. Instead,
904 assign a temporary variable to C<$db->{foo}>, then pass that to each().
908 As with hashes, you can treat any DBM::Deep object like a normal Perl array
909 reference. This includes inserting, removing and manipulating elements,
910 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
911 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
912 or simply be a nested array reference inside a hash. Example:
914 my $db = DBM::Deep->new(
915 file => "foo-array.db",
916 type => DBM::Deep->TYPE_ARRAY
920 push @$db, "bar", "baz";
923 my $last_elem = pop @$db; # baz
924 my $first_elem = shift @$db; # bah
925 my $second_elem = $db->[1]; # bar
927 my $num_elements = scalar @$db;
931 In addition to the I<tie()> interface, you can also use a standard OO interface
932 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
933 array) has its own methods, but both types share the following common methods:
934 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
938 =item * new() / clone()
940 These are the constructor and copy-functions.
942 =item * put() / store()
944 Stores a new hash key/value pair, or sets an array element value. Takes two
945 arguments, the hash key or array index, and the new value. The value can be
946 a scalar, hash ref or array ref. Returns true on success, false on failure.
948 $db->put("foo", "bar"); # for hashes
949 $db->put(1, "bar"); # for arrays
951 =item * get() / fetch()
953 Fetches the value of a hash key or array element. Takes one argument: the hash
954 key or array index. Returns a scalar, hash ref or array ref, depending on the
957 my $value = $db->get("foo"); # for hashes
958 my $value = $db->get(1); # for arrays
962 Checks if a hash key or array index exists. Takes one argument: the hash key
963 or array index. Returns true if it exists, false if not.
965 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
966 if ($db->exists(1)) { print "yay!\n"; } # for arrays
970 Deletes one hash key/value pair or array element. Takes one argument: the hash
971 key or array index. Returns true on success, false if not found. For arrays,
972 the remaining elements located after the deleted element are NOT moved over.
973 The deleted element is essentially just undefined, which is exactly how Perl's
974 internal arrays work. Please note that the space occupied by the deleted
975 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
976 below for details and workarounds.
978 $db->delete("foo"); # for hashes
979 $db->delete(1); # for arrays
983 Deletes B<all> hash keys or array elements. Takes no arguments. No return
984 value. Please note that the space occupied by the deleted keys/values or
985 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
986 details and workarounds.
988 $db->clear(); # hashes or arrays
990 =item * lock() / unlock()
996 Recover lost disk space.
998 =item * import() / export()
1000 Data going in and out.
1006 For hashes, DBM::Deep supports all the common methods described above, and the
1007 following additional methods: C<first_key()> and C<next_key()>.
1013 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1014 fetched in an undefined order (which appears random). Takes no arguments,
1015 returns the key as a scalar value.
1017 my $key = $db->first_key();
1021 Returns the "next" key in the hash, given the previous one as the sole argument.
1022 Returns undef if there are no more keys to be fetched.
1024 $key = $db->next_key($key);
1028 Here are some examples of using hashes:
1030 my $db = DBM::Deep->new( "foo.db" );
1032 $db->put("foo", "bar");
1033 print "foo: " . $db->get("foo") . "\n";
1035 $db->put("baz", {}); # new child hash ref
1036 $db->get("baz")->put("buz", "biz");
1037 print "buz: " . $db->get("baz")->get("buz") . "\n";
1039 my $key = $db->first_key();
1041 print "$key: " . $db->get($key) . "\n";
1042 $key = $db->next_key($key);
1045 if ($db->exists("foo")) { $db->delete("foo"); }
1049 For arrays, DBM::Deep supports all the common methods described above, and the
1050 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1051 C<unshift()> and C<splice()>.
1057 Returns the number of elements in the array. Takes no arguments.
1059 my $len = $db->length();
1063 Adds one or more elements onto the end of the array. Accepts scalars, hash
1064 refs or array refs. No return value.
1066 $db->push("foo", "bar", {});
1070 Fetches the last element in the array, and deletes it. Takes no arguments.
1071 Returns undef if array is empty. Returns the element value.
1073 my $elem = $db->pop();
1077 Fetches the first element in the array, deletes it, then shifts all the
1078 remaining elements over to take up the space. Returns the element value. This
1079 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1082 my $elem = $db->shift();
1086 Inserts one or more elements onto the beginning of the array, shifting all
1087 existing elements over to make room. Accepts scalars, hash refs or array refs.
1088 No return value. This method is not recommended with large arrays -- see
1089 <LARGE ARRAYS> below for details.
1091 $db->unshift("foo", "bar", {});
1095 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1096 -f splice> for usage -- it is too complicated to document here. This method is
1097 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1101 Here are some examples of using arrays:
1103 my $db = DBM::Deep->new(
1105 type => DBM::Deep->TYPE_ARRAY
1108 $db->push("bar", "baz");
1109 $db->unshift("foo");
1112 my $len = $db->length();
1113 print "length: $len\n"; # 4
1115 for (my $k=0; $k<$len; $k++) {
1116 print "$k: " . $db->get($k) . "\n";
1119 $db->splice(1, 2, "biz", "baf");
1121 while (my $elem = shift @$db) {
1122 print "shifted: $elem\n";
1127 Enable automatic file locking by passing a true value to the C<locking>
1128 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1130 my $db = DBM::Deep->new(
1135 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1136 mode for writes, and shared mode for reads. This is required if you have
1137 multiple processes accessing the same database file, to avoid file corruption.
1138 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1139 NFS> below for more.
1141 =head2 EXPLICIT LOCKING
1143 You can explicitly lock a database, so it remains locked for multiple
1144 transactions. This is done by calling the C<lock()> method, and passing an
1145 optional lock mode argument (defaults to exclusive mode). This is particularly
1146 useful for things like counters, where the current value needs to be fetched,
1147 then incremented, then stored again.
1150 my $counter = $db->get("counter");
1152 $db->put("counter", $counter);
1161 You can pass C<lock()> an optional argument, which specifies which mode to use
1162 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1163 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1164 same as the constants defined in Perl's C<Fcntl> module.
1166 $db->lock( DBM::Deep->LOCK_SH );
1170 =head1 IMPORTING/EXPORTING
1172 You can import existing complex structures by calling the C<import()> method,
1173 and export an entire database into an in-memory structure using the C<export()>
1174 method. Both are examined here.
1178 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1179 walking the structure and adding keys/elements to the database as you go,
1180 simply pass a reference to the C<import()> method. This recursively adds
1181 everything to an existing DBM::Deep object for you. Here is an example:
1186 array1 => [ "elem0", "elem1", "elem2" ],
1188 subkey1 => "subvalue1",
1189 subkey2 => "subvalue2"
1193 my $db = DBM::Deep->new( "foo.db" );
1194 $db->import( $struct );
1196 print $db->{key1} . "\n"; # prints "value1"
1198 This recursively imports the entire C<$struct> object into C<$db>, including
1199 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1200 keys are merged with the existing ones, replacing if they already exist.
1201 The C<import()> method can be called on any database level (not just the base
1202 level), and works with both hash and array DB types.
1204 B<Note:> Make sure your existing structure has no circular references in it.
1205 These will cause an infinite loop when importing.
1209 Calling the C<export()> method on an existing DBM::Deep object will return
1210 a reference to a new in-memory copy of the database. The export is done
1211 recursively, so all nested hashes/arrays are all exported to standard Perl
1212 objects. Here is an example:
1214 my $db = DBM::Deep->new( "foo.db" );
1216 $db->{key1} = "value1";
1217 $db->{key2} = "value2";
1219 $db->{hash1}->{subkey1} = "subvalue1";
1220 $db->{hash1}->{subkey2} = "subvalue2";
1222 my $struct = $db->export();
1224 print $struct->{key1} . "\n"; # prints "value1"
1226 This makes a complete copy of the database in memory, and returns a reference
1227 to it. The C<export()> method can be called on any database level (not just
1228 the base level), and works with both hash and array DB types. Be careful of
1229 large databases -- you can store a lot more data in a DBM::Deep object than an
1230 in-memory Perl structure.
1232 B<Note:> Make sure your database has no circular references in it.
1233 These will cause an infinite loop when exporting.
1237 DBM::Deep has a number of hooks where you can specify your own Perl function
1238 to perform filtering on incoming or outgoing data. This is a perfect
1239 way to extend the engine, and implement things like real-time compression or
1240 encryption. Filtering applies to the base DB level, and all child hashes /
1241 arrays. Filter hooks can be specified when your DBM::Deep object is first
1242 constructed, or by calling the C<set_filter()> method at any time. There are
1243 four available filter hooks, described below:
1247 =item * filter_store_key
1249 This filter is called whenever a hash key is stored. It
1250 is passed the incoming key, and expected to return a transformed key.
1252 =item * filter_store_value
1254 This filter is called whenever a hash key or array element is stored. It
1255 is passed the incoming value, and expected to return a transformed value.
1257 =item * filter_fetch_key
1259 This filter is called whenever a hash key is fetched (i.e. via
1260 C<first_key()> or C<next_key()>). It is passed the transformed key,
1261 and expected to return the plain key.
1263 =item * filter_fetch_value
1265 This filter is called whenever a hash key or array element is fetched.
1266 It is passed the transformed value, and expected to return the plain value.
1270 Here are the two ways to setup a filter hook:
1272 my $db = DBM::Deep->new(
1274 filter_store_value => \&my_filter_store,
1275 filter_fetch_value => \&my_filter_fetch
1280 $db->set_filter( "filter_store_value", \&my_filter_store );
1281 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1283 Your filter function will be called only when dealing with SCALAR keys or
1284 values. When nested hashes and arrays are being stored/fetched, filtering
1285 is bypassed. Filters are called as static functions, passed a single SCALAR
1286 argument, and expected to return a single SCALAR value. If you want to
1287 remove a filter, set the function reference to C<undef>:
1289 $db->set_filter( "filter_store_value", undef );
1291 =head2 REAL-TIME ENCRYPTION EXAMPLE
1293 Here is a working example that uses the I<Crypt::Blowfish> module to
1294 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1295 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1296 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1299 use Crypt::Blowfish;
1302 my $cipher = Crypt::CBC->new({
1303 'key' => 'my secret key',
1304 'cipher' => 'Blowfish',
1306 'regenerate_key' => 0,
1307 'padding' => 'space',
1311 my $db = DBM::Deep->new(
1312 file => "foo-encrypt.db",
1313 filter_store_key => \&my_encrypt,
1314 filter_store_value => \&my_encrypt,
1315 filter_fetch_key => \&my_decrypt,
1316 filter_fetch_value => \&my_decrypt,
1319 $db->{key1} = "value1";
1320 $db->{key2} = "value2";
1321 print "key1: " . $db->{key1} . "\n";
1322 print "key2: " . $db->{key2} . "\n";
1328 return $cipher->encrypt( $_[0] );
1331 return $cipher->decrypt( $_[0] );
1334 =head2 REAL-TIME COMPRESSION EXAMPLE
1336 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1337 compression / decompression of keys & values with DBM::Deep Filters.
1338 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1339 more on I<Compress::Zlib>.
1344 my $db = DBM::Deep->new(
1345 file => "foo-compress.db",
1346 filter_store_key => \&my_compress,
1347 filter_store_value => \&my_compress,
1348 filter_fetch_key => \&my_decompress,
1349 filter_fetch_value => \&my_decompress,
1352 $db->{key1} = "value1";
1353 $db->{key2} = "value2";
1354 print "key1: " . $db->{key1} . "\n";
1355 print "key2: " . $db->{key2} . "\n";
1361 return Compress::Zlib::memGzip( $_[0] ) ;
1364 return Compress::Zlib::memGunzip( $_[0] ) ;
1367 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1368 actually numerical index numbers, and are not filtered.
1370 =head1 ERROR HANDLING
1372 Most DBM::Deep methods return a true value for success, and call die() on
1373 failure. You can wrap calls in an eval block to catch the die.
1375 my $db = DBM::Deep->new( "foo.db" ); # create hash
1376 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1378 print $@; # prints error message
1380 =head1 LARGEFILE SUPPORT
1382 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1383 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1384 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1385 by specifying the 'pack_size' parameter when constructing the file.
1388 filename => $filename,
1389 pack_size => 'large',
1392 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1393 instead of 32-bit longs. After setting these values your DB files have a
1394 theoretical maximum size of 16 XB (exabytes).
1396 You can also use C<pack_size =E<gt> 'small'> in order to use 16-bit file
1399 B<Note:> Changing these values will B<NOT> work for existing database files.
1400 Only change this for new files. Once the value has been set, it is stored in
1401 the file's header and cannot be changed for the life of the file. These
1402 parameters are per-file, meaning you can access 32-bit and 64-bit files, as
1405 B<Note:> We have not personally tested files larger than 2 GB -- all my
1406 systems have only a 32-bit Perl. However, I have received user reports that
1407 this does indeed work!
1409 =head1 LOW-LEVEL ACCESS
1411 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1412 you can call the C<_fh()> method, which returns the handle:
1414 my $fh = $db->_fh();
1416 This method can be called on the root level of the datbase, or any child
1417 hashes or arrays. All levels share a I<root> structure, which contains things
1418 like the filehandle, a reference counter, and all the options specified
1419 when you created the object. You can get access to this file object by
1420 calling the C<_fileobj()> method.
1422 my $file_obj = $db->_fileobj();
1424 This is useful for changing options after the object has already been created,
1425 such as enabling/disabling locking. You can also store your own temporary user
1426 data in this structure (be wary of name collision), which is then accessible from
1427 any child hash or array.
1429 =head1 CUSTOM DIGEST ALGORITHM
1431 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1432 keys. However you can override this, and use another algorithm (such as SHA-256)
1433 or even write your own. But please note that DBM::Deep currently expects zero
1434 collisions, so your algorithm has to be I<perfect>, so to speak. Collision
1435 detection may be introduced in a later version.
1437 You can specify a custom digest algorithm by passing it into the parameter
1438 list for new(), passing a reference to a subroutine as the 'digest' parameter,
1439 and the length of the algorithm's hashes (in bytes) as the 'hash_size'
1440 parameter. Here is a working example that uses a 256-bit hash from the
1441 I<Digest::SHA256> module. Please see
1442 L<http://search.cpan.org/search?module=Digest::SHA256> for more information.
1447 my $context = Digest::SHA256::new(256);
1449 my $db = DBM::Deep->new(
1450 filename => "foo-sha.db",
1451 digest => \&my_digest,
1455 $db->{key1} = "value1";
1456 $db->{key2} = "value2";
1457 print "key1: " . $db->{key1} . "\n";
1458 print "key2: " . $db->{key2} . "\n";
1464 return substr( $context->hash($_[0]), 0, 32 );
1467 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1468 of bytes you specify in the hash_size parameter (in this case 32).
1470 B<Note:> If you do choose to use a custom digest algorithm, you must set it
1471 every time you access this file. Otherwise, the default (MD5) will be used.
1473 =head1 CIRCULAR REFERENCES
1475 DBM::Deep has B<experimental> support for circular references. Meaning you
1476 can have a nested hash key or array element that points to a parent object.
1477 This relationship is stored in the DB file, and is preserved between sessions.
1480 my $db = DBM::Deep->new( "foo.db" );
1483 $db->{circle} = $db; # ref to self
1485 print $db->{foo} . "\n"; # prints "bar"
1486 print $db->{circle}->{foo} . "\n"; # prints "bar" again
1488 B<Note>: Passing the object to a function that recursively walks the
1489 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1490 C<export()> methods) will result in an infinite loop. This will be fixed in
1493 =head1 CAVEATS / ISSUES / BUGS
1495 This section describes all the known issues with DBM::Deep. It you have found
1496 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1498 =head2 UNUSED SPACE RECOVERY
1500 One major caveat with DBM::Deep is that space occupied by existing keys and
1501 values is not recovered when they are deleted. Meaning if you keep deleting
1502 and adding new keys, your file will continuously grow. I am working on this,
1503 but in the meantime you can call the built-in C<optimize()> method from time to
1504 time (perhaps in a crontab or something) to recover all your unused space.
1506 $db->optimize(); # returns true on success
1508 This rebuilds the ENTIRE database into a new file, then moves it on top of
1509 the original. The new file will have no unused space, thus it will take up as
1510 little disk space as possible. Please note that this operation can take
1511 a long time for large files, and you need enough disk space to temporarily hold
1512 2 copies of your DB file. The temporary file is created in the same directory
1513 as the original, named with a ".tmp" extension, and is deleted when the
1514 operation completes. Oh, and if locking is enabled, the DB is automatically
1515 locked for the entire duration of the copy.
1517 B<WARNING:> Only call optimize() on the top-level node of the database, and
1518 make sure there are no child references lying around. DBM::Deep keeps a reference
1519 counter, and if it is greater than 1, optimize() will abort and return undef.
1523 (The reasons given assume a high level of Perl understanding, specifically of
1524 references. You can safely skip this section.)
1526 Currently, the only references supported are HASH and ARRAY. The other reference
1527 types (SCALAR, CODE, GLOB, and REF) cannot be supported for various reasons.
1533 These are things like filehandles and other sockets. They can't be supported
1534 because it's completely unclear how DBM::Deep should serialize them.
1536 =item * SCALAR / REF
1538 The discussion here refers to the following type of example:
1545 # In some other process ...
1547 my $val = ${ $db->{key1} };
1549 is( $val, 50, "What actually gets stored in the DB file?" );
1551 The problem is one of synchronization. When the variable being referred to
1552 changes value, the reference isn't notified. This means that the new value won't
1553 be stored in the datafile for other processes to read. There is no TIEREF.
1555 It is theoretically possible to store references to values already within a
1556 DBM::Deep object because everything already is synchronized, but the change to
1557 the internals would be quite large. Specifically, DBM::Deep would have to tie
1558 every single value that is stored. This would bloat the RAM footprint of
1559 DBM::Deep at least twofold (if not more) and be a significant performance drain,
1560 all to support a feature that has never been requested.
1564 L<http://search.cpan.org/search?module=Data::Dump::Streamer> provides a
1565 mechanism for serializing coderefs, including saving off all closure state.
1566 However, just as for SCALAR and REF, that closure state may change without
1567 notifying the DBM::Deep object storing the reference.
1571 =head2 FILE CORRUPTION
1573 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1574 for a 32-bit signature when opened, but other corruption in files can cause
1575 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1576 stuck in an infinite loop depending on the level of corruption. File write
1577 operations are not checked for failure (for speed), so if you happen to run
1578 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1579 be addressed in a later version of DBM::Deep.
1583 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1584 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1585 about setting up your NFS server with a locking daemon, then using lockf() to
1586 lock your files, but your mileage may vary there as well. From what I
1587 understand, there is no real way to do it. However, if you need access to the
1588 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1589 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1591 =head2 COPYING OBJECTS
1593 Beware of copying tied objects in Perl. Very strange things can happen.
1594 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1595 returns a new, blessed, tied hash or array to the same level in the DB.
1597 my $copy = $db->clone();
1599 B<Note>: Since clone() here is cloning the object, not the database location, any
1600 modifications to either $db or $copy will be visible in both.
1604 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1605 These functions cause every element in the array to move, which can be murder
1606 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1607 a different location. This will be addressed in the forthcoming version 1.00.
1609 =head2 WRITEONLY FILES
1611 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1612 writeonly mode. STORE will verify that the filehandle is writable. However, there
1613 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1614 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1618 This section discusses DBM::Deep's speed and memory usage.
1622 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1623 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1624 multi-level hash/array support, and cross-platform FTPable files. Even so,
1625 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1626 with huge databases. Here is some test data:
1628 Adding 1,000,000 keys to new DB file...
1630 At 100 keys, avg. speed is 2,703 keys/sec
1631 At 200 keys, avg. speed is 2,642 keys/sec
1632 At 300 keys, avg. speed is 2,598 keys/sec
1633 At 400 keys, avg. speed is 2,578 keys/sec
1634 At 500 keys, avg. speed is 2,722 keys/sec
1635 At 600 keys, avg. speed is 2,628 keys/sec
1636 At 700 keys, avg. speed is 2,700 keys/sec
1637 At 800 keys, avg. speed is 2,607 keys/sec
1638 At 900 keys, avg. speed is 2,190 keys/sec
1639 At 1,000 keys, avg. speed is 2,570 keys/sec
1640 At 2,000 keys, avg. speed is 2,417 keys/sec
1641 At 3,000 keys, avg. speed is 1,982 keys/sec
1642 At 4,000 keys, avg. speed is 1,568 keys/sec
1643 At 5,000 keys, avg. speed is 1,533 keys/sec
1644 At 6,000 keys, avg. speed is 1,787 keys/sec
1645 At 7,000 keys, avg. speed is 1,977 keys/sec
1646 At 8,000 keys, avg. speed is 2,028 keys/sec
1647 At 9,000 keys, avg. speed is 2,077 keys/sec
1648 At 10,000 keys, avg. speed is 2,031 keys/sec
1649 At 20,000 keys, avg. speed is 1,970 keys/sec
1650 At 30,000 keys, avg. speed is 2,050 keys/sec
1651 At 40,000 keys, avg. speed is 2,073 keys/sec
1652 At 50,000 keys, avg. speed is 1,973 keys/sec
1653 At 60,000 keys, avg. speed is 1,914 keys/sec
1654 At 70,000 keys, avg. speed is 2,091 keys/sec
1655 At 80,000 keys, avg. speed is 2,103 keys/sec
1656 At 90,000 keys, avg. speed is 1,886 keys/sec
1657 At 100,000 keys, avg. speed is 1,970 keys/sec
1658 At 200,000 keys, avg. speed is 2,053 keys/sec
1659 At 300,000 keys, avg. speed is 1,697 keys/sec
1660 At 400,000 keys, avg. speed is 1,838 keys/sec
1661 At 500,000 keys, avg. speed is 1,941 keys/sec
1662 At 600,000 keys, avg. speed is 1,930 keys/sec
1663 At 700,000 keys, avg. speed is 1,735 keys/sec
1664 At 800,000 keys, avg. speed is 1,795 keys/sec
1665 At 900,000 keys, avg. speed is 1,221 keys/sec
1666 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1668 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1669 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1670 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1671 Run time was 12 min 3 sec.
1675 One of the great things about DBM::Deep is that it uses very little memory.
1676 Even with huge databases (1,000,000+ keys) you will not see much increased
1677 memory on your process. DBM::Deep relies solely on the filesystem for storing
1678 and fetching data. Here is output from I</usr/bin/top> before even opening a
1681 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1682 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1684 Basically the process is taking 2,716K of memory. And here is the same
1685 process after storing and fetching 1,000,000 keys:
1687 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1688 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1690 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1691 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1693 =head1 DB FILE FORMAT
1695 In case you were interested in the underlying DB file format, it is documented
1696 here in this section. You don't need to know this to use the module, it's just
1697 included for reference.
1701 DBM::Deep files always start with a 32-bit signature to identify the file type.
1702 This is at offset 0. The signature is "DPDB" in network byte order. This is
1703 checked for when the file is opened and an error will be thrown if it's not found.
1707 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1708 has a standard header containing the type of data, the length of data, and then
1709 the data itself. The type is a single character (1 byte), the length is a
1710 32-bit unsigned long in network byte order, and the data is, well, the data.
1711 Here is how it unfolds:
1715 Immediately after the 32-bit file signature is the I<Master Index> record.
1716 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1717 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1718 depending on how the DBM::Deep object was constructed.
1720 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1721 number). The first 8-bit char of the MD5 signature is the offset into the
1722 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1723 index element is a file offset of the next tag for the key/element in question,
1724 which is usually a I<Bucket List> tag (see below).
1726 The next tag I<could> be another index, depending on how many keys/elements
1727 exist. See L<RE-INDEXING> below for details.
1731 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1732 file offsets to where the actual data is stored. It starts with a standard
1733 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1734 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1735 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1736 When the list fills up, a I<Re-Index> operation is performed (See
1737 L<RE-INDEXING> below).
1741 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1742 index/value pair (in array mode). It starts with a standard tag header with
1743 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1744 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1745 header. The size reported in the tag header is only for the value, but then,
1746 just after the value is another size (32-bit unsigned long) and then the plain
1747 key itself. Since the value is likely to be fetched more often than the plain
1748 key, I figured it would be I<slightly> faster to store the value first.
1750 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1751 record for the nested structure, where the process begins all over again.
1755 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1756 exhausted. Then, when another key/element comes in, the list is converted to a
1757 new index record. However, this index will look at the next char in the MD5
1758 hash, and arrange new Bucket List pointers accordingly. This process is called
1759 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1760 17 (16 + new one) keys/elements are removed from the old Bucket List and
1761 inserted into the new index. Several new Bucket Lists are created in the
1762 process, as a new MD5 char from the key is being examined (it is unlikely that
1763 the keys will all share the same next char of their MD5s).
1765 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1766 when the Bucket Lists will turn into indexes, but the first round tends to
1767 happen right around 4,000 keys. You will see a I<slight> decrease in
1768 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1769 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1770 right around 900,000 keys. This process can continue nearly indefinitely --
1771 right up until the point the I<MD5> signatures start colliding with each other,
1772 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1773 getting struck by lightning while you are walking to cash in your tickets.
1774 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1775 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1776 this is 340 unodecillion, but don't quote me).
1780 When a new key/element is stored, the key (or index number) is first run through
1781 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1782 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1783 for the first char of the signature (in this case I<b0>). If it does not exist,
1784 a new I<Bucket List> is created for our key (and the next 15 future keys that
1785 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1786 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1787 this point, unless we are replacing an existing I<Bucket>), where the actual
1788 data will be stored.
1792 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1793 (or index number), then walking along the indexes. If there are enough
1794 keys/elements in this DB level, there might be nested indexes, each linked to
1795 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1796 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1797 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1798 plain key are stored.
1800 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1801 methods. In this process the indexes are walked systematically, and each key
1802 fetched in increasing MD5 order (which is why it appears random). Once the
1803 I<Bucket> is found, the value is skipped and the plain key returned instead.
1804 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1805 alphabetically sorted. This only happens on an index-level -- as soon as the
1806 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1807 so it's pretty much undefined how the keys will come out -- just like Perl's
1810 =head1 CODE COVERAGE
1812 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1813 B<Devel::Cover> report on this module's test suite.
1815 ----------------------------------- ------ ------ ------ ------ ------ ------
1816 File stmt bran cond sub time total
1817 ----------------------------------- ------ ------ ------ ------ ------ ------
1818 blib/lib/DBM/Deep.pm 94.9 80.6 73.0 100.0 37.9 90.4
1819 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 18.2 98.1
1820 blib/lib/DBM/Deep/Engine.pm 98.9 87.3 80.0 100.0 34.2 95.2
1821 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 9.7 97.3
1822 Total 97.9 85.9 79.7 100.0 100.0 94.3
1823 ----------------------------------- ------ ------ ------ ------ ------ ------
1825 =head1 MORE INFORMATION
1827 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1828 or send email to L<DBM-Deep@googlegroups.com>.
1832 Joseph Huckaby, L<jhuckaby@cpan.org>
1834 Rob Kinyon, L<rkinyon@cpan.org>
1836 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1840 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1841 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1845 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1846 This is free software, you may use it and distribute it under the
1847 same terms as Perl itself.