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);
45 # Setup file and tag signatures. These should never change.
47 sub SIG_FILE () { 'DPDB' }
48 sub SIG_HASH () { 'H' }
49 sub SIG_ARRAY () { 'A' }
50 sub SIG_SCALAR () { 'S' }
51 sub SIG_NULL () { 'N' }
52 sub SIG_DATA () { 'D' }
53 sub SIG_INDEX () { 'I' }
54 sub SIG_BLIST () { 'B' }
58 # Setup constants for users to pass to new()
60 sub TYPE_HASH () { SIG_HASH }
61 sub TYPE_ARRAY () { SIG_ARRAY }
62 sub TYPE_SCALAR () { SIG_SCALAR }
70 $proto->_throw_error( "Odd number of parameters to " . (caller(1))[2] );
75 unless ( eval { local $SIG{'__DIE__'}; %{$_[0]} || 1 } ) {
76 $proto->_throw_error( "Not a hashref in args to " . (caller(1))[2] );
81 $args = { file => shift };
89 # Class constructor method for Perl OO interface.
90 # Calls tie() and returns blessed reference to tied hash or array,
91 # providing a hybrid OO/tie interface.
94 my $args = $class->_get_args( @_ );
97 # Check if we want a tied hash or array.
100 if (defined($args->{type}) && $args->{type} eq TYPE_ARRAY) {
101 $class = 'DBM::Deep::Array';
102 require DBM::Deep::Array;
103 tie @$self, $class, %$args;
106 $class = 'DBM::Deep::Hash';
107 require DBM::Deep::Hash;
108 tie %$self, $class, %$args;
111 return bless $self, $class;
116 # Setup $self and bless into this class.
121 # These are the defaults to be optionally overridden below
124 base_offset => length(SIG_FILE),
125 engine => DBM::Deep::Engine->new,
128 foreach my $param ( keys %$self ) {
129 next unless exists $args->{$param};
130 $self->{$param} = delete $args->{$param}
133 # locking implicitly enables autoflush
134 if ($args->{locking}) { $args->{autoflush} = 1; }
136 $self->{root} = exists $args->{root}
138 : DBM::Deep::_::Root->new( $args );
140 $self->{engine}->setup_fh( $self );
147 require DBM::Deep::Hash;
148 return DBM::Deep::Hash->TIEHASH( @_ );
153 require DBM::Deep::Array;
154 return DBM::Deep::Array->TIEARRAY( @_ );
157 #XXX Unneeded now ...
163 # If db locking is set, flock() the db file. If called multiple
164 # times before unlock(), then the same number of unlocks() must
165 # be called before the lock is released.
167 my $self = $_[0]->_get_self;
169 $type = LOCK_EX unless defined $type;
171 if (!defined($self->_fh)) { return; }
173 if ($self->_root->{locking}) {
174 if (!$self->_root->{locked}) {
175 flock($self->_fh, $type);
177 # refresh end counter in case file has changed size
178 my @stats = stat($self->_root->{file});
179 $self->_root->{end} = $stats[7];
181 # double-check file inode, in case another process
182 # has optimize()d our file while we were waiting.
183 if ($stats[1] != $self->_root->{inode}) {
184 $self->{engine}->close_fh( $self );
185 $self->{engine}->setup_fh( $self );
186 flock($self->_fh, $type); # re-lock
188 # This may not be necessary after re-opening
189 $self->_root->{end} = (stat($self->_fh))[7]; # re-end
192 $self->_root->{locked}++;
202 # If db locking is set, unlock the db file. See note in lock()
203 # regarding calling lock() multiple times.
205 my $self = $_[0]->_get_self;
207 if (!defined($self->_fh)) { return; }
209 if ($self->_root->{locking} && $self->_root->{locked} > 0) {
210 $self->_root->{locked}--;
211 if (!$self->_root->{locked}) { flock($self->_fh, LOCK_UN); }
220 my $self = shift->_get_self;
221 my ($spot, $value) = @_;
226 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
227 my $type = $value->_type;
228 ${$spot} = $type eq TYPE_HASH ? {} : [];
229 $value->_copy_node( ${$spot} );
232 my $r = Scalar::Util::reftype( $value );
233 my $c = Scalar::Util::blessed( $value );
234 if ( $r eq 'ARRAY' ) {
235 ${$spot} = [ @{$value} ];
238 ${$spot} = { %{$value} };
240 ${$spot} = bless ${$spot}, $c
249 # Copy single level of keys or elements to new DB handle.
250 # Recurse for nested structures
252 my $self = shift->_get_self;
255 if ($self->_type eq TYPE_HASH) {
256 my $key = $self->first_key();
258 my $value = $self->get($key);
259 $self->_copy_value( \$db_temp->{$key}, $value );
260 $key = $self->next_key($key);
264 my $length = $self->length();
265 for (my $index = 0; $index < $length; $index++) {
266 my $value = $self->get($index);
267 $self->_copy_value( \$db_temp->[$index], $value );
276 # Recursively export into standard Perl hashes and arrays.
278 my $self = $_[0]->_get_self;
281 if ($self->_type eq TYPE_HASH) { $temp = {}; }
282 elsif ($self->_type eq TYPE_ARRAY) { $temp = []; }
285 $self->_copy_node( $temp );
293 # Recursively import Perl hash/array structure
295 #XXX This use of ref() seems to be ok
296 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
298 my $self = $_[0]->_get_self;
301 #XXX This use of ref() seems to be ok
304 # struct is not a reference, so just import based on our type
308 if ($self->_type eq TYPE_HASH) { $struct = {@_}; }
309 elsif ($self->_type eq TYPE_ARRAY) { $struct = [@_]; }
312 my $r = Scalar::Util::reftype($struct) || '';
313 if ($r eq "HASH" && $self->_type eq TYPE_HASH) {
314 foreach my $key (keys %$struct) { $self->put($key, $struct->{$key}); }
316 elsif ($r eq "ARRAY" && $self->_type eq TYPE_ARRAY) {
317 $self->push( @$struct );
320 return $self->_throw_error("Cannot import: type mismatch");
328 # Rebuild entire database into new file, then move
329 # it back on top of original.
331 my $self = $_[0]->_get_self;
333 #XXX Need to create a new test for this
334 # if ($self->_root->{links} > 1) {
335 # return $self->_throw_error("Cannot optimize: reference count is greater than 1");
338 my $db_temp = DBM::Deep->new(
339 file => $self->_root->{file} . '.tmp',
343 return $self->_throw_error("Cannot optimize: failed to open temp file: $!");
347 $self->_copy_node( $db_temp );
351 # Attempt to copy user, group and permissions over to new file
353 my @stats = stat($self->_fh);
354 my $perms = $stats[2] & 07777;
357 chown( $uid, $gid, $self->_root->{file} . '.tmp' );
358 chmod( $perms, $self->_root->{file} . '.tmp' );
360 # q.v. perlport for more information on this variable
361 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
363 # Potential race condition when optmizing on Win32 with locking.
364 # The Windows filesystem requires that the filehandle be closed
365 # before it is overwritten with rename(). This could be redone
369 $self->{engine}->close_fh( $self );
372 if (!rename $self->_root->{file} . '.tmp', $self->_root->{file}) {
373 unlink $self->_root->{file} . '.tmp';
375 return $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
379 $self->{engine}->close_fh( $self );
380 $self->{engine}->setup_fh( $self );
387 # Make copy of object and return
389 my $self = $_[0]->_get_self;
391 return DBM::Deep->new(
392 type => $self->_type,
393 base_offset => $self->_base_offset,
399 my %is_legal_filter = map {
402 store_key store_value
403 fetch_key fetch_value
408 # Setup filter function for storing or fetching the key or value
410 my $self = $_[0]->_get_self;
412 my $func = $_[2] ? $_[2] : undef;
414 if ( $is_legal_filter{$type} ) {
415 $self->_root->{"filter_$type"} = $func;
429 # Get access to the root structure
431 my $self = $_[0]->_get_self;
432 return $self->{root};
437 # Get access to the raw fh
439 #XXX It will be useful, though, when we split out HASH and ARRAY
440 my $self = $_[0]->_get_self;
441 return $self->_root->{fh};
446 # Get type of current node (TYPE_HASH or TYPE_ARRAY)
448 my $self = $_[0]->_get_self;
449 return $self->{type};
454 # Get base_offset of current node (TYPE_HASH or TYPE_ARRAY)
456 my $self = $_[0]->_get_self;
457 return $self->{base_offset};
465 die "DBM::Deep: $_[1]\n";
470 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
475 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
479 # tie() methods (hashes and arrays)
484 # Store single hash key/value or array element in database.
486 my $self = shift->_get_self;
487 my ($key, $value) = @_;
489 unless ( _is_writable( $self->_fh ) ) {
490 $self->_throw_error( 'Cannot write to a readonly filehandle' );
494 # Request exclusive lock for writing
496 $self->lock( LOCK_EX );
498 my $md5 = $self->{engine}{digest}->($key);
500 my $tag = $self->{engine}->find_bucket_list( $self, $md5, { create => 1 } );
502 # User may be storing a hash, in which case we do not want it run
503 # through the filtering system
504 if ( !ref($value) && $self->_root->{filter_store_value} ) {
505 $value = $self->_root->{filter_store_value}->( $value );
509 # Add key/value to bucket list
511 my $result = $self->{engine}->add_bucket( $self, $tag, $md5, $key, $value );
520 # Fetch single value or element given plain key or array index
522 my $self = shift->_get_self;
525 my $md5 = $self->{engine}{digest}->($key);
528 # Request shared lock for reading
530 $self->lock( LOCK_SH );
532 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
539 # Get value from bucket list
541 my $result = $self->{engine}->get_bucket_value( $self, $tag, $md5 );
545 #XXX What is ref() checking here?
546 #YYY Filters only apply on scalar values, so the ref check is making
547 #YYY sure the fetched bucket is a scalar, not a child hash or array.
548 return ($result && !ref($result) && $self->_root->{filter_fetch_value})
549 ? $self->_root->{filter_fetch_value}->($result)
555 # Delete single key/value pair or element given plain key or array index
557 my $self = $_[0]->_get_self;
560 my $md5 = $self->{engine}{digest}->($key);
563 # Request exclusive lock for writing
565 $self->lock( LOCK_EX );
567 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
576 my $value = $self->{engine}->get_bucket_value($self, $tag, $md5 );
577 if ($value && !ref($value) && $self->_root->{filter_fetch_value}) {
578 $value = $self->_root->{filter_fetch_value}->($value);
581 my $result = $self->{engine}->delete_bucket( $self, $tag, $md5 );
584 # If this object is an array and the key deleted was on the end of the stack,
585 # decrement the length variable.
595 # Check if a single key or element exists given plain key or array index
597 my $self = $_[0]->_get_self;
600 my $md5 = $self->{engine}{digest}->($key);
603 # Request shared lock for reading
605 $self->lock( LOCK_SH );
607 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
612 # For some reason, the built-in exists() function returns '' for false
618 # Check if bucket exists and return 1 or ''
620 my $result = $self->{engine}->bucket_exists( $self, $tag, $md5 ) || '';
629 # Clear all keys from hash, or all elements from array.
631 my $self = $_[0]->_get_self;
634 # Request exclusive lock for writing
636 $self->lock( LOCK_EX );
640 seek($fh, $self->_base_offset + $self->_root->{file_offset}, SEEK_SET);
646 $self->{engine}->create_tag($self, $self->_base_offset, $self->_type, chr(0) x $self->{engine}{index_size});
654 # Public method aliases
656 sub put { (shift)->STORE( @_ ) }
657 sub store { (shift)->STORE( @_ ) }
658 sub get { (shift)->FETCH( @_ ) }
659 sub fetch { (shift)->FETCH( @_ ) }
660 sub delete { (shift)->DELETE( @_ ) }
661 sub exists { (shift)->EXISTS( @_ ) }
662 sub clear { (shift)->CLEAR( @_ ) }
664 package DBM::Deep::_::Root;
678 filter_store_key => undef,
679 filter_store_value => undef,
680 filter_fetch_key => undef,
681 filter_fetch_value => undef,
686 if ( $self->{fh} && !$self->{file_offset} ) {
687 $self->{file_offset} = tell( $self->{fh} );
697 close $self->{fh} if $self->{fh};
708 DBM::Deep - A pure perl multi-level hash/array DBM
713 my $db = DBM::Deep->new( "foo.db" );
715 $db->{key} = 'value'; # tie() style
718 $db->put('key' => 'value'); # OO style
719 print $db->get('key');
721 # true multi-level support
722 $db->{my_complex} = [
723 'hello', { perl => 'rules' },
729 A unique flat-file database module, written in pure perl. True
730 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
731 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
732 handle millions of keys and unlimited hash levels without significant
733 slow-down. Written from the ground-up in pure perl -- this is NOT a
734 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
735 Mac OS X and Windows.
739 Hopefully you are using Perl's excellent CPAN module, which will download
740 and install the module for you. If not, get the tarball, and run these
752 Construction can be done OO-style (which is the recommended way), or using
753 Perl's tie() function. Both are examined here.
755 =head2 OO CONSTRUCTION
757 The recommended way to construct a DBM::Deep object is to use the new()
758 method, which gets you a blessed, tied hash or array reference.
760 my $db = DBM::Deep->new( "foo.db" );
762 This opens a new database handle, mapped to the file "foo.db". If this
763 file does not exist, it will automatically be created. DB files are
764 opened in "r+" (read/write) mode, and the type of object returned is a
765 hash, unless otherwise specified (see L<OPTIONS> below).
767 You can pass a number of options to the constructor to specify things like
768 locking, autoflush, etc. This is done by passing an inline hash:
770 my $db = DBM::Deep->new(
776 Notice that the filename is now specified I<inside> the hash with
777 the "file" parameter, as opposed to being the sole argument to the
778 constructor. This is required if any options are specified.
779 See L<OPTIONS> below for the complete list.
783 You can also start with an array instead of a hash. For this, you must
784 specify the C<type> parameter:
786 my $db = DBM::Deep->new(
788 type => DBM::Deep->TYPE_ARRAY
791 B<Note:> Specifing the C<type> parameter only takes effect when beginning
792 a new DB file. If you create a DBM::Deep object with an existing file, the
793 C<type> will be loaded from the file header, and an error will be thrown if
794 the wrong type is passed in.
796 =head2 TIE CONSTRUCTION
798 Alternately, you can create a DBM::Deep handle by using Perl's built-in
799 tie() function. The object returned from tie() can be used to call methods,
800 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
801 file (as expected with most tie'd objects).
804 my $db = tie %hash, "DBM::Deep", "foo.db";
807 my $db = tie @array, "DBM::Deep", "bar.db";
809 As with the OO constructor, you can replace the DB filename parameter with
810 a hash containing one or more options (see L<OPTIONS> just below for the
813 tie %hash, "DBM::Deep", {
821 There are a number of options that can be passed in when constructing your
822 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
828 Filename of the DB file to link the handle to. You can pass a full absolute
829 filesystem path, partial path, or a plain filename if the file is in the
830 current working directory. This is a required parameter (though q.v. fh).
834 If you want, you can pass in the fh instead of the file. This is most useful for doing
837 my $db = DBM::Deep->new( { fh => \*DATA } );
839 You are responsible for making sure that the fh has been opened appropriately for your
840 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
841 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
842 needs to read from the fh.
846 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
847 not need to set this. However, it's there if you want it.
849 If you pass in fh and do not set this, it will be set appropriately.
853 This parameter specifies what type of object to create, a hash or array. Use
854 one of these two constants: C<DBM::Deep-E<gt>TYPE_HASH> or C<DBM::Deep-E<gt>TYPE_ARRAY>.
855 This only takes effect when beginning a new file. This is an optional
856 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
860 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
861 function to lock the database in exclusive mode for writes, and shared mode for
862 reads. Pass any true value to enable. This affects the base DB handle I<and
863 any child hashes or arrays> that use the same DB file. This is an optional
864 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
868 Specifies whether autoflush is to be enabled on the underlying filehandle.
869 This obviously slows down write operations, but is required if you may have
870 multiple processes accessing the same DB file (also consider enable I<locking>).
871 Pass any true value to enable. This is an optional parameter, and defaults to 0
876 If I<autobless> mode is enabled, DBM::Deep will preserve blessed hashes, and
877 restore them when fetched. This is an B<experimental> feature, and does have
878 side-effects. Basically, when hashes are re-blessed into their original
879 classes, they are no longer blessed into the DBM::Deep class! So you won't be
880 able to call any DBM::Deep methods on them. You have been warned.
881 This is an optional parameter, and defaults to 0 (disabled).
885 See L<FILTERS> below.
891 With DBM::Deep you can access your databases using Perl's standard hash/array
892 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
893 treat them as such. DBM::Deep will intercept all reads/writes and direct them
894 to the right place -- the DB file. This has nothing to do with the
895 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
896 using regular hashes and arrays, rather than calling functions like C<get()>
897 and C<put()> (although those work too). It is entirely up to you how to want
898 to access your databases.
902 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
903 or even nested hashes (or arrays) using standard Perl syntax:
905 my $db = DBM::Deep->new( "foo.db" );
907 $db->{mykey} = "myvalue";
909 $db->{myhash}->{subkey} = "subvalue";
911 print $db->{myhash}->{subkey} . "\n";
913 You can even step through hash keys using the normal Perl C<keys()> function:
915 foreach my $key (keys %$db) {
916 print "$key: " . $db->{$key} . "\n";
919 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
920 pushes them onto an array, all before the loop even begins. If you have an
921 extra large hash, this may exhaust Perl's memory. Instead, consider using
922 Perl's C<each()> function, which pulls keys/values one at a time, using very
925 while (my ($key, $value) = each %$db) {
926 print "$key: $value\n";
929 Please note that when using C<each()>, you should always pass a direct
930 hash reference, not a lookup. Meaning, you should B<never> do this:
933 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
935 This causes an infinite loop, because for each iteration, Perl is calling
936 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
937 it effectively keeps returning the first key over and over again. Instead,
938 assign a temporary variable to C<$db->{foo}>, then pass that to each().
942 As with hashes, you can treat any DBM::Deep object like a normal Perl array
943 reference. This includes inserting, removing and manipulating elements,
944 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
945 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
946 or simply be a nested array reference inside a hash. Example:
948 my $db = DBM::Deep->new(
949 file => "foo-array.db",
950 type => DBM::Deep->TYPE_ARRAY
954 push @$db, "bar", "baz";
957 my $last_elem = pop @$db; # baz
958 my $first_elem = shift @$db; # bah
959 my $second_elem = $db->[1]; # bar
961 my $num_elements = scalar @$db;
965 In addition to the I<tie()> interface, you can also use a standard OO interface
966 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
967 array) has its own methods, but both types share the following common methods:
968 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
972 =item * new() / clone()
974 These are the constructor and copy-functions.
976 =item * put() / store()
978 Stores a new hash key/value pair, or sets an array element value. Takes two
979 arguments, the hash key or array index, and the new value. The value can be
980 a scalar, hash ref or array ref. Returns true on success, false on failure.
982 $db->put("foo", "bar"); # for hashes
983 $db->put(1, "bar"); # for arrays
985 =item * get() / fetch()
987 Fetches the value of a hash key or array element. Takes one argument: the hash
988 key or array index. Returns a scalar, hash ref or array ref, depending on the
991 my $value = $db->get("foo"); # for hashes
992 my $value = $db->get(1); # for arrays
996 Checks if a hash key or array index exists. Takes one argument: the hash key
997 or array index. Returns true if it exists, false if not.
999 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
1000 if ($db->exists(1)) { print "yay!\n"; } # for arrays
1004 Deletes one hash key/value pair or array element. Takes one argument: the hash
1005 key or array index. Returns true on success, false if not found. For arrays,
1006 the remaining elements located after the deleted element are NOT moved over.
1007 The deleted element is essentially just undefined, which is exactly how Perl's
1008 internal arrays work. Please note that the space occupied by the deleted
1009 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
1010 below for details and workarounds.
1012 $db->delete("foo"); # for hashes
1013 $db->delete(1); # for arrays
1017 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1018 value. Please note that the space occupied by the deleted keys/values or
1019 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1020 details and workarounds.
1022 $db->clear(); # hashes or arrays
1024 =item * lock() / unlock()
1030 Recover lost disk space.
1032 =item * import() / export()
1034 Data going in and out.
1036 =item * set_digest() / set_pack() / set_filter()
1038 q.v. adjusting the interal parameters.
1044 For hashes, DBM::Deep supports all the common methods described above, and the
1045 following additional methods: C<first_key()> and C<next_key()>.
1051 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1052 fetched in an undefined order (which appears random). Takes no arguments,
1053 returns the key as a scalar value.
1055 my $key = $db->first_key();
1059 Returns the "next" key in the hash, given the previous one as the sole argument.
1060 Returns undef if there are no more keys to be fetched.
1062 $key = $db->next_key($key);
1066 Here are some examples of using hashes:
1068 my $db = DBM::Deep->new( "foo.db" );
1070 $db->put("foo", "bar");
1071 print "foo: " . $db->get("foo") . "\n";
1073 $db->put("baz", {}); # new child hash ref
1074 $db->get("baz")->put("buz", "biz");
1075 print "buz: " . $db->get("baz")->get("buz") . "\n";
1077 my $key = $db->first_key();
1079 print "$key: " . $db->get($key) . "\n";
1080 $key = $db->next_key($key);
1083 if ($db->exists("foo")) { $db->delete("foo"); }
1087 For arrays, DBM::Deep supports all the common methods described above, and the
1088 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1089 C<unshift()> and C<splice()>.
1095 Returns the number of elements in the array. Takes no arguments.
1097 my $len = $db->length();
1101 Adds one or more elements onto the end of the array. Accepts scalars, hash
1102 refs or array refs. No return value.
1104 $db->push("foo", "bar", {});
1108 Fetches the last element in the array, and deletes it. Takes no arguments.
1109 Returns undef if array is empty. Returns the element value.
1111 my $elem = $db->pop();
1115 Fetches the first element in the array, deletes it, then shifts all the
1116 remaining elements over to take up the space. Returns the element value. This
1117 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1120 my $elem = $db->shift();
1124 Inserts one or more elements onto the beginning of the array, shifting all
1125 existing elements over to make room. Accepts scalars, hash refs or array refs.
1126 No return value. This method is not recommended with large arrays -- see
1127 <LARGE ARRAYS> below for details.
1129 $db->unshift("foo", "bar", {});
1133 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1134 -f splice> for usage -- it is too complicated to document here. This method is
1135 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1139 Here are some examples of using arrays:
1141 my $db = DBM::Deep->new(
1143 type => DBM::Deep->TYPE_ARRAY
1146 $db->push("bar", "baz");
1147 $db->unshift("foo");
1150 my $len = $db->length();
1151 print "length: $len\n"; # 4
1153 for (my $k=0; $k<$len; $k++) {
1154 print "$k: " . $db->get($k) . "\n";
1157 $db->splice(1, 2, "biz", "baf");
1159 while (my $elem = shift @$db) {
1160 print "shifted: $elem\n";
1165 Enable automatic file locking by passing a true value to the C<locking>
1166 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1168 my $db = DBM::Deep->new(
1173 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1174 mode for writes, and shared mode for reads. This is required if you have
1175 multiple processes accessing the same database file, to avoid file corruption.
1176 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1177 NFS> below for more.
1179 =head2 EXPLICIT LOCKING
1181 You can explicitly lock a database, so it remains locked for multiple
1182 transactions. This is done by calling the C<lock()> method, and passing an
1183 optional lock mode argument (defaults to exclusive mode). This is particularly
1184 useful for things like counters, where the current value needs to be fetched,
1185 then incremented, then stored again.
1188 my $counter = $db->get("counter");
1190 $db->put("counter", $counter);
1199 You can pass C<lock()> an optional argument, which specifies which mode to use
1200 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1201 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1202 same as the constants defined in Perl's C<Fcntl> module.
1204 $db->lock( DBM::Deep->LOCK_SH );
1208 =head1 IMPORTING/EXPORTING
1210 You can import existing complex structures by calling the C<import()> method,
1211 and export an entire database into an in-memory structure using the C<export()>
1212 method. Both are examined here.
1216 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1217 walking the structure and adding keys/elements to the database as you go,
1218 simply pass a reference to the C<import()> method. This recursively adds
1219 everything to an existing DBM::Deep object for you. Here is an example:
1224 array1 => [ "elem0", "elem1", "elem2" ],
1226 subkey1 => "subvalue1",
1227 subkey2 => "subvalue2"
1231 my $db = DBM::Deep->new( "foo.db" );
1232 $db->import( $struct );
1234 print $db->{key1} . "\n"; # prints "value1"
1236 This recursively imports the entire C<$struct> object into C<$db>, including
1237 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1238 keys are merged with the existing ones, replacing if they already exist.
1239 The C<import()> method can be called on any database level (not just the base
1240 level), and works with both hash and array DB types.
1242 B<Note:> Make sure your existing structure has no circular references in it.
1243 These will cause an infinite loop when importing.
1247 Calling the C<export()> method on an existing DBM::Deep object will return
1248 a reference to a new in-memory copy of the database. The export is done
1249 recursively, so all nested hashes/arrays are all exported to standard Perl
1250 objects. Here is an example:
1252 my $db = DBM::Deep->new( "foo.db" );
1254 $db->{key1} = "value1";
1255 $db->{key2} = "value2";
1257 $db->{hash1}->{subkey1} = "subvalue1";
1258 $db->{hash1}->{subkey2} = "subvalue2";
1260 my $struct = $db->export();
1262 print $struct->{key1} . "\n"; # prints "value1"
1264 This makes a complete copy of the database in memory, and returns a reference
1265 to it. The C<export()> method can be called on any database level (not just
1266 the base level), and works with both hash and array DB types. Be careful of
1267 large databases -- you can store a lot more data in a DBM::Deep object than an
1268 in-memory Perl structure.
1270 B<Note:> Make sure your database has no circular references in it.
1271 These will cause an infinite loop when exporting.
1275 DBM::Deep has a number of hooks where you can specify your own Perl function
1276 to perform filtering on incoming or outgoing data. This is a perfect
1277 way to extend the engine, and implement things like real-time compression or
1278 encryption. Filtering applies to the base DB level, and all child hashes /
1279 arrays. Filter hooks can be specified when your DBM::Deep object is first
1280 constructed, or by calling the C<set_filter()> method at any time. There are
1281 four available filter hooks, described below:
1285 =item * filter_store_key
1287 This filter is called whenever a hash key is stored. It
1288 is passed the incoming key, and expected to return a transformed key.
1290 =item * filter_store_value
1292 This filter is called whenever a hash key or array element is stored. It
1293 is passed the incoming value, and expected to return a transformed value.
1295 =item * filter_fetch_key
1297 This filter is called whenever a hash key is fetched (i.e. via
1298 C<first_key()> or C<next_key()>). It is passed the transformed key,
1299 and expected to return the plain key.
1301 =item * filter_fetch_value
1303 This filter is called whenever a hash key or array element is fetched.
1304 It is passed the transformed value, and expected to return the plain value.
1308 Here are the two ways to setup a filter hook:
1310 my $db = DBM::Deep->new(
1312 filter_store_value => \&my_filter_store,
1313 filter_fetch_value => \&my_filter_fetch
1318 $db->set_filter( "filter_store_value", \&my_filter_store );
1319 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1321 Your filter function will be called only when dealing with SCALAR keys or
1322 values. When nested hashes and arrays are being stored/fetched, filtering
1323 is bypassed. Filters are called as static functions, passed a single SCALAR
1324 argument, and expected to return a single SCALAR value. If you want to
1325 remove a filter, set the function reference to C<undef>:
1327 $db->set_filter( "filter_store_value", undef );
1329 =head2 REAL-TIME ENCRYPTION EXAMPLE
1331 Here is a working example that uses the I<Crypt::Blowfish> module to
1332 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1333 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1334 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1337 use Crypt::Blowfish;
1340 my $cipher = Crypt::CBC->new({
1341 'key' => 'my secret key',
1342 'cipher' => 'Blowfish',
1344 'regenerate_key' => 0,
1345 'padding' => 'space',
1349 my $db = DBM::Deep->new(
1350 file => "foo-encrypt.db",
1351 filter_store_key => \&my_encrypt,
1352 filter_store_value => \&my_encrypt,
1353 filter_fetch_key => \&my_decrypt,
1354 filter_fetch_value => \&my_decrypt,
1357 $db->{key1} = "value1";
1358 $db->{key2} = "value2";
1359 print "key1: " . $db->{key1} . "\n";
1360 print "key2: " . $db->{key2} . "\n";
1366 return $cipher->encrypt( $_[0] );
1369 return $cipher->decrypt( $_[0] );
1372 =head2 REAL-TIME COMPRESSION EXAMPLE
1374 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1375 compression / decompression of keys & values with DBM::Deep Filters.
1376 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1377 more on I<Compress::Zlib>.
1382 my $db = DBM::Deep->new(
1383 file => "foo-compress.db",
1384 filter_store_key => \&my_compress,
1385 filter_store_value => \&my_compress,
1386 filter_fetch_key => \&my_decompress,
1387 filter_fetch_value => \&my_decompress,
1390 $db->{key1} = "value1";
1391 $db->{key2} = "value2";
1392 print "key1: " . $db->{key1} . "\n";
1393 print "key2: " . $db->{key2} . "\n";
1399 return Compress::Zlib::memGzip( $_[0] ) ;
1402 return Compress::Zlib::memGunzip( $_[0] ) ;
1405 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1406 actually numerical index numbers, and are not filtered.
1408 =head1 ERROR HANDLING
1410 Most DBM::Deep methods return a true value for success, and call die() on
1411 failure. You can wrap calls in an eval block to catch the die.
1413 my $db = DBM::Deep->new( "foo.db" ); # create hash
1414 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1416 print $@; # prints error message
1418 =head1 LARGEFILE SUPPORT
1420 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1421 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1422 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1423 by calling the static C<set_pack()> method before you do anything else.
1425 DBM::Deep::set_pack(8, 'Q');
1427 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1428 instead of 32-bit longs. After setting these values your DB files have a
1429 theoretical maximum size of 16 XB (exabytes).
1431 B<Note:> Changing these values will B<NOT> work for existing database files.
1432 Only change this for new files, and make sure it stays set consistently
1433 throughout the file's life. If you do set these values, you can no longer
1434 access 32-bit DB files. You can, however, call C<set_pack(4, 'N')> to change
1435 back to 32-bit mode.
1437 B<Note:> I have not personally tested files > 2 GB -- all my systems have
1438 only a 32-bit Perl. However, I have received user reports that this does
1441 =head1 LOW-LEVEL ACCESS
1443 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1444 you can call the C<_fh()> method, which returns the handle:
1446 my $fh = $db->_fh();
1448 This method can be called on the root level of the datbase, or any child
1449 hashes or arrays. All levels share a I<root> structure, which contains things
1450 like the filehandle, a reference counter, and all the options specified
1451 when you created the object. You can get access to this root structure by
1452 calling the C<root()> method.
1454 my $root = $db->_root();
1456 This is useful for changing options after the object has already been created,
1457 such as enabling/disabling locking. You can also store your own temporary user
1458 data in this structure (be wary of name collision), which is then accessible from
1459 any child hash or array.
1461 =head1 CUSTOM DIGEST ALGORITHM
1463 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1464 keys. However you can override this, and use another algorithm (such as SHA-256)
1465 or even write your own. But please note that DBM::Deep currently expects zero
1466 collisions, so your algorithm has to be I<perfect>, so to speak.
1467 Collision detection may be introduced in a later version.
1471 You can specify a custom digest algorithm by calling the static C<set_digest()>
1472 function, passing a reference to a subroutine, and the length of the algorithm's
1473 hashes (in bytes). This is a global static function, which affects ALL DBM::Deep
1474 objects. Here is a working example that uses a 256-bit hash from the
1475 I<Digest::SHA256> module. Please see
1476 L<http://search.cpan.org/search?module=Digest::SHA256> for more.
1481 my $context = Digest::SHA256::new(256);
1483 DBM::Deep::set_digest( \&my_digest, 32 );
1485 my $db = DBM::Deep->new( "foo-sha.db" );
1487 $db->{key1} = "value1";
1488 $db->{key2} = "value2";
1489 print "key1: " . $db->{key1} . "\n";
1490 print "key2: " . $db->{key2} . "\n";
1496 return substr( $context->hash($_[0]), 0, 32 );
1499 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1500 of bytes you specify in the C<set_digest()> function (in this case 32).
1502 =head1 CIRCULAR REFERENCES
1504 DBM::Deep has B<experimental> support for circular references. Meaning you
1505 can have a nested hash key or array element that points to a parent object.
1506 This relationship is stored in the DB file, and is preserved between sessions.
1509 my $db = DBM::Deep->new( "foo.db" );
1512 $db->{circle} = $db; # ref to self
1514 print $db->{foo} . "\n"; # prints "foo"
1515 print $db->{circle}->{foo} . "\n"; # prints "foo" again
1517 One catch is, passing the object to a function that recursively walks the
1518 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1519 C<export()> methods) will result in an infinite loop. The other catch is,
1520 if you fetch the I<key> of a circular reference (i.e. using the C<first_key()>
1521 or C<next_key()> methods), you will get the I<target object's key>, not the
1522 ref's key. This gets even more interesting with the above example, where
1523 the I<circle> key points to the base DB object, which technically doesn't
1524 have a key. So I made DBM::Deep return "[base]" as the key name in that
1527 =head1 CAVEATS / ISSUES / BUGS
1529 This section describes all the known issues with DBM::Deep. It you have found
1530 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1532 =head2 UNUSED SPACE RECOVERY
1534 One major caveat with DBM::Deep is that space occupied by existing keys and
1535 values is not recovered when they are deleted. Meaning if you keep deleting
1536 and adding new keys, your file will continuously grow. I am working on this,
1537 but in the meantime you can call the built-in C<optimize()> method from time to
1538 time (perhaps in a crontab or something) to recover all your unused space.
1540 $db->optimize(); # returns true on success
1542 This rebuilds the ENTIRE database into a new file, then moves it on top of
1543 the original. The new file will have no unused space, thus it will take up as
1544 little disk space as possible. Please note that this operation can take
1545 a long time for large files, and you need enough disk space to temporarily hold
1546 2 copies of your DB file. The temporary file is created in the same directory
1547 as the original, named with a ".tmp" extension, and is deleted when the
1548 operation completes. Oh, and if locking is enabled, the DB is automatically
1549 locked for the entire duration of the copy.
1551 B<WARNING:> Only call optimize() on the top-level node of the database, and
1552 make sure there are no child references lying around. DBM::Deep keeps a reference
1553 counter, and if it is greater than 1, optimize() will abort and return undef.
1555 =head2 AUTOVIVIFICATION
1557 Unfortunately, autovivification doesn't work with tied hashes. This appears to
1558 be a bug in Perl's tie() system, as I<Jakob Schmidt> encountered the very same
1559 issue with his I<DWH_FIle> module (see L<http://search.cpan.org/search?module=DWH_File>),
1560 and it is also mentioned in the BUGS section for the I<MLDBM> module <see
1561 L<http://search.cpan.org/search?module=MLDBM>). Basically, on a new db file,
1564 $db->{foo}->{bar} = "hello";
1566 Since "foo" doesn't exist, you cannot add "bar" to it. You end up with "foo"
1567 being an empty hash. Try this instead, which works fine:
1569 $db->{foo} = { bar => "hello" };
1571 As of Perl 5.8.7, this bug still exists. I have walked very carefully through
1572 the execution path, and Perl indeed passes an empty hash to the STORE() method.
1573 Probably a bug in Perl.
1575 =head2 FILE CORRUPTION
1577 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1578 for a 32-bit signature when opened, but other corruption in files can cause
1579 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1580 stuck in an infinite loop depending on the level of corruption. File write
1581 operations are not checked for failure (for speed), so if you happen to run
1582 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1583 be addressed in a later version of DBM::Deep.
1587 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1588 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1589 about setting up your NFS server with a locking daemon, then using lockf() to
1590 lock your files, but your mileage may vary there as well. From what I
1591 understand, there is no real way to do it. However, if you need access to the
1592 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1593 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1595 =head2 COPYING OBJECTS
1597 Beware of copying tied objects in Perl. Very strange things can happen.
1598 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1599 returns a new, blessed, tied hash or array to the same level in the DB.
1601 my $copy = $db->clone();
1603 B<Note>: Since clone() here is cloning the object, not the database location, any
1604 modifications to either $db or $copy will be visible in both.
1608 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1609 These functions cause every element in the array to move, which can be murder
1610 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1611 a different location. This will be addressed in the forthcoming version 1.00.
1613 =head2 WRITEONLY FILES
1615 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1616 writeonly mode. STORE will verify that the filehandle is writable. However, there
1617 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1618 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1622 This section discusses DBM::Deep's speed and memory usage.
1626 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1627 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1628 multi-level hash/array support, and cross-platform FTPable files. Even so,
1629 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1630 with huge databases. Here is some test data:
1632 Adding 1,000,000 keys to new DB file...
1634 At 100 keys, avg. speed is 2,703 keys/sec
1635 At 200 keys, avg. speed is 2,642 keys/sec
1636 At 300 keys, avg. speed is 2,598 keys/sec
1637 At 400 keys, avg. speed is 2,578 keys/sec
1638 At 500 keys, avg. speed is 2,722 keys/sec
1639 At 600 keys, avg. speed is 2,628 keys/sec
1640 At 700 keys, avg. speed is 2,700 keys/sec
1641 At 800 keys, avg. speed is 2,607 keys/sec
1642 At 900 keys, avg. speed is 2,190 keys/sec
1643 At 1,000 keys, avg. speed is 2,570 keys/sec
1644 At 2,000 keys, avg. speed is 2,417 keys/sec
1645 At 3,000 keys, avg. speed is 1,982 keys/sec
1646 At 4,000 keys, avg. speed is 1,568 keys/sec
1647 At 5,000 keys, avg. speed is 1,533 keys/sec
1648 At 6,000 keys, avg. speed is 1,787 keys/sec
1649 At 7,000 keys, avg. speed is 1,977 keys/sec
1650 At 8,000 keys, avg. speed is 2,028 keys/sec
1651 At 9,000 keys, avg. speed is 2,077 keys/sec
1652 At 10,000 keys, avg. speed is 2,031 keys/sec
1653 At 20,000 keys, avg. speed is 1,970 keys/sec
1654 At 30,000 keys, avg. speed is 2,050 keys/sec
1655 At 40,000 keys, avg. speed is 2,073 keys/sec
1656 At 50,000 keys, avg. speed is 1,973 keys/sec
1657 At 60,000 keys, avg. speed is 1,914 keys/sec
1658 At 70,000 keys, avg. speed is 2,091 keys/sec
1659 At 80,000 keys, avg. speed is 2,103 keys/sec
1660 At 90,000 keys, avg. speed is 1,886 keys/sec
1661 At 100,000 keys, avg. speed is 1,970 keys/sec
1662 At 200,000 keys, avg. speed is 2,053 keys/sec
1663 At 300,000 keys, avg. speed is 1,697 keys/sec
1664 At 400,000 keys, avg. speed is 1,838 keys/sec
1665 At 500,000 keys, avg. speed is 1,941 keys/sec
1666 At 600,000 keys, avg. speed is 1,930 keys/sec
1667 At 700,000 keys, avg. speed is 1,735 keys/sec
1668 At 800,000 keys, avg. speed is 1,795 keys/sec
1669 At 900,000 keys, avg. speed is 1,221 keys/sec
1670 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1672 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1673 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1674 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1675 Run time was 12 min 3 sec.
1679 One of the great things about DBM::Deep is that it uses very little memory.
1680 Even with huge databases (1,000,000+ keys) you will not see much increased
1681 memory on your process. DBM::Deep relies solely on the filesystem for storing
1682 and fetching data. Here is output from I</usr/bin/top> before even opening a
1685 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1686 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1688 Basically the process is taking 2,716K of memory. And here is the same
1689 process after storing and fetching 1,000,000 keys:
1691 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1692 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1694 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1695 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1697 =head1 DB FILE FORMAT
1699 In case you were interested in the underlying DB file format, it is documented
1700 here in this section. You don't need to know this to use the module, it's just
1701 included for reference.
1705 DBM::Deep files always start with a 32-bit signature to identify the file type.
1706 This is at offset 0. The signature is "DPDB" in network byte order. This is
1707 checked for when the file is opened and an error will be thrown if it's not found.
1711 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1712 has a standard header containing the type of data, the length of data, and then
1713 the data itself. The type is a single character (1 byte), the length is a
1714 32-bit unsigned long in network byte order, and the data is, well, the data.
1715 Here is how it unfolds:
1719 Immediately after the 32-bit file signature is the I<Master Index> record.
1720 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1721 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1722 depending on how the DBM::Deep object was constructed.
1724 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1725 number). The first 8-bit char of the MD5 signature is the offset into the
1726 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1727 index element is a file offset of the next tag for the key/element in question,
1728 which is usually a I<Bucket List> tag (see below).
1730 The next tag I<could> be another index, depending on how many keys/elements
1731 exist. See L<RE-INDEXING> below for details.
1735 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1736 file offsets to where the actual data is stored. It starts with a standard
1737 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1738 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1739 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1740 When the list fills up, a I<Re-Index> operation is performed (See
1741 L<RE-INDEXING> below).
1745 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1746 index/value pair (in array mode). It starts with a standard tag header with
1747 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1748 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1749 header. The size reported in the tag header is only for the value, but then,
1750 just after the value is another size (32-bit unsigned long) and then the plain
1751 key itself. Since the value is likely to be fetched more often than the plain
1752 key, I figured it would be I<slightly> faster to store the value first.
1754 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1755 record for the nested structure, where the process begins all over again.
1759 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1760 exhausted. Then, when another key/element comes in, the list is converted to a
1761 new index record. However, this index will look at the next char in the MD5
1762 hash, and arrange new Bucket List pointers accordingly. This process is called
1763 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1764 17 (16 + new one) keys/elements are removed from the old Bucket List and
1765 inserted into the new index. Several new Bucket Lists are created in the
1766 process, as a new MD5 char from the key is being examined (it is unlikely that
1767 the keys will all share the same next char of their MD5s).
1769 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1770 when the Bucket Lists will turn into indexes, but the first round tends to
1771 happen right around 4,000 keys. You will see a I<slight> decrease in
1772 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1773 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1774 right around 900,000 keys. This process can continue nearly indefinitely --
1775 right up until the point the I<MD5> signatures start colliding with each other,
1776 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1777 getting struck by lightning while you are walking to cash in your tickets.
1778 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1779 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1780 this is 340 unodecillion, but don't quote me).
1784 When a new key/element is stored, the key (or index number) is first run through
1785 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1786 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1787 for the first char of the signature (in this case I<b0>). If it does not exist,
1788 a new I<Bucket List> is created for our key (and the next 15 future keys that
1789 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1790 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1791 this point, unless we are replacing an existing I<Bucket>), where the actual
1792 data will be stored.
1796 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1797 (or index number), then walking along the indexes. If there are enough
1798 keys/elements in this DB level, there might be nested indexes, each linked to
1799 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1800 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1801 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1802 plain key are stored.
1804 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1805 methods. In this process the indexes are walked systematically, and each key
1806 fetched in increasing MD5 order (which is why it appears random). Once the
1807 I<Bucket> is found, the value is skipped and the plain key returned instead.
1808 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1809 alphabetically sorted. This only happens on an index-level -- as soon as the
1810 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1811 so it's pretty much undefined how the keys will come out -- just like Perl's
1814 =head1 CODE COVERAGE
1816 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1817 B<Devel::Cover> report on this module's test suite.
1819 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1820 File stmt bran cond sub pod time total
1821 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1822 blib/lib/DBM/Deep.pm 95.1 81.6 70.3 100.0 100.0 33.4 91.0
1823 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 n/a 27.8 98.0
1824 blib/lib/DBM/Deep/Engine.pm 97.8 85.6 75.0 100.0 0.0 25.8 90.8
1825 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 n/a 13.0 97.2
1826 Total 97.5 85.4 76.6 100.0 46.9 100.0 92.5
1827 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1829 =head1 MORE INFORMATION
1831 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1832 or send email to L<DBM-Deep@googlegroups.com>.
1836 Joseph Huckaby, L<jhuckaby@cpan.org>
1838 Rob Kinyon, L<rkinyon@cpan.org>
1840 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1844 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1845 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1849 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1850 This is free software, you may use it and distribute it under the
1851 same terms as Perl itself.