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( $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( $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( $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 = $_[0]->_get_self;
489 # User may be storing a hash, in which case we do not want it run
490 # through the filtering system
491 my $value = ($self->_root->{filter_store_value} && !ref($_[2]))
492 ? $self->_root->{filter_store_value}->($_[2])
495 my $md5 = $self->{engine}{digest}->($key);
497 unless ( _is_writable( $self->_fh ) ) {
498 $self->_throw_error( 'Cannot write to a readonly filehandle' );
502 # Request exclusive lock for writing
504 $self->lock( LOCK_EX );
509 # Locate offset for bucket list using digest index system
511 my $tag = $self->{engine}->load_tag($self, $self->_base_offset);
513 $tag = $self->{engine}->create_tag($self, $self->_base_offset, SIG_INDEX, chr(0) x $self->{engine}{index_size});
517 while ($tag->{signature} ne SIG_BLIST) {
518 my $num = ord(substr($md5, $ch, 1));
520 my $ref_loc = $tag->{offset} + ($num * $DBM::Deep::Engine::LONG_SIZE);
521 my $new_tag = $self->{engine}->index_lookup($self, $tag, $num);
524 seek($fh, $ref_loc + $self->_root->{file_offset}, SEEK_SET);
525 print( $fh pack($DBM::Deep::Engine::LONG_PACK, $self->_root->{end}) );
527 $tag = $self->{engine}->create_tag($self, $self->_root->{end}, SIG_BLIST, chr(0) x $DBM::Deep::Engine::BUCKET_LIST_SIZE);
529 $tag->{ref_loc} = $ref_loc;
537 $tag->{ref_loc} = $ref_loc;
544 # Add key/value to bucket list
546 my $result = $self->{engine}->add_bucket( $self, $tag, $md5, $key, $value );
555 # Fetch single value or element given plain key or array index
557 my $self = shift->_get_self;
560 my $md5 = $self->{engine}{digest}->($key);
563 # Request shared lock for reading
565 $self->lock( LOCK_SH );
567 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
574 # Get value from bucket list
576 my $result = $self->{engine}->get_bucket_value( $self, $tag, $md5 );
580 #XXX What is ref() checking here?
581 #YYY Filters only apply on scalar values, so the ref check is making
582 #YYY sure the fetched bucket is a scalar, not a child hash or array.
583 return ($result && !ref($result) && $self->_root->{filter_fetch_value})
584 ? $self->_root->{filter_fetch_value}->($result)
590 # Delete single key/value pair or element given plain key or array index
592 my $self = $_[0]->_get_self;
595 my $md5 = $self->{engine}{digest}->($key);
598 # Request exclusive lock for writing
600 $self->lock( LOCK_EX );
602 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
611 my $value = $self->{engine}->get_bucket_value($self, $tag, $md5 );
612 if ($value && !ref($value) && $self->_root->{filter_fetch_value}) {
613 $value = $self->_root->{filter_fetch_value}->($value);
616 my $result = $self->{engine}->delete_bucket( $self, $tag, $md5 );
619 # If this object is an array and the key deleted was on the end of the stack,
620 # decrement the length variable.
630 # Check if a single key or element exists given plain key or array index
632 my $self = $_[0]->_get_self;
635 my $md5 = $self->{engine}{digest}->($key);
638 # Request shared lock for reading
640 $self->lock( LOCK_SH );
642 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
645 # For some reason, the built-in exists() function returns '' for false
653 # Check if bucket exists and return 1 or ''
655 my $result = $self->{engine}->bucket_exists( $self, $tag, $md5 ) || '';
664 # Clear all keys from hash, or all elements from array.
666 my $self = $_[0]->_get_self;
669 # Request exclusive lock for writing
671 $self->lock( LOCK_EX );
675 seek($fh, $self->_base_offset + $self->_root->{file_offset}, SEEK_SET);
681 $self->{engine}->create_tag($self, $self->_base_offset, $self->_type, chr(0) x $self->{engine}{index_size});
689 # Public method aliases
691 sub put { (shift)->STORE( @_ ) }
692 sub store { (shift)->STORE( @_ ) }
693 sub get { (shift)->FETCH( @_ ) }
694 sub fetch { (shift)->FETCH( @_ ) }
695 sub delete { (shift)->DELETE( @_ ) }
696 sub exists { (shift)->EXISTS( @_ ) }
697 sub clear { (shift)->CLEAR( @_ ) }
699 package DBM::Deep::_::Root;
713 filter_store_key => undef,
714 filter_store_value => undef,
715 filter_fetch_key => undef,
716 filter_fetch_value => undef,
721 if ( $self->{fh} && !$self->{file_offset} ) {
722 $self->{file_offset} = tell( $self->{fh} );
732 close $self->{fh} if $self->{fh};
743 DBM::Deep - A pure perl multi-level hash/array DBM
748 my $db = DBM::Deep->new( "foo.db" );
750 $db->{key} = 'value'; # tie() style
753 $db->put('key' => 'value'); # OO style
754 print $db->get('key');
756 # true multi-level support
757 $db->{my_complex} = [
758 'hello', { perl => 'rules' },
764 A unique flat-file database module, written in pure perl. True
765 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
766 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
767 handle millions of keys and unlimited hash levels without significant
768 slow-down. Written from the ground-up in pure perl -- this is NOT a
769 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
770 Mac OS X and Windows.
774 Hopefully you are using Perl's excellent CPAN module, which will download
775 and install the module for you. If not, get the tarball, and run these
787 Construction can be done OO-style (which is the recommended way), or using
788 Perl's tie() function. Both are examined here.
790 =head2 OO CONSTRUCTION
792 The recommended way to construct a DBM::Deep object is to use the new()
793 method, which gets you a blessed, tied hash or array reference.
795 my $db = DBM::Deep->new( "foo.db" );
797 This opens a new database handle, mapped to the file "foo.db". If this
798 file does not exist, it will automatically be created. DB files are
799 opened in "r+" (read/write) mode, and the type of object returned is a
800 hash, unless otherwise specified (see L<OPTIONS> below).
802 You can pass a number of options to the constructor to specify things like
803 locking, autoflush, etc. This is done by passing an inline hash:
805 my $db = DBM::Deep->new(
811 Notice that the filename is now specified I<inside> the hash with
812 the "file" parameter, as opposed to being the sole argument to the
813 constructor. This is required if any options are specified.
814 See L<OPTIONS> below for the complete list.
818 You can also start with an array instead of a hash. For this, you must
819 specify the C<type> parameter:
821 my $db = DBM::Deep->new(
823 type => DBM::Deep->TYPE_ARRAY
826 B<Note:> Specifing the C<type> parameter only takes effect when beginning
827 a new DB file. If you create a DBM::Deep object with an existing file, the
828 C<type> will be loaded from the file header, and an error will be thrown if
829 the wrong type is passed in.
831 =head2 TIE CONSTRUCTION
833 Alternately, you can create a DBM::Deep handle by using Perl's built-in
834 tie() function. The object returned from tie() can be used to call methods,
835 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
836 file (as expected with most tie'd objects).
839 my $db = tie %hash, "DBM::Deep", "foo.db";
842 my $db = tie @array, "DBM::Deep", "bar.db";
844 As with the OO constructor, you can replace the DB filename parameter with
845 a hash containing one or more options (see L<OPTIONS> just below for the
848 tie %hash, "DBM::Deep", {
856 There are a number of options that can be passed in when constructing your
857 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
863 Filename of the DB file to link the handle to. You can pass a full absolute
864 filesystem path, partial path, or a plain filename if the file is in the
865 current working directory. This is a required parameter (though q.v. fh).
869 If you want, you can pass in the fh instead of the file. This is most useful for doing
872 my $db = DBM::Deep->new( { fh => \*DATA } );
874 You are responsible for making sure that the fh has been opened appropriately for your
875 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
876 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
877 needs to read from the fh.
881 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
882 not need to set this. However, it's there if you want it.
884 If you pass in fh and do not set this, it will be set appropriately.
888 This parameter specifies what type of object to create, a hash or array. Use
889 one of these two constants: C<DBM::Deep-E<gt>TYPE_HASH> or C<DBM::Deep-E<gt>TYPE_ARRAY>.
890 This only takes effect when beginning a new file. This is an optional
891 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
895 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
896 function to lock the database in exclusive mode for writes, and shared mode for
897 reads. Pass any true value to enable. This affects the base DB handle I<and
898 any child hashes or arrays> that use the same DB file. This is an optional
899 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
903 Specifies whether autoflush is to be enabled on the underlying filehandle.
904 This obviously slows down write operations, but is required if you may have
905 multiple processes accessing the same DB file (also consider enable I<locking>).
906 Pass any true value to enable. This is an optional parameter, and defaults to 0
911 If I<autobless> mode is enabled, DBM::Deep will preserve blessed hashes, and
912 restore them when fetched. This is an B<experimental> feature, and does have
913 side-effects. Basically, when hashes are re-blessed into their original
914 classes, they are no longer blessed into the DBM::Deep class! So you won't be
915 able to call any DBM::Deep methods on them. You have been warned.
916 This is an optional parameter, and defaults to 0 (disabled).
920 See L<FILTERS> below.
926 With DBM::Deep you can access your databases using Perl's standard hash/array
927 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
928 treat them as such. DBM::Deep will intercept all reads/writes and direct them
929 to the right place -- the DB file. This has nothing to do with the
930 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
931 using regular hashes and arrays, rather than calling functions like C<get()>
932 and C<put()> (although those work too). It is entirely up to you how to want
933 to access your databases.
937 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
938 or even nested hashes (or arrays) using standard Perl syntax:
940 my $db = DBM::Deep->new( "foo.db" );
942 $db->{mykey} = "myvalue";
944 $db->{myhash}->{subkey} = "subvalue";
946 print $db->{myhash}->{subkey} . "\n";
948 You can even step through hash keys using the normal Perl C<keys()> function:
950 foreach my $key (keys %$db) {
951 print "$key: " . $db->{$key} . "\n";
954 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
955 pushes them onto an array, all before the loop even begins. If you have an
956 extra large hash, this may exhaust Perl's memory. Instead, consider using
957 Perl's C<each()> function, which pulls keys/values one at a time, using very
960 while (my ($key, $value) = each %$db) {
961 print "$key: $value\n";
964 Please note that when using C<each()>, you should always pass a direct
965 hash reference, not a lookup. Meaning, you should B<never> do this:
968 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
970 This causes an infinite loop, because for each iteration, Perl is calling
971 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
972 it effectively keeps returning the first key over and over again. Instead,
973 assign a temporary variable to C<$db->{foo}>, then pass that to each().
977 As with hashes, you can treat any DBM::Deep object like a normal Perl array
978 reference. This includes inserting, removing and manipulating elements,
979 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
980 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
981 or simply be a nested array reference inside a hash. Example:
983 my $db = DBM::Deep->new(
984 file => "foo-array.db",
985 type => DBM::Deep->TYPE_ARRAY
989 push @$db, "bar", "baz";
992 my $last_elem = pop @$db; # baz
993 my $first_elem = shift @$db; # bah
994 my $second_elem = $db->[1]; # bar
996 my $num_elements = scalar @$db;
1000 In addition to the I<tie()> interface, you can also use a standard OO interface
1001 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
1002 array) has its own methods, but both types share the following common methods:
1003 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
1007 =item * new() / clone()
1009 These are the constructor and copy-functions.
1011 =item * put() / store()
1013 Stores a new hash key/value pair, or sets an array element value. Takes two
1014 arguments, the hash key or array index, and the new value. The value can be
1015 a scalar, hash ref or array ref. Returns true on success, false on failure.
1017 $db->put("foo", "bar"); # for hashes
1018 $db->put(1, "bar"); # for arrays
1020 =item * get() / fetch()
1022 Fetches the value of a hash key or array element. Takes one argument: the hash
1023 key or array index. Returns a scalar, hash ref or array ref, depending on the
1026 my $value = $db->get("foo"); # for hashes
1027 my $value = $db->get(1); # for arrays
1031 Checks if a hash key or array index exists. Takes one argument: the hash key
1032 or array index. Returns true if it exists, false if not.
1034 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
1035 if ($db->exists(1)) { print "yay!\n"; } # for arrays
1039 Deletes one hash key/value pair or array element. Takes one argument: the hash
1040 key or array index. Returns true on success, false if not found. For arrays,
1041 the remaining elements located after the deleted element are NOT moved over.
1042 The deleted element is essentially just undefined, which is exactly how Perl's
1043 internal arrays work. Please note that the space occupied by the deleted
1044 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
1045 below for details and workarounds.
1047 $db->delete("foo"); # for hashes
1048 $db->delete(1); # for arrays
1052 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1053 value. Please note that the space occupied by the deleted keys/values or
1054 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1055 details and workarounds.
1057 $db->clear(); # hashes or arrays
1059 =item * lock() / unlock()
1065 Recover lost disk space.
1067 =item * import() / export()
1069 Data going in and out.
1071 =item * set_digest() / set_pack() / set_filter()
1073 q.v. adjusting the interal parameters.
1079 For hashes, DBM::Deep supports all the common methods described above, and the
1080 following additional methods: C<first_key()> and C<next_key()>.
1086 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1087 fetched in an undefined order (which appears random). Takes no arguments,
1088 returns the key as a scalar value.
1090 my $key = $db->first_key();
1094 Returns the "next" key in the hash, given the previous one as the sole argument.
1095 Returns undef if there are no more keys to be fetched.
1097 $key = $db->next_key($key);
1101 Here are some examples of using hashes:
1103 my $db = DBM::Deep->new( "foo.db" );
1105 $db->put("foo", "bar");
1106 print "foo: " . $db->get("foo") . "\n";
1108 $db->put("baz", {}); # new child hash ref
1109 $db->get("baz")->put("buz", "biz");
1110 print "buz: " . $db->get("baz")->get("buz") . "\n";
1112 my $key = $db->first_key();
1114 print "$key: " . $db->get($key) . "\n";
1115 $key = $db->next_key($key);
1118 if ($db->exists("foo")) { $db->delete("foo"); }
1122 For arrays, DBM::Deep supports all the common methods described above, and the
1123 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1124 C<unshift()> and C<splice()>.
1130 Returns the number of elements in the array. Takes no arguments.
1132 my $len = $db->length();
1136 Adds one or more elements onto the end of the array. Accepts scalars, hash
1137 refs or array refs. No return value.
1139 $db->push("foo", "bar", {});
1143 Fetches the last element in the array, and deletes it. Takes no arguments.
1144 Returns undef if array is empty. Returns the element value.
1146 my $elem = $db->pop();
1150 Fetches the first element in the array, deletes it, then shifts all the
1151 remaining elements over to take up the space. Returns the element value. This
1152 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1155 my $elem = $db->shift();
1159 Inserts one or more elements onto the beginning of the array, shifting all
1160 existing elements over to make room. Accepts scalars, hash refs or array refs.
1161 No return value. This method is not recommended with large arrays -- see
1162 <LARGE ARRAYS> below for details.
1164 $db->unshift("foo", "bar", {});
1168 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1169 -f splice> for usage -- it is too complicated to document here. This method is
1170 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1174 Here are some examples of using arrays:
1176 my $db = DBM::Deep->new(
1178 type => DBM::Deep->TYPE_ARRAY
1181 $db->push("bar", "baz");
1182 $db->unshift("foo");
1185 my $len = $db->length();
1186 print "length: $len\n"; # 4
1188 for (my $k=0; $k<$len; $k++) {
1189 print "$k: " . $db->get($k) . "\n";
1192 $db->splice(1, 2, "biz", "baf");
1194 while (my $elem = shift @$db) {
1195 print "shifted: $elem\n";
1200 Enable automatic file locking by passing a true value to the C<locking>
1201 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1203 my $db = DBM::Deep->new(
1208 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1209 mode for writes, and shared mode for reads. This is required if you have
1210 multiple processes accessing the same database file, to avoid file corruption.
1211 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1212 NFS> below for more.
1214 =head2 EXPLICIT LOCKING
1216 You can explicitly lock a database, so it remains locked for multiple
1217 transactions. This is done by calling the C<lock()> method, and passing an
1218 optional lock mode argument (defaults to exclusive mode). This is particularly
1219 useful for things like counters, where the current value needs to be fetched,
1220 then incremented, then stored again.
1223 my $counter = $db->get("counter");
1225 $db->put("counter", $counter);
1234 You can pass C<lock()> an optional argument, which specifies which mode to use
1235 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1236 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1237 same as the constants defined in Perl's C<Fcntl> module.
1239 $db->lock( DBM::Deep->LOCK_SH );
1243 =head1 IMPORTING/EXPORTING
1245 You can import existing complex structures by calling the C<import()> method,
1246 and export an entire database into an in-memory structure using the C<export()>
1247 method. Both are examined here.
1251 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1252 walking the structure and adding keys/elements to the database as you go,
1253 simply pass a reference to the C<import()> method. This recursively adds
1254 everything to an existing DBM::Deep object for you. Here is an example:
1259 array1 => [ "elem0", "elem1", "elem2" ],
1261 subkey1 => "subvalue1",
1262 subkey2 => "subvalue2"
1266 my $db = DBM::Deep->new( "foo.db" );
1267 $db->import( $struct );
1269 print $db->{key1} . "\n"; # prints "value1"
1271 This recursively imports the entire C<$struct> object into C<$db>, including
1272 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1273 keys are merged with the existing ones, replacing if they already exist.
1274 The C<import()> method can be called on any database level (not just the base
1275 level), and works with both hash and array DB types.
1277 B<Note:> Make sure your existing structure has no circular references in it.
1278 These will cause an infinite loop when importing.
1282 Calling the C<export()> method on an existing DBM::Deep object will return
1283 a reference to a new in-memory copy of the database. The export is done
1284 recursively, so all nested hashes/arrays are all exported to standard Perl
1285 objects. Here is an example:
1287 my $db = DBM::Deep->new( "foo.db" );
1289 $db->{key1} = "value1";
1290 $db->{key2} = "value2";
1292 $db->{hash1}->{subkey1} = "subvalue1";
1293 $db->{hash1}->{subkey2} = "subvalue2";
1295 my $struct = $db->export();
1297 print $struct->{key1} . "\n"; # prints "value1"
1299 This makes a complete copy of the database in memory, and returns a reference
1300 to it. The C<export()> method can be called on any database level (not just
1301 the base level), and works with both hash and array DB types. Be careful of
1302 large databases -- you can store a lot more data in a DBM::Deep object than an
1303 in-memory Perl structure.
1305 B<Note:> Make sure your database has no circular references in it.
1306 These will cause an infinite loop when exporting.
1310 DBM::Deep has a number of hooks where you can specify your own Perl function
1311 to perform filtering on incoming or outgoing data. This is a perfect
1312 way to extend the engine, and implement things like real-time compression or
1313 encryption. Filtering applies to the base DB level, and all child hashes /
1314 arrays. Filter hooks can be specified when your DBM::Deep object is first
1315 constructed, or by calling the C<set_filter()> method at any time. There are
1316 four available filter hooks, described below:
1320 =item * filter_store_key
1322 This filter is called whenever a hash key is stored. It
1323 is passed the incoming key, and expected to return a transformed key.
1325 =item * filter_store_value
1327 This filter is called whenever a hash key or array element is stored. It
1328 is passed the incoming value, and expected to return a transformed value.
1330 =item * filter_fetch_key
1332 This filter is called whenever a hash key is fetched (i.e. via
1333 C<first_key()> or C<next_key()>). It is passed the transformed key,
1334 and expected to return the plain key.
1336 =item * filter_fetch_value
1338 This filter is called whenever a hash key or array element is fetched.
1339 It is passed the transformed value, and expected to return the plain value.
1343 Here are the two ways to setup a filter hook:
1345 my $db = DBM::Deep->new(
1347 filter_store_value => \&my_filter_store,
1348 filter_fetch_value => \&my_filter_fetch
1353 $db->set_filter( "filter_store_value", \&my_filter_store );
1354 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1356 Your filter function will be called only when dealing with SCALAR keys or
1357 values. When nested hashes and arrays are being stored/fetched, filtering
1358 is bypassed. Filters are called as static functions, passed a single SCALAR
1359 argument, and expected to return a single SCALAR value. If you want to
1360 remove a filter, set the function reference to C<undef>:
1362 $db->set_filter( "filter_store_value", undef );
1364 =head2 REAL-TIME ENCRYPTION EXAMPLE
1366 Here is a working example that uses the I<Crypt::Blowfish> module to
1367 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1368 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1369 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1372 use Crypt::Blowfish;
1375 my $cipher = Crypt::CBC->new({
1376 'key' => 'my secret key',
1377 'cipher' => 'Blowfish',
1379 'regenerate_key' => 0,
1380 'padding' => 'space',
1384 my $db = DBM::Deep->new(
1385 file => "foo-encrypt.db",
1386 filter_store_key => \&my_encrypt,
1387 filter_store_value => \&my_encrypt,
1388 filter_fetch_key => \&my_decrypt,
1389 filter_fetch_value => \&my_decrypt,
1392 $db->{key1} = "value1";
1393 $db->{key2} = "value2";
1394 print "key1: " . $db->{key1} . "\n";
1395 print "key2: " . $db->{key2} . "\n";
1401 return $cipher->encrypt( $_[0] );
1404 return $cipher->decrypt( $_[0] );
1407 =head2 REAL-TIME COMPRESSION EXAMPLE
1409 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1410 compression / decompression of keys & values with DBM::Deep Filters.
1411 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1412 more on I<Compress::Zlib>.
1417 my $db = DBM::Deep->new(
1418 file => "foo-compress.db",
1419 filter_store_key => \&my_compress,
1420 filter_store_value => \&my_compress,
1421 filter_fetch_key => \&my_decompress,
1422 filter_fetch_value => \&my_decompress,
1425 $db->{key1} = "value1";
1426 $db->{key2} = "value2";
1427 print "key1: " . $db->{key1} . "\n";
1428 print "key2: " . $db->{key2} . "\n";
1434 return Compress::Zlib::memGzip( $_[0] ) ;
1437 return Compress::Zlib::memGunzip( $_[0] ) ;
1440 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1441 actually numerical index numbers, and are not filtered.
1443 =head1 ERROR HANDLING
1445 Most DBM::Deep methods return a true value for success, and call die() on
1446 failure. You can wrap calls in an eval block to catch the die.
1448 my $db = DBM::Deep->new( "foo.db" ); # create hash
1449 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1451 print $@; # prints error message
1453 =head1 LARGEFILE SUPPORT
1455 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1456 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1457 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1458 by calling the static C<set_pack()> method before you do anything else.
1460 DBM::Deep::set_pack(8, 'Q');
1462 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1463 instead of 32-bit longs. After setting these values your DB files have a
1464 theoretical maximum size of 16 XB (exabytes).
1466 B<Note:> Changing these values will B<NOT> work for existing database files.
1467 Only change this for new files, and make sure it stays set consistently
1468 throughout the file's life. If you do set these values, you can no longer
1469 access 32-bit DB files. You can, however, call C<set_pack(4, 'N')> to change
1470 back to 32-bit mode.
1472 B<Note:> I have not personally tested files > 2 GB -- all my systems have
1473 only a 32-bit Perl. However, I have received user reports that this does
1476 =head1 LOW-LEVEL ACCESS
1478 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1479 you can call the C<_fh()> method, which returns the handle:
1481 my $fh = $db->_fh();
1483 This method can be called on the root level of the datbase, or any child
1484 hashes or arrays. All levels share a I<root> structure, which contains things
1485 like the filehandle, a reference counter, and all the options specified
1486 when you created the object. You can get access to this root structure by
1487 calling the C<root()> method.
1489 my $root = $db->_root();
1491 This is useful for changing options after the object has already been created,
1492 such as enabling/disabling locking. You can also store your own temporary user
1493 data in this structure (be wary of name collision), which is then accessible from
1494 any child hash or array.
1496 =head1 CUSTOM DIGEST ALGORITHM
1498 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1499 keys. However you can override this, and use another algorithm (such as SHA-256)
1500 or even write your own. But please note that DBM::Deep currently expects zero
1501 collisions, so your algorithm has to be I<perfect>, so to speak.
1502 Collision detection may be introduced in a later version.
1506 You can specify a custom digest algorithm by calling the static C<set_digest()>
1507 function, passing a reference to a subroutine, and the length of the algorithm's
1508 hashes (in bytes). This is a global static function, which affects ALL DBM::Deep
1509 objects. Here is a working example that uses a 256-bit hash from the
1510 I<Digest::SHA256> module. Please see
1511 L<http://search.cpan.org/search?module=Digest::SHA256> for more.
1516 my $context = Digest::SHA256::new(256);
1518 DBM::Deep::set_digest( \&my_digest, 32 );
1520 my $db = DBM::Deep->new( "foo-sha.db" );
1522 $db->{key1} = "value1";
1523 $db->{key2} = "value2";
1524 print "key1: " . $db->{key1} . "\n";
1525 print "key2: " . $db->{key2} . "\n";
1531 return substr( $context->hash($_[0]), 0, 32 );
1534 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1535 of bytes you specify in the C<set_digest()> function (in this case 32).
1537 =head1 CIRCULAR REFERENCES
1539 DBM::Deep has B<experimental> support for circular references. Meaning you
1540 can have a nested hash key or array element that points to a parent object.
1541 This relationship is stored in the DB file, and is preserved between sessions.
1544 my $db = DBM::Deep->new( "foo.db" );
1547 $db->{circle} = $db; # ref to self
1549 print $db->{foo} . "\n"; # prints "foo"
1550 print $db->{circle}->{foo} . "\n"; # prints "foo" again
1552 One catch is, passing the object to a function that recursively walks the
1553 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1554 C<export()> methods) will result in an infinite loop. The other catch is,
1555 if you fetch the I<key> of a circular reference (i.e. using the C<first_key()>
1556 or C<next_key()> methods), you will get the I<target object's key>, not the
1557 ref's key. This gets even more interesting with the above example, where
1558 the I<circle> key points to the base DB object, which technically doesn't
1559 have a key. So I made DBM::Deep return "[base]" as the key name in that
1562 =head1 CAVEATS / ISSUES / BUGS
1564 This section describes all the known issues with DBM::Deep. It you have found
1565 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1567 =head2 UNUSED SPACE RECOVERY
1569 One major caveat with DBM::Deep is that space occupied by existing keys and
1570 values is not recovered when they are deleted. Meaning if you keep deleting
1571 and adding new keys, your file will continuously grow. I am working on this,
1572 but in the meantime you can call the built-in C<optimize()> method from time to
1573 time (perhaps in a crontab or something) to recover all your unused space.
1575 $db->optimize(); # returns true on success
1577 This rebuilds the ENTIRE database into a new file, then moves it on top of
1578 the original. The new file will have no unused space, thus it will take up as
1579 little disk space as possible. Please note that this operation can take
1580 a long time for large files, and you need enough disk space to temporarily hold
1581 2 copies of your DB file. The temporary file is created in the same directory
1582 as the original, named with a ".tmp" extension, and is deleted when the
1583 operation completes. Oh, and if locking is enabled, the DB is automatically
1584 locked for the entire duration of the copy.
1586 B<WARNING:> Only call optimize() on the top-level node of the database, and
1587 make sure there are no child references lying around. DBM::Deep keeps a reference
1588 counter, and if it is greater than 1, optimize() will abort and return undef.
1590 =head2 AUTOVIVIFICATION
1592 Unfortunately, autovivification doesn't work with tied hashes. This appears to
1593 be a bug in Perl's tie() system, as I<Jakob Schmidt> encountered the very same
1594 issue with his I<DWH_FIle> module (see L<http://search.cpan.org/search?module=DWH_File>),
1595 and it is also mentioned in the BUGS section for the I<MLDBM> module <see
1596 L<http://search.cpan.org/search?module=MLDBM>). Basically, on a new db file,
1599 $db->{foo}->{bar} = "hello";
1601 Since "foo" doesn't exist, you cannot add "bar" to it. You end up with "foo"
1602 being an empty hash. Try this instead, which works fine:
1604 $db->{foo} = { bar => "hello" };
1606 As of Perl 5.8.7, this bug still exists. I have walked very carefully through
1607 the execution path, and Perl indeed passes an empty hash to the STORE() method.
1608 Probably a bug in Perl.
1610 =head2 FILE CORRUPTION
1612 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1613 for a 32-bit signature when opened, but other corruption in files can cause
1614 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1615 stuck in an infinite loop depending on the level of corruption. File write
1616 operations are not checked for failure (for speed), so if you happen to run
1617 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1618 be addressed in a later version of DBM::Deep.
1622 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1623 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1624 about setting up your NFS server with a locking daemon, then using lockf() to
1625 lock your files, but your mileage may vary there as well. From what I
1626 understand, there is no real way to do it. However, if you need access to the
1627 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1628 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1630 =head2 COPYING OBJECTS
1632 Beware of copying tied objects in Perl. Very strange things can happen.
1633 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1634 returns a new, blessed, tied hash or array to the same level in the DB.
1636 my $copy = $db->clone();
1638 B<Note>: Since clone() here is cloning the object, not the database location, any
1639 modifications to either $db or $copy will be visible in both.
1643 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1644 These functions cause every element in the array to move, which can be murder
1645 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1646 a different location. This will be addressed in the forthcoming version 1.00.
1648 =head2 WRITEONLY FILES
1650 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1651 writeonly mode. STORE will verify that the filehandle is writable. However, there
1652 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1653 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1657 This section discusses DBM::Deep's speed and memory usage.
1661 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1662 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1663 multi-level hash/array support, and cross-platform FTPable files. Even so,
1664 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1665 with huge databases. Here is some test data:
1667 Adding 1,000,000 keys to new DB file...
1669 At 100 keys, avg. speed is 2,703 keys/sec
1670 At 200 keys, avg. speed is 2,642 keys/sec
1671 At 300 keys, avg. speed is 2,598 keys/sec
1672 At 400 keys, avg. speed is 2,578 keys/sec
1673 At 500 keys, avg. speed is 2,722 keys/sec
1674 At 600 keys, avg. speed is 2,628 keys/sec
1675 At 700 keys, avg. speed is 2,700 keys/sec
1676 At 800 keys, avg. speed is 2,607 keys/sec
1677 At 900 keys, avg. speed is 2,190 keys/sec
1678 At 1,000 keys, avg. speed is 2,570 keys/sec
1679 At 2,000 keys, avg. speed is 2,417 keys/sec
1680 At 3,000 keys, avg. speed is 1,982 keys/sec
1681 At 4,000 keys, avg. speed is 1,568 keys/sec
1682 At 5,000 keys, avg. speed is 1,533 keys/sec
1683 At 6,000 keys, avg. speed is 1,787 keys/sec
1684 At 7,000 keys, avg. speed is 1,977 keys/sec
1685 At 8,000 keys, avg. speed is 2,028 keys/sec
1686 At 9,000 keys, avg. speed is 2,077 keys/sec
1687 At 10,000 keys, avg. speed is 2,031 keys/sec
1688 At 20,000 keys, avg. speed is 1,970 keys/sec
1689 At 30,000 keys, avg. speed is 2,050 keys/sec
1690 At 40,000 keys, avg. speed is 2,073 keys/sec
1691 At 50,000 keys, avg. speed is 1,973 keys/sec
1692 At 60,000 keys, avg. speed is 1,914 keys/sec
1693 At 70,000 keys, avg. speed is 2,091 keys/sec
1694 At 80,000 keys, avg. speed is 2,103 keys/sec
1695 At 90,000 keys, avg. speed is 1,886 keys/sec
1696 At 100,000 keys, avg. speed is 1,970 keys/sec
1697 At 200,000 keys, avg. speed is 2,053 keys/sec
1698 At 300,000 keys, avg. speed is 1,697 keys/sec
1699 At 400,000 keys, avg. speed is 1,838 keys/sec
1700 At 500,000 keys, avg. speed is 1,941 keys/sec
1701 At 600,000 keys, avg. speed is 1,930 keys/sec
1702 At 700,000 keys, avg. speed is 1,735 keys/sec
1703 At 800,000 keys, avg. speed is 1,795 keys/sec
1704 At 900,000 keys, avg. speed is 1,221 keys/sec
1705 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1707 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1708 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1709 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1710 Run time was 12 min 3 sec.
1714 One of the great things about DBM::Deep is that it uses very little memory.
1715 Even with huge databases (1,000,000+ keys) you will not see much increased
1716 memory on your process. DBM::Deep relies solely on the filesystem for storing
1717 and fetching data. Here is output from I</usr/bin/top> before even opening a
1720 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1721 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1723 Basically the process is taking 2,716K of memory. And here is the same
1724 process after storing and fetching 1,000,000 keys:
1726 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1727 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1729 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1730 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1732 =head1 DB FILE FORMAT
1734 In case you were interested in the underlying DB file format, it is documented
1735 here in this section. You don't need to know this to use the module, it's just
1736 included for reference.
1740 DBM::Deep files always start with a 32-bit signature to identify the file type.
1741 This is at offset 0. The signature is "DPDB" in network byte order. This is
1742 checked for when the file is opened and an error will be thrown if it's not found.
1746 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1747 has a standard header containing the type of data, the length of data, and then
1748 the data itself. The type is a single character (1 byte), the length is a
1749 32-bit unsigned long in network byte order, and the data is, well, the data.
1750 Here is how it unfolds:
1754 Immediately after the 32-bit file signature is the I<Master Index> record.
1755 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1756 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1757 depending on how the DBM::Deep object was constructed.
1759 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1760 number). The first 8-bit char of the MD5 signature is the offset into the
1761 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1762 index element is a file offset of the next tag for the key/element in question,
1763 which is usually a I<Bucket List> tag (see below).
1765 The next tag I<could> be another index, depending on how many keys/elements
1766 exist. See L<RE-INDEXING> below for details.
1770 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1771 file offsets to where the actual data is stored. It starts with a standard
1772 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1773 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1774 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1775 When the list fills up, a I<Re-Index> operation is performed (See
1776 L<RE-INDEXING> below).
1780 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1781 index/value pair (in array mode). It starts with a standard tag header with
1782 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1783 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1784 header. The size reported in the tag header is only for the value, but then,
1785 just after the value is another size (32-bit unsigned long) and then the plain
1786 key itself. Since the value is likely to be fetched more often than the plain
1787 key, I figured it would be I<slightly> faster to store the value first.
1789 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1790 record for the nested structure, where the process begins all over again.
1794 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1795 exhausted. Then, when another key/element comes in, the list is converted to a
1796 new index record. However, this index will look at the next char in the MD5
1797 hash, and arrange new Bucket List pointers accordingly. This process is called
1798 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1799 17 (16 + new one) keys/elements are removed from the old Bucket List and
1800 inserted into the new index. Several new Bucket Lists are created in the
1801 process, as a new MD5 char from the key is being examined (it is unlikely that
1802 the keys will all share the same next char of their MD5s).
1804 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1805 when the Bucket Lists will turn into indexes, but the first round tends to
1806 happen right around 4,000 keys. You will see a I<slight> decrease in
1807 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1808 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1809 right around 900,000 keys. This process can continue nearly indefinitely --
1810 right up until the point the I<MD5> signatures start colliding with each other,
1811 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1812 getting struck by lightning while you are walking to cash in your tickets.
1813 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1814 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1815 this is 340 unodecillion, but don't quote me).
1819 When a new key/element is stored, the key (or index number) is first run through
1820 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1821 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1822 for the first char of the signature (in this case I<b0>). If it does not exist,
1823 a new I<Bucket List> is created for our key (and the next 15 future keys that
1824 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1825 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1826 this point, unless we are replacing an existing I<Bucket>), where the actual
1827 data will be stored.
1831 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1832 (or index number), then walking along the indexes. If there are enough
1833 keys/elements in this DB level, there might be nested indexes, each linked to
1834 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1835 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1836 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1837 plain key are stored.
1839 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1840 methods. In this process the indexes are walked systematically, and each key
1841 fetched in increasing MD5 order (which is why it appears random). Once the
1842 I<Bucket> is found, the value is skipped and the plain key returned instead.
1843 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1844 alphabetically sorted. This only happens on an index-level -- as soon as the
1845 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1846 so it's pretty much undefined how the keys will come out -- just like Perl's
1849 =head1 CODE COVERAGE
1851 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1852 B<Devel::Cover> report on this module's test suite.
1854 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1855 File stmt bran cond sub pod time total
1856 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1857 blib/lib/DBM/Deep.pm 95.2 83.8 70.0 98.2 100.0 58.0 91.0
1858 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 n/a 26.7 98.0
1859 blib/lib/DBM/Deep/Hash.pm 95.3 80.0 100.0 100.0 n/a 15.3 92.4
1860 Total 96.2 84.8 74.4 98.8 100.0 100.0 92.4
1861 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1863 =head1 MORE INFORMATION
1865 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1866 or send email to L<DBM-Deep@googlegroups.com>.
1870 Joseph Huckaby, L<jhuckaby@cpan.org>
1872 Rob Kinyon, L<rkinyon@cpan.org>
1874 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1878 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1879 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1883 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1884 This is free software, you may use it and distribute it under the
1885 same terms as Perl itself.