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',
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( $args );
140 if (!defined($self->_fh)) { $self->{engine}->open( $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}->open( $self ); # re-open
185 flock($self->_fh, $type); # re-lock
186 $self->_root->{end} = (stat($self->_fh))[7]; # re-end
189 $self->_root->{locked}++;
199 # If db locking is set, unlock the db file. See note in lock()
200 # regarding calling lock() multiple times.
202 my $self = $_[0]->_get_self;
204 if (!defined($self->_fh)) { return; }
206 if ($self->_root->{locking} && $self->_root->{locked} > 0) {
207 $self->_root->{locked}--;
208 if (!$self->_root->{locked}) { flock($self->_fh, LOCK_UN); }
217 my $self = shift->_get_self;
218 my ($spot, $value) = @_;
223 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
224 my $type = $value->_type;
225 ${$spot} = $type eq TYPE_HASH ? {} : [];
226 $value->_copy_node( ${$spot} );
229 my $r = Scalar::Util::reftype( $value );
230 my $c = Scalar::Util::blessed( $value );
231 if ( $r eq 'ARRAY' ) {
232 ${$spot} = [ @{$value} ];
235 ${$spot} = { %{$value} };
237 ${$spot} = bless ${$spot}, $c
246 # Copy single level of keys or elements to new DB handle.
247 # Recurse for nested structures
249 my $self = shift->_get_self;
252 if ($self->_type eq TYPE_HASH) {
253 my $key = $self->first_key();
255 my $value = $self->get($key);
256 $self->_copy_value( \$db_temp->{$key}, $value );
257 $key = $self->next_key($key);
261 my $length = $self->length();
262 for (my $index = 0; $index < $length; $index++) {
263 my $value = $self->get($index);
264 $self->_copy_value( \$db_temp->[$index], $value );
273 # Recursively export into standard Perl hashes and arrays.
275 my $self = $_[0]->_get_self;
278 if ($self->_type eq TYPE_HASH) { $temp = {}; }
279 elsif ($self->_type eq TYPE_ARRAY) { $temp = []; }
282 $self->_copy_node( $temp );
290 # Recursively import Perl hash/array structure
292 #XXX This use of ref() seems to be ok
293 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
295 my $self = $_[0]->_get_self;
298 #XXX This use of ref() seems to be ok
301 # struct is not a reference, so just import based on our type
305 if ($self->_type eq TYPE_HASH) { $struct = {@_}; }
306 elsif ($self->_type eq TYPE_ARRAY) { $struct = [@_]; }
309 my $r = Scalar::Util::reftype($struct) || '';
310 if ($r eq "HASH" && $self->_type eq TYPE_HASH) {
311 foreach my $key (keys %$struct) { $self->put($key, $struct->{$key}); }
313 elsif ($r eq "ARRAY" && $self->_type eq TYPE_ARRAY) {
314 $self->push( @$struct );
317 return $self->_throw_error("Cannot import: type mismatch");
325 # Rebuild entire database into new file, then move
326 # it back on top of original.
328 my $self = $_[0]->_get_self;
330 #XXX Need to create a new test for this
331 # if ($self->_root->{links} > 1) {
332 # return $self->_throw_error("Cannot optimize: reference count is greater than 1");
335 my $db_temp = DBM::Deep->new(
336 file => $self->_root->{file} . '.tmp',
340 return $self->_throw_error("Cannot optimize: failed to open temp file: $!");
344 $self->_copy_node( $db_temp );
348 # Attempt to copy user, group and permissions over to new file
350 my @stats = stat($self->_fh);
351 my $perms = $stats[2] & 07777;
354 chown( $uid, $gid, $self->_root->{file} . '.tmp' );
355 chmod( $perms, $self->_root->{file} . '.tmp' );
357 # q.v. perlport for more information on this variable
358 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
360 # Potential race condition when optmizing on Win32 with locking.
361 # The Windows filesystem requires that the filehandle be closed
362 # before it is overwritten with rename(). This could be redone
366 $self->{engine}->close( $self );
369 if (!rename $self->_root->{file} . '.tmp', $self->_root->{file}) {
370 unlink $self->_root->{file} . '.tmp';
372 return $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
376 $self->{engine}->close( $self );
377 $self->{engine}->open( $self );
384 # Make copy of object and return
386 my $self = $_[0]->_get_self;
388 return DBM::Deep->new(
389 type => $self->_type,
390 base_offset => $self->_base_offset,
396 my %is_legal_filter = map {
399 store_key store_value
400 fetch_key fetch_value
405 # Setup filter function for storing or fetching the key or value
407 my $self = $_[0]->_get_self;
409 my $func = $_[2] ? $_[2] : undef;
411 if ( $is_legal_filter{$type} ) {
412 $self->_root->{"filter_$type"} = $func;
426 # Get access to the root structure
428 my $self = $_[0]->_get_self;
429 return $self->{root};
434 # Get access to the raw fh
436 #XXX It will be useful, though, when we split out HASH and ARRAY
437 my $self = $_[0]->_get_self;
438 return $self->_root->{fh};
443 # Get type of current node (TYPE_HASH or TYPE_ARRAY)
445 my $self = $_[0]->_get_self;
446 return $self->{type};
451 # Get base_offset of current node (TYPE_HASH or TYPE_ARRAY)
453 my $self = $_[0]->_get_self;
454 return $self->{base_offset};
462 die "DBM::Deep: $_[1]\n";
467 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
472 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
476 # tie() methods (hashes and arrays)
481 # Store single hash key/value or array element in database.
483 my $self = $_[0]->_get_self;
486 # User may be storing a hash, in which case we do not want it run
487 # through the filtering system
488 my $value = ($self->_root->{filter_store_value} && !ref($_[2]))
489 ? $self->_root->{filter_store_value}->($_[2])
492 my $md5 = $DBM::Deep::Engine::DIGEST_FUNC->($key);
494 unless ( _is_writable( $self->_fh ) ) {
495 $self->_throw_error( 'Cannot write to a readonly filehandle' );
499 # Request exclusive lock for writing
501 $self->lock( LOCK_EX );
506 # Locate offset for bucket list using digest index system
508 my $tag = $self->{engine}->load_tag($self, $self->_base_offset);
510 $tag = $self->{engine}->create_tag($self, $self->_base_offset, SIG_INDEX, chr(0) x $DBM::Deep::Engine::INDEX_SIZE);
514 while ($tag->{signature} ne SIG_BLIST) {
515 my $num = ord(substr($md5, $ch, 1));
517 my $ref_loc = $tag->{offset} + ($num * $DBM::Deep::Engine::LONG_SIZE);
518 my $new_tag = $self->{engine}->index_lookup($self, $tag, $num);
521 seek($fh, $ref_loc + $self->_root->{file_offset}, SEEK_SET);
522 print( $fh pack($DBM::Deep::Engine::LONG_PACK, $self->_root->{end}) );
524 $tag = $self->{engine}->create_tag($self, $self->_root->{end}, SIG_BLIST, chr(0) x $DBM::Deep::Engine::BUCKET_LIST_SIZE);
526 $tag->{ref_loc} = $ref_loc;
534 $tag->{ref_loc} = $ref_loc;
541 # Add key/value to bucket list
543 my $result = $self->{engine}->add_bucket( $self, $tag, $md5, $key, $value );
552 # Fetch single value or element given plain key or array index
554 my $self = shift->_get_self;
557 my $md5 = $DBM::Deep::Engine::DIGEST_FUNC->($key);
560 # Request shared lock for reading
562 $self->lock( LOCK_SH );
564 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
571 # Get value from bucket list
573 my $result = $self->{engine}->get_bucket_value( $self, $tag, $md5 );
577 #XXX What is ref() checking here?
578 #YYY Filters only apply on scalar values, so the ref check is making
579 #YYY sure the fetched bucket is a scalar, not a child hash or array.
580 return ($result && !ref($result) && $self->_root->{filter_fetch_value})
581 ? $self->_root->{filter_fetch_value}->($result)
587 # Delete single key/value pair or element given plain key or array index
589 my $self = $_[0]->_get_self;
592 my $md5 = $DBM::Deep::Engine::DIGEST_FUNC->($key);
595 # Request exclusive lock for writing
597 $self->lock( LOCK_EX );
599 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
608 my $value = $self->{engine}->get_bucket_value($self, $tag, $md5 );
609 if ($value && !ref($value) && $self->_root->{filter_fetch_value}) {
610 $value = $self->_root->{filter_fetch_value}->($value);
613 my $result = $self->{engine}->delete_bucket( $self, $tag, $md5 );
616 # If this object is an array and the key deleted was on the end of the stack,
617 # decrement the length variable.
627 # Check if a single key or element exists given plain key or array index
629 my $self = $_[0]->_get_self;
632 my $md5 = $DBM::Deep::Engine::DIGEST_FUNC->($key);
635 # Request shared lock for reading
637 $self->lock( LOCK_SH );
639 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
642 # For some reason, the built-in exists() function returns '' for false
650 # Check if bucket exists and return 1 or ''
652 my $result = $self->{engine}->bucket_exists( $self, $tag, $md5 ) || '';
661 # Clear all keys from hash, or all elements from array.
663 my $self = $_[0]->_get_self;
666 # Request exclusive lock for writing
668 $self->lock( LOCK_EX );
672 seek($fh, $self->_base_offset + $self->_root->{file_offset}, SEEK_SET);
678 $self->{engine}->create_tag($self, $self->_base_offset, $self->_type, chr(0) x $DBM::Deep::Engine::INDEX_SIZE);
686 # Public method aliases
688 sub put { (shift)->STORE( @_ ) }
689 sub store { (shift)->STORE( @_ ) }
690 sub get { (shift)->FETCH( @_ ) }
691 sub fetch { (shift)->FETCH( @_ ) }
692 sub delete { (shift)->DELETE( @_ ) }
693 sub exists { (shift)->EXISTS( @_ ) }
694 sub clear { (shift)->CLEAR( @_ ) }
696 package DBM::Deep::_::Root;
710 filter_store_key => undef,
711 filter_store_value => undef,
712 filter_fetch_key => undef,
713 filter_fetch_value => undef,
719 if ( $self->{fh} && !$self->{file_offset} ) {
720 $self->{file_offset} = tell( $self->{fh} );
730 close $self->{fh} if $self->{fh};
741 DBM::Deep - A pure perl multi-level hash/array DBM
746 my $db = DBM::Deep->new( "foo.db" );
748 $db->{key} = 'value'; # tie() style
751 $db->put('key' => 'value'); # OO style
752 print $db->get('key');
754 # true multi-level support
755 $db->{my_complex} = [
756 'hello', { perl => 'rules' },
762 A unique flat-file database module, written in pure perl. True
763 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
764 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
765 handle millions of keys and unlimited hash levels without significant
766 slow-down. Written from the ground-up in pure perl -- this is NOT a
767 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
768 Mac OS X and Windows.
772 Hopefully you are using Perl's excellent CPAN module, which will download
773 and install the module for you. If not, get the tarball, and run these
785 Construction can be done OO-style (which is the recommended way), or using
786 Perl's tie() function. Both are examined here.
788 =head2 OO CONSTRUCTION
790 The recommended way to construct a DBM::Deep object is to use the new()
791 method, which gets you a blessed, tied hash or array reference.
793 my $db = DBM::Deep->new( "foo.db" );
795 This opens a new database handle, mapped to the file "foo.db". If this
796 file does not exist, it will automatically be created. DB files are
797 opened in "r+" (read/write) mode, and the type of object returned is a
798 hash, unless otherwise specified (see L<OPTIONS> below).
800 You can pass a number of options to the constructor to specify things like
801 locking, autoflush, etc. This is done by passing an inline hash:
803 my $db = DBM::Deep->new(
809 Notice that the filename is now specified I<inside> the hash with
810 the "file" parameter, as opposed to being the sole argument to the
811 constructor. This is required if any options are specified.
812 See L<OPTIONS> below for the complete list.
816 You can also start with an array instead of a hash. For this, you must
817 specify the C<type> parameter:
819 my $db = DBM::Deep->new(
821 type => DBM::Deep->TYPE_ARRAY
824 B<Note:> Specifing the C<type> parameter only takes effect when beginning
825 a new DB file. If you create a DBM::Deep object with an existing file, the
826 C<type> will be loaded from the file header, and an error will be thrown if
827 the wrong type is passed in.
829 =head2 TIE CONSTRUCTION
831 Alternately, you can create a DBM::Deep handle by using Perl's built-in
832 tie() function. The object returned from tie() can be used to call methods,
833 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
834 file (as expected with most tie'd objects).
837 my $db = tie %hash, "DBM::Deep", "foo.db";
840 my $db = tie @array, "DBM::Deep", "bar.db";
842 As with the OO constructor, you can replace the DB filename parameter with
843 a hash containing one or more options (see L<OPTIONS> just below for the
846 tie %hash, "DBM::Deep", {
854 There are a number of options that can be passed in when constructing your
855 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
861 Filename of the DB file to link the handle to. You can pass a full absolute
862 filesystem path, partial path, or a plain filename if the file is in the
863 current working directory. This is a required parameter (though q.v. fh).
867 If you want, you can pass in the fh instead of the file. This is most useful for doing
870 my $db = DBM::Deep->new( { fh => \*DATA } );
872 You are responsible for making sure that the fh has been opened appropriately for your
873 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
874 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
875 needs to read from the fh.
879 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
880 not need to set this. However, it's there if you want it.
882 If you pass in fh and do not set this, it will be set appropriately.
886 This parameter specifies what type of object to create, a hash or array. Use
887 one of these two constants: C<DBM::Deep-E<gt>TYPE_HASH> or C<DBM::Deep-E<gt>TYPE_ARRAY>.
888 This only takes effect when beginning a new file. This is an optional
889 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
893 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
894 function to lock the database in exclusive mode for writes, and shared mode for
895 reads. Pass any true value to enable. This affects the base DB handle I<and
896 any child hashes or arrays> that use the same DB file. This is an optional
897 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
901 Specifies whether autoflush is to be enabled on the underlying filehandle.
902 This obviously slows down write operations, but is required if you may have
903 multiple processes accessing the same DB file (also consider enable I<locking>).
904 Pass any true value to enable. This is an optional parameter, and defaults to 0
909 If I<autobless> mode is enabled, DBM::Deep will preserve blessed hashes, and
910 restore them when fetched. This is an B<experimental> feature, and does have
911 side-effects. Basically, when hashes are re-blessed into their original
912 classes, they are no longer blessed into the DBM::Deep class! So you won't be
913 able to call any DBM::Deep methods on them. You have been warned.
914 This is an optional parameter, and defaults to 0 (disabled).
918 See L<FILTERS> below.
922 Setting I<debug> mode will make all errors non-fatal, dump them out to
923 STDERR, and continue on. This is for debugging purposes only, and probably
924 not what you want. This is an optional parameter, and defaults to 0 (disabled).
926 B<NOTE>: This parameter is considered deprecated and should not be used anymore.
932 With DBM::Deep you can access your databases using Perl's standard hash/array
933 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
934 treat them as such. DBM::Deep will intercept all reads/writes and direct them
935 to the right place -- the DB file. This has nothing to do with the
936 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
937 using regular hashes and arrays, rather than calling functions like C<get()>
938 and C<put()> (although those work too). It is entirely up to you how to want
939 to access your databases.
943 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
944 or even nested hashes (or arrays) using standard Perl syntax:
946 my $db = DBM::Deep->new( "foo.db" );
948 $db->{mykey} = "myvalue";
950 $db->{myhash}->{subkey} = "subvalue";
952 print $db->{myhash}->{subkey} . "\n";
954 You can even step through hash keys using the normal Perl C<keys()> function:
956 foreach my $key (keys %$db) {
957 print "$key: " . $db->{$key} . "\n";
960 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
961 pushes them onto an array, all before the loop even begins. If you have an
962 extra large hash, this may exhaust Perl's memory. Instead, consider using
963 Perl's C<each()> function, which pulls keys/values one at a time, using very
966 while (my ($key, $value) = each %$db) {
967 print "$key: $value\n";
970 Please note that when using C<each()>, you should always pass a direct
971 hash reference, not a lookup. Meaning, you should B<never> do this:
974 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
976 This causes an infinite loop, because for each iteration, Perl is calling
977 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
978 it effectively keeps returning the first key over and over again. Instead,
979 assign a temporary variable to C<$db->{foo}>, then pass that to each().
983 As with hashes, you can treat any DBM::Deep object like a normal Perl array
984 reference. This includes inserting, removing and manipulating elements,
985 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
986 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
987 or simply be a nested array reference inside a hash. Example:
989 my $db = DBM::Deep->new(
990 file => "foo-array.db",
991 type => DBM::Deep->TYPE_ARRAY
995 push @$db, "bar", "baz";
998 my $last_elem = pop @$db; # baz
999 my $first_elem = shift @$db; # bah
1000 my $second_elem = $db->[1]; # bar
1002 my $num_elements = scalar @$db;
1006 In addition to the I<tie()> interface, you can also use a standard OO interface
1007 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
1008 array) has its own methods, but both types share the following common methods:
1009 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
1013 =item * new() / clone()
1015 These are the constructor and copy-functions.
1017 =item * put() / store()
1019 Stores a new hash key/value pair, or sets an array element value. Takes two
1020 arguments, the hash key or array index, and the new value. The value can be
1021 a scalar, hash ref or array ref. Returns true on success, false on failure.
1023 $db->put("foo", "bar"); # for hashes
1024 $db->put(1, "bar"); # for arrays
1026 =item * get() / fetch()
1028 Fetches the value of a hash key or array element. Takes one argument: the hash
1029 key or array index. Returns a scalar, hash ref or array ref, depending on the
1032 my $value = $db->get("foo"); # for hashes
1033 my $value = $db->get(1); # for arrays
1037 Checks if a hash key or array index exists. Takes one argument: the hash key
1038 or array index. Returns true if it exists, false if not.
1040 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
1041 if ($db->exists(1)) { print "yay!\n"; } # for arrays
1045 Deletes one hash key/value pair or array element. Takes one argument: the hash
1046 key or array index. Returns true on success, false if not found. For arrays,
1047 the remaining elements located after the deleted element are NOT moved over.
1048 The deleted element is essentially just undefined, which is exactly how Perl's
1049 internal arrays work. Please note that the space occupied by the deleted
1050 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
1051 below for details and workarounds.
1053 $db->delete("foo"); # for hashes
1054 $db->delete(1); # for arrays
1058 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1059 value. Please note that the space occupied by the deleted keys/values or
1060 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1061 details and workarounds.
1063 $db->clear(); # hashes or arrays
1065 =item * lock() / unlock()
1071 Recover lost disk space.
1073 =item * import() / export()
1075 Data going in and out.
1077 =item * set_digest() / set_pack() / set_filter()
1079 q.v. adjusting the interal parameters.
1085 For hashes, DBM::Deep supports all the common methods described above, and the
1086 following additional methods: C<first_key()> and C<next_key()>.
1092 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1093 fetched in an undefined order (which appears random). Takes no arguments,
1094 returns the key as a scalar value.
1096 my $key = $db->first_key();
1100 Returns the "next" key in the hash, given the previous one as the sole argument.
1101 Returns undef if there are no more keys to be fetched.
1103 $key = $db->next_key($key);
1107 Here are some examples of using hashes:
1109 my $db = DBM::Deep->new( "foo.db" );
1111 $db->put("foo", "bar");
1112 print "foo: " . $db->get("foo") . "\n";
1114 $db->put("baz", {}); # new child hash ref
1115 $db->get("baz")->put("buz", "biz");
1116 print "buz: " . $db->get("baz")->get("buz") . "\n";
1118 my $key = $db->first_key();
1120 print "$key: " . $db->get($key) . "\n";
1121 $key = $db->next_key($key);
1124 if ($db->exists("foo")) { $db->delete("foo"); }
1128 For arrays, DBM::Deep supports all the common methods described above, and the
1129 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1130 C<unshift()> and C<splice()>.
1136 Returns the number of elements in the array. Takes no arguments.
1138 my $len = $db->length();
1142 Adds one or more elements onto the end of the array. Accepts scalars, hash
1143 refs or array refs. No return value.
1145 $db->push("foo", "bar", {});
1149 Fetches the last element in the array, and deletes it. Takes no arguments.
1150 Returns undef if array is empty. Returns the element value.
1152 my $elem = $db->pop();
1156 Fetches the first element in the array, deletes it, then shifts all the
1157 remaining elements over to take up the space. Returns the element value. This
1158 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1161 my $elem = $db->shift();
1165 Inserts one or more elements onto the beginning of the array, shifting all
1166 existing elements over to make room. Accepts scalars, hash refs or array refs.
1167 No return value. This method is not recommended with large arrays -- see
1168 <LARGE ARRAYS> below for details.
1170 $db->unshift("foo", "bar", {});
1174 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1175 -f splice> for usage -- it is too complicated to document here. This method is
1176 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1180 Here are some examples of using arrays:
1182 my $db = DBM::Deep->new(
1184 type => DBM::Deep->TYPE_ARRAY
1187 $db->push("bar", "baz");
1188 $db->unshift("foo");
1191 my $len = $db->length();
1192 print "length: $len\n"; # 4
1194 for (my $k=0; $k<$len; $k++) {
1195 print "$k: " . $db->get($k) . "\n";
1198 $db->splice(1, 2, "biz", "baf");
1200 while (my $elem = shift @$db) {
1201 print "shifted: $elem\n";
1206 Enable automatic file locking by passing a true value to the C<locking>
1207 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1209 my $db = DBM::Deep->new(
1214 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1215 mode for writes, and shared mode for reads. This is required if you have
1216 multiple processes accessing the same database file, to avoid file corruption.
1217 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1218 NFS> below for more.
1220 =head2 EXPLICIT LOCKING
1222 You can explicitly lock a database, so it remains locked for multiple
1223 transactions. This is done by calling the C<lock()> method, and passing an
1224 optional lock mode argument (defaults to exclusive mode). This is particularly
1225 useful for things like counters, where the current value needs to be fetched,
1226 then incremented, then stored again.
1229 my $counter = $db->get("counter");
1231 $db->put("counter", $counter);
1240 You can pass C<lock()> an optional argument, which specifies which mode to use
1241 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1242 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1243 same as the constants defined in Perl's C<Fcntl> module.
1245 $db->lock( DBM::Deep->LOCK_SH );
1249 =head1 IMPORTING/EXPORTING
1251 You can import existing complex structures by calling the C<import()> method,
1252 and export an entire database into an in-memory structure using the C<export()>
1253 method. Both are examined here.
1257 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1258 walking the structure and adding keys/elements to the database as you go,
1259 simply pass a reference to the C<import()> method. This recursively adds
1260 everything to an existing DBM::Deep object for you. Here is an example:
1265 array1 => [ "elem0", "elem1", "elem2" ],
1267 subkey1 => "subvalue1",
1268 subkey2 => "subvalue2"
1272 my $db = DBM::Deep->new( "foo.db" );
1273 $db->import( $struct );
1275 print $db->{key1} . "\n"; # prints "value1"
1277 This recursively imports the entire C<$struct> object into C<$db>, including
1278 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1279 keys are merged with the existing ones, replacing if they already exist.
1280 The C<import()> method can be called on any database level (not just the base
1281 level), and works with both hash and array DB types.
1283 B<Note:> Make sure your existing structure has no circular references in it.
1284 These will cause an infinite loop when importing.
1288 Calling the C<export()> method on an existing DBM::Deep object will return
1289 a reference to a new in-memory copy of the database. The export is done
1290 recursively, so all nested hashes/arrays are all exported to standard Perl
1291 objects. Here is an example:
1293 my $db = DBM::Deep->new( "foo.db" );
1295 $db->{key1} = "value1";
1296 $db->{key2} = "value2";
1298 $db->{hash1}->{subkey1} = "subvalue1";
1299 $db->{hash1}->{subkey2} = "subvalue2";
1301 my $struct = $db->export();
1303 print $struct->{key1} . "\n"; # prints "value1"
1305 This makes a complete copy of the database in memory, and returns a reference
1306 to it. The C<export()> method can be called on any database level (not just
1307 the base level), and works with both hash and array DB types. Be careful of
1308 large databases -- you can store a lot more data in a DBM::Deep object than an
1309 in-memory Perl structure.
1311 B<Note:> Make sure your database has no circular references in it.
1312 These will cause an infinite loop when exporting.
1316 DBM::Deep has a number of hooks where you can specify your own Perl function
1317 to perform filtering on incoming or outgoing data. This is a perfect
1318 way to extend the engine, and implement things like real-time compression or
1319 encryption. Filtering applies to the base DB level, and all child hashes /
1320 arrays. Filter hooks can be specified when your DBM::Deep object is first
1321 constructed, or by calling the C<set_filter()> method at any time. There are
1322 four available filter hooks, described below:
1326 =item * filter_store_key
1328 This filter is called whenever a hash key is stored. It
1329 is passed the incoming key, and expected to return a transformed key.
1331 =item * filter_store_value
1333 This filter is called whenever a hash key or array element is stored. It
1334 is passed the incoming value, and expected to return a transformed value.
1336 =item * filter_fetch_key
1338 This filter is called whenever a hash key is fetched (i.e. via
1339 C<first_key()> or C<next_key()>). It is passed the transformed key,
1340 and expected to return the plain key.
1342 =item * filter_fetch_value
1344 This filter is called whenever a hash key or array element is fetched.
1345 It is passed the transformed value, and expected to return the plain value.
1349 Here are the two ways to setup a filter hook:
1351 my $db = DBM::Deep->new(
1353 filter_store_value => \&my_filter_store,
1354 filter_fetch_value => \&my_filter_fetch
1359 $db->set_filter( "filter_store_value", \&my_filter_store );
1360 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1362 Your filter function will be called only when dealing with SCALAR keys or
1363 values. When nested hashes and arrays are being stored/fetched, filtering
1364 is bypassed. Filters are called as static functions, passed a single SCALAR
1365 argument, and expected to return a single SCALAR value. If you want to
1366 remove a filter, set the function reference to C<undef>:
1368 $db->set_filter( "filter_store_value", undef );
1370 =head2 REAL-TIME ENCRYPTION EXAMPLE
1372 Here is a working example that uses the I<Crypt::Blowfish> module to
1373 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1374 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1375 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1378 use Crypt::Blowfish;
1381 my $cipher = Crypt::CBC->new({
1382 'key' => 'my secret key',
1383 'cipher' => 'Blowfish',
1385 'regenerate_key' => 0,
1386 'padding' => 'space',
1390 my $db = DBM::Deep->new(
1391 file => "foo-encrypt.db",
1392 filter_store_key => \&my_encrypt,
1393 filter_store_value => \&my_encrypt,
1394 filter_fetch_key => \&my_decrypt,
1395 filter_fetch_value => \&my_decrypt,
1398 $db->{key1} = "value1";
1399 $db->{key2} = "value2";
1400 print "key1: " . $db->{key1} . "\n";
1401 print "key2: " . $db->{key2} . "\n";
1407 return $cipher->encrypt( $_[0] );
1410 return $cipher->decrypt( $_[0] );
1413 =head2 REAL-TIME COMPRESSION EXAMPLE
1415 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1416 compression / decompression of keys & values with DBM::Deep Filters.
1417 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1418 more on I<Compress::Zlib>.
1423 my $db = DBM::Deep->new(
1424 file => "foo-compress.db",
1425 filter_store_key => \&my_compress,
1426 filter_store_value => \&my_compress,
1427 filter_fetch_key => \&my_decompress,
1428 filter_fetch_value => \&my_decompress,
1431 $db->{key1} = "value1";
1432 $db->{key2} = "value2";
1433 print "key1: " . $db->{key1} . "\n";
1434 print "key2: " . $db->{key2} . "\n";
1440 return Compress::Zlib::memGzip( $_[0] ) ;
1443 return Compress::Zlib::memGunzip( $_[0] ) ;
1446 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1447 actually numerical index numbers, and are not filtered.
1449 =head1 ERROR HANDLING
1451 Most DBM::Deep methods return a true value for success, and call die() on
1452 failure. You can wrap calls in an eval block to catch the die.
1454 my $db = DBM::Deep->new( "foo.db" ); # create hash
1455 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1457 print $@; # prints error message
1459 =head1 LARGEFILE SUPPORT
1461 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1462 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1463 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1464 by calling the static C<set_pack()> method before you do anything else.
1466 DBM::Deep::set_pack(8, 'Q');
1468 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1469 instead of 32-bit longs. After setting these values your DB files have a
1470 theoretical maximum size of 16 XB (exabytes).
1472 B<Note:> Changing these values will B<NOT> work for existing database files.
1473 Only change this for new files, and make sure it stays set consistently
1474 throughout the file's life. If you do set these values, you can no longer
1475 access 32-bit DB files. You can, however, call C<set_pack(4, 'N')> to change
1476 back to 32-bit mode.
1478 B<Note:> I have not personally tested files > 2 GB -- all my systems have
1479 only a 32-bit Perl. However, I have received user reports that this does
1482 =head1 LOW-LEVEL ACCESS
1484 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1485 you can call the C<_fh()> method, which returns the handle:
1487 my $fh = $db->_fh();
1489 This method can be called on the root level of the datbase, or any child
1490 hashes or arrays. All levels share a I<root> structure, which contains things
1491 like the filehandle, a reference counter, and all the options specified
1492 when you created the object. You can get access to this root structure by
1493 calling the C<root()> method.
1495 my $root = $db->_root();
1497 This is useful for changing options after the object has already been created,
1498 such as enabling/disabling locking, or debug modes. You can also
1499 store your own temporary user data in this structure (be wary of name
1500 collision), which is then accessible from any child hash or array.
1502 =head1 CUSTOM DIGEST ALGORITHM
1504 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1505 keys. However you can override this, and use another algorithm (such as SHA-256)
1506 or even write your own. But please note that DBM::Deep currently expects zero
1507 collisions, so your algorithm has to be I<perfect>, so to speak.
1508 Collision detection may be introduced in a later version.
1512 You can specify a custom digest algorithm by calling the static C<set_digest()>
1513 function, passing a reference to a subroutine, and the length of the algorithm's
1514 hashes (in bytes). This is a global static function, which affects ALL DBM::Deep
1515 objects. Here is a working example that uses a 256-bit hash from the
1516 I<Digest::SHA256> module. Please see
1517 L<http://search.cpan.org/search?module=Digest::SHA256> for more.
1522 my $context = Digest::SHA256::new(256);
1524 DBM::Deep::set_digest( \&my_digest, 32 );
1526 my $db = DBM::Deep->new( "foo-sha.db" );
1528 $db->{key1} = "value1";
1529 $db->{key2} = "value2";
1530 print "key1: " . $db->{key1} . "\n";
1531 print "key2: " . $db->{key2} . "\n";
1537 return substr( $context->hash($_[0]), 0, 32 );
1540 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1541 of bytes you specify in the C<set_digest()> function (in this case 32).
1543 =head1 CIRCULAR REFERENCES
1545 DBM::Deep has B<experimental> support for circular references. Meaning you
1546 can have a nested hash key or array element that points to a parent object.
1547 This relationship is stored in the DB file, and is preserved between sessions.
1550 my $db = DBM::Deep->new( "foo.db" );
1553 $db->{circle} = $db; # ref to self
1555 print $db->{foo} . "\n"; # prints "foo"
1556 print $db->{circle}->{foo} . "\n"; # prints "foo" again
1558 One catch is, passing the object to a function that recursively walks the
1559 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1560 C<export()> methods) will result in an infinite loop. The other catch is,
1561 if you fetch the I<key> of a circular reference (i.e. using the C<first_key()>
1562 or C<next_key()> methods), you will get the I<target object's key>, not the
1563 ref's key. This gets even more interesting with the above example, where
1564 the I<circle> key points to the base DB object, which technically doesn't
1565 have a key. So I made DBM::Deep return "[base]" as the key name in that
1568 =head1 CAVEATS / ISSUES / BUGS
1570 This section describes all the known issues with DBM::Deep. It you have found
1571 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1573 =head2 UNUSED SPACE RECOVERY
1575 One major caveat with DBM::Deep is that space occupied by existing keys and
1576 values is not recovered when they are deleted. Meaning if you keep deleting
1577 and adding new keys, your file will continuously grow. I am working on this,
1578 but in the meantime you can call the built-in C<optimize()> method from time to
1579 time (perhaps in a crontab or something) to recover all your unused space.
1581 $db->optimize(); # returns true on success
1583 This rebuilds the ENTIRE database into a new file, then moves it on top of
1584 the original. The new file will have no unused space, thus it will take up as
1585 little disk space as possible. Please note that this operation can take
1586 a long time for large files, and you need enough disk space to temporarily hold
1587 2 copies of your DB file. The temporary file is created in the same directory
1588 as the original, named with a ".tmp" extension, and is deleted when the
1589 operation completes. Oh, and if locking is enabled, the DB is automatically
1590 locked for the entire duration of the copy.
1592 B<WARNING:> Only call optimize() on the top-level node of the database, and
1593 make sure there are no child references lying around. DBM::Deep keeps a reference
1594 counter, and if it is greater than 1, optimize() will abort and return undef.
1596 =head2 AUTOVIVIFICATION
1598 Unfortunately, autovivification doesn't work with tied hashes. This appears to
1599 be a bug in Perl's tie() system, as I<Jakob Schmidt> encountered the very same
1600 issue with his I<DWH_FIle> module (see L<http://search.cpan.org/search?module=DWH_File>),
1601 and it is also mentioned in the BUGS section for the I<MLDBM> module <see
1602 L<http://search.cpan.org/search?module=MLDBM>). Basically, on a new db file,
1605 $db->{foo}->{bar} = "hello";
1607 Since "foo" doesn't exist, you cannot add "bar" to it. You end up with "foo"
1608 being an empty hash. Try this instead, which works fine:
1610 $db->{foo} = { bar => "hello" };
1612 As of Perl 5.8.7, this bug still exists. I have walked very carefully through
1613 the execution path, and Perl indeed passes an empty hash to the STORE() method.
1614 Probably a bug in Perl.
1616 =head2 FILE CORRUPTION
1618 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1619 for a 32-bit signature when opened, but other corruption in files can cause
1620 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1621 stuck in an infinite loop depending on the level of corruption. File write
1622 operations are not checked for failure (for speed), so if you happen to run
1623 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1624 be addressed in a later version of DBM::Deep.
1628 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1629 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1630 about setting up your NFS server with a locking daemon, then using lockf() to
1631 lock your files, but your mileage may vary there as well. From what I
1632 understand, there is no real way to do it. However, if you need access to the
1633 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1634 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1636 =head2 COPYING OBJECTS
1638 Beware of copying tied objects in Perl. Very strange things can happen.
1639 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1640 returns a new, blessed, tied hash or array to the same level in the DB.
1642 my $copy = $db->clone();
1644 B<Note>: Since clone() here is cloning the object, not the database location, any
1645 modifications to either $db or $copy will be visible in both.
1649 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1650 These functions cause every element in the array to move, which can be murder
1651 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1652 a different location. This will be addressed in the forthcoming version 1.00.
1654 =head2 WRITEONLY FILES
1656 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1657 writeonly mode. STORE will verify that the filehandle is writable. However, there
1658 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1659 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1663 This section discusses DBM::Deep's speed and memory usage.
1667 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1668 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1669 multi-level hash/array support, and cross-platform FTPable files. Even so,
1670 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1671 with huge databases. Here is some test data:
1673 Adding 1,000,000 keys to new DB file...
1675 At 100 keys, avg. speed is 2,703 keys/sec
1676 At 200 keys, avg. speed is 2,642 keys/sec
1677 At 300 keys, avg. speed is 2,598 keys/sec
1678 At 400 keys, avg. speed is 2,578 keys/sec
1679 At 500 keys, avg. speed is 2,722 keys/sec
1680 At 600 keys, avg. speed is 2,628 keys/sec
1681 At 700 keys, avg. speed is 2,700 keys/sec
1682 At 800 keys, avg. speed is 2,607 keys/sec
1683 At 900 keys, avg. speed is 2,190 keys/sec
1684 At 1,000 keys, avg. speed is 2,570 keys/sec
1685 At 2,000 keys, avg. speed is 2,417 keys/sec
1686 At 3,000 keys, avg. speed is 1,982 keys/sec
1687 At 4,000 keys, avg. speed is 1,568 keys/sec
1688 At 5,000 keys, avg. speed is 1,533 keys/sec
1689 At 6,000 keys, avg. speed is 1,787 keys/sec
1690 At 7,000 keys, avg. speed is 1,977 keys/sec
1691 At 8,000 keys, avg. speed is 2,028 keys/sec
1692 At 9,000 keys, avg. speed is 2,077 keys/sec
1693 At 10,000 keys, avg. speed is 2,031 keys/sec
1694 At 20,000 keys, avg. speed is 1,970 keys/sec
1695 At 30,000 keys, avg. speed is 2,050 keys/sec
1696 At 40,000 keys, avg. speed is 2,073 keys/sec
1697 At 50,000 keys, avg. speed is 1,973 keys/sec
1698 At 60,000 keys, avg. speed is 1,914 keys/sec
1699 At 70,000 keys, avg. speed is 2,091 keys/sec
1700 At 80,000 keys, avg. speed is 2,103 keys/sec
1701 At 90,000 keys, avg. speed is 1,886 keys/sec
1702 At 100,000 keys, avg. speed is 1,970 keys/sec
1703 At 200,000 keys, avg. speed is 2,053 keys/sec
1704 At 300,000 keys, avg. speed is 1,697 keys/sec
1705 At 400,000 keys, avg. speed is 1,838 keys/sec
1706 At 500,000 keys, avg. speed is 1,941 keys/sec
1707 At 600,000 keys, avg. speed is 1,930 keys/sec
1708 At 700,000 keys, avg. speed is 1,735 keys/sec
1709 At 800,000 keys, avg. speed is 1,795 keys/sec
1710 At 900,000 keys, avg. speed is 1,221 keys/sec
1711 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1713 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1714 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1715 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1716 Run time was 12 min 3 sec.
1720 One of the great things about DBM::Deep is that it uses very little memory.
1721 Even with huge databases (1,000,000+ keys) you will not see much increased
1722 memory on your process. DBM::Deep relies solely on the filesystem for storing
1723 and fetching data. Here is output from I</usr/bin/top> before even opening a
1726 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1727 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1729 Basically the process is taking 2,716K of memory. And here is the same
1730 process after storing and fetching 1,000,000 keys:
1732 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1733 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1735 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1736 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1738 =head1 DB FILE FORMAT
1740 In case you were interested in the underlying DB file format, it is documented
1741 here in this section. You don't need to know this to use the module, it's just
1742 included for reference.
1746 DBM::Deep files always start with a 32-bit signature to identify the file type.
1747 This is at offset 0. The signature is "DPDB" in network byte order. This is
1748 checked for when the file is opened and an error will be thrown if it's not found.
1752 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1753 has a standard header containing the type of data, the length of data, and then
1754 the data itself. The type is a single character (1 byte), the length is a
1755 32-bit unsigned long in network byte order, and the data is, well, the data.
1756 Here is how it unfolds:
1760 Immediately after the 32-bit file signature is the I<Master Index> record.
1761 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1762 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1763 depending on how the DBM::Deep object was constructed.
1765 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1766 number). The first 8-bit char of the MD5 signature is the offset into the
1767 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1768 index element is a file offset of the next tag for the key/element in question,
1769 which is usually a I<Bucket List> tag (see below).
1771 The next tag I<could> be another index, depending on how many keys/elements
1772 exist. See L<RE-INDEXING> below for details.
1776 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1777 file offsets to where the actual data is stored. It starts with a standard
1778 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1779 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1780 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1781 When the list fills up, a I<Re-Index> operation is performed (See
1782 L<RE-INDEXING> below).
1786 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1787 index/value pair (in array mode). It starts with a standard tag header with
1788 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1789 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1790 header. The size reported in the tag header is only for the value, but then,
1791 just after the value is another size (32-bit unsigned long) and then the plain
1792 key itself. Since the value is likely to be fetched more often than the plain
1793 key, I figured it would be I<slightly> faster to store the value first.
1795 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1796 record for the nested structure, where the process begins all over again.
1800 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1801 exhausted. Then, when another key/element comes in, the list is converted to a
1802 new index record. However, this index will look at the next char in the MD5
1803 hash, and arrange new Bucket List pointers accordingly. This process is called
1804 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1805 17 (16 + new one) keys/elements are removed from the old Bucket List and
1806 inserted into the new index. Several new Bucket Lists are created in the
1807 process, as a new MD5 char from the key is being examined (it is unlikely that
1808 the keys will all share the same next char of their MD5s).
1810 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1811 when the Bucket Lists will turn into indexes, but the first round tends to
1812 happen right around 4,000 keys. You will see a I<slight> decrease in
1813 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1814 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1815 right around 900,000 keys. This process can continue nearly indefinitely --
1816 right up until the point the I<MD5> signatures start colliding with each other,
1817 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1818 getting struck by lightning while you are walking to cash in your tickets.
1819 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1820 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1821 this is 340 unodecillion, but don't quote me).
1825 When a new key/element is stored, the key (or index number) is first run through
1826 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1827 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1828 for the first char of the signature (in this case I<b0>). If it does not exist,
1829 a new I<Bucket List> is created for our key (and the next 15 future keys that
1830 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1831 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1832 this point, unless we are replacing an existing I<Bucket>), where the actual
1833 data will be stored.
1837 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1838 (or index number), then walking along the indexes. If there are enough
1839 keys/elements in this DB level, there might be nested indexes, each linked to
1840 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1841 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1842 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1843 plain key are stored.
1845 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1846 methods. In this process the indexes are walked systematically, and each key
1847 fetched in increasing MD5 order (which is why it appears random). Once the
1848 I<Bucket> is found, the value is skipped and the plain key returned instead.
1849 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1850 alphabetically sorted. This only happens on an index-level -- as soon as the
1851 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1852 so it's pretty much undefined how the keys will come out -- just like Perl's
1855 =head1 CODE COVERAGE
1857 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1858 B<Devel::Cover> report on this module's test suite.
1860 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1861 File stmt bran cond sub pod time total
1862 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1863 blib/lib/DBM/Deep.pm 95.2 83.8 70.0 98.2 100.0 58.0 91.0
1864 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 n/a 26.7 98.0
1865 blib/lib/DBM/Deep/Hash.pm 95.3 80.0 100.0 100.0 n/a 15.3 92.4
1866 Total 96.2 84.8 74.4 98.8 100.0 100.0 92.4
1867 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1869 =head1 MORE INFORMATION
1871 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1872 or send email to L<DBM-Deep@googlegroups.com>.
1876 Joseph Huckaby, L<jhuckaby@cpan.org>
1878 Rob Kinyon, L<rkinyon@cpan.org>
1880 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1884 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1885 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1889 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1890 This is free software, you may use it and distribute it under the
1891 same terms as Perl itself.