7 # Multi-level database module for storing hash trees, arrays and simple
8 # key/value pairs into FTP-able, cross-platform binary database files.
10 # Type `perldoc DBM::Deep` for complete documentation.
14 # tie %db, 'DBM::Deep', 'my_database.db'; # standard tie() method
16 # my $db = new DBM::Deep( 'my_database.db' ); # preferred OO method
18 # $db->{my_scalar} = 'hello world';
19 # $db->{my_hash} = { larry => 'genius', hashes => 'fast' };
20 # $db->{my_array} = [ 1, 2, 3, time() ];
21 # $db->{my_complex} = [ 'hello', { perl => 'rules' }, 42, 99 ];
22 # push @{$db->{my_array}}, 'another value';
23 # my @key_list = keys %{$db->{my_hash}};
24 # print "This module " . $db->{my_complex}->[1]->{perl} . "!\n";
27 # (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
28 # This program is free software; you can redistribute it and/or
29 # modify it under the same terms as Perl itself.
34 use Fcntl qw( :DEFAULT :flock :seek );
38 use DBM::Deep::Engine;
40 use vars qw( $VERSION );
41 $VERSION = q(0.99_01);
45 # Setup file and tag signatures. These should never change.
47 sub SIG_FILE () { 'DPDB' }
48 sub SIG_HASH () { 'H' }
49 sub SIG_ARRAY () { 'A' }
50 sub SIG_SCALAR () { 'S' }
51 sub SIG_NULL () { 'N' }
52 sub SIG_DATA () { 'D' }
53 sub SIG_INDEX () { 'I' }
54 sub SIG_BLIST () { 'B' }
58 # Setup constants for users to pass to new()
60 sub TYPE_HASH () { SIG_HASH }
61 sub TYPE_ARRAY () { SIG_ARRAY }
62 sub TYPE_SCALAR () { SIG_SCALAR }
70 $proto->_throw_error( "Odd number of parameters to " . (caller(1))[2] );
75 unless ( eval { local $SIG{'__DIE__'}; %{$_[0]} || 1 } ) {
76 $proto->_throw_error( "Not a hashref in args to " . (caller(1))[2] );
81 $args = { file => shift };
89 # Class constructor method for Perl OO interface.
90 # Calls tie() and returns blessed reference to tied hash or array,
91 # providing a hybrid OO/tie interface.
94 my $args = $class->_get_args( @_ );
97 # Check if we want a tied hash or array.
100 if (defined($args->{type}) && $args->{type} eq TYPE_ARRAY) {
101 $class = 'DBM::Deep::Array';
102 require DBM::Deep::Array;
103 tie @$self, $class, %$args;
106 $class = 'DBM::Deep::Hash';
107 require DBM::Deep::Hash;
108 tie %$self, $class, %$args;
111 return bless $self, $class;
116 # Setup $self and bless into this class.
121 # These are the defaults to be optionally overridden below
124 base_offset => length(SIG_FILE),
125 engine => DBM::Deep::Engine->new,
128 foreach my $param ( keys %$self ) {
129 next unless exists $args->{$param};
130 $self->{$param} = delete $args->{$param}
133 # locking implicitly enables autoflush
134 if ($args->{locking}) { $args->{autoflush} = 1; }
136 $self->{root} = exists $args->{root}
138 : DBM::Deep::_::Root->new( $args );
140 $self->{engine}->setup_fh( $self );
147 require DBM::Deep::Hash;
148 return DBM::Deep::Hash->TIEHASH( @_ );
153 require DBM::Deep::Array;
154 return DBM::Deep::Array->TIEARRAY( @_ );
157 #XXX Unneeded now ...
163 # If db locking is set, flock() the db file. If called multiple
164 # times before unlock(), then the same number of unlocks() must
165 # be called before the lock is released.
167 my $self = $_[0]->_get_self;
169 $type = LOCK_EX unless defined $type;
171 if (!defined($self->_fh)) { return; }
173 if ($self->_root->{locking}) {
174 if (!$self->_root->{locked}) {
175 flock($self->_fh, $type);
177 # refresh end counter in case file has changed size
178 my @stats = stat($self->_root->{file});
179 $self->_root->{end} = $stats[7];
181 # double-check file inode, in case another process
182 # has optimize()d our file while we were waiting.
183 if ($stats[1] != $self->_root->{inode}) {
184 $self->{engine}->close_fh( $self );
185 $self->{engine}->setup_fh( $self );
186 flock($self->_fh, $type); # re-lock
188 # This may not be necessary after re-opening
189 $self->_root->{end} = (stat($self->_fh))[7]; # re-end
192 $self->_root->{locked}++;
202 # If db locking is set, unlock the db file. See note in lock()
203 # regarding calling lock() multiple times.
205 my $self = $_[0]->_get_self;
207 if (!defined($self->_fh)) { return; }
209 if ($self->_root->{locking} && $self->_root->{locked} > 0) {
210 $self->_root->{locked}--;
211 if (!$self->_root->{locked}) { flock($self->_fh, LOCK_UN); }
220 my $self = shift->_get_self;
221 my ($spot, $value) = @_;
226 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
227 my $type = $value->_type;
228 ${$spot} = $type eq TYPE_HASH ? {} : [];
229 $value->_copy_node( ${$spot} );
232 my $r = Scalar::Util::reftype( $value );
233 my $c = Scalar::Util::blessed( $value );
234 if ( $r eq 'ARRAY' ) {
235 ${$spot} = [ @{$value} ];
238 ${$spot} = { %{$value} };
240 ${$spot} = bless ${$spot}, $c
249 # Copy single level of keys or elements to new DB handle.
250 # Recurse for nested structures
252 my $self = shift->_get_self;
255 if ($self->_type eq TYPE_HASH) {
256 my $key = $self->first_key();
258 my $value = $self->get($key);
259 $self->_copy_value( \$db_temp->{$key}, $value );
260 $key = $self->next_key($key);
264 my $length = $self->length();
265 for (my $index = 0; $index < $length; $index++) {
266 my $value = $self->get($index);
267 $self->_copy_value( \$db_temp->[$index], $value );
276 # Recursively export into standard Perl hashes and arrays.
278 my $self = $_[0]->_get_self;
281 if ($self->_type eq TYPE_HASH) { $temp = {}; }
282 elsif ($self->_type eq TYPE_ARRAY) { $temp = []; }
285 $self->_copy_node( $temp );
293 # Recursively import Perl hash/array structure
295 #XXX This use of ref() seems to be ok
296 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
298 my $self = $_[0]->_get_self;
301 #XXX This use of ref() seems to be ok
304 # struct is not a reference, so just import based on our type
308 if ($self->_type eq TYPE_HASH) { $struct = {@_}; }
309 elsif ($self->_type eq TYPE_ARRAY) { $struct = [@_]; }
312 my $r = Scalar::Util::reftype($struct) || '';
313 if ($r eq "HASH" && $self->_type eq TYPE_HASH) {
314 foreach my $key (keys %$struct) { $self->put($key, $struct->{$key}); }
316 elsif ($r eq "ARRAY" && $self->_type eq TYPE_ARRAY) {
317 $self->push( @$struct );
320 $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 # $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 $self->_throw_error("Cannot optimize: failed to open temp file: $!");
347 $self->_copy_node( $db_temp );
351 # Attempt to copy user, group and permissions over to new file
353 my @stats = stat($self->_fh);
354 my $perms = $stats[2] & 07777;
357 chown( $uid, $gid, $self->_root->{file} . '.tmp' );
358 chmod( $perms, $self->_root->{file} . '.tmp' );
360 # q.v. perlport for more information on this variable
361 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
363 # Potential race condition when optmizing on Win32 with locking.
364 # The Windows filesystem requires that the filehandle be closed
365 # before it is overwritten with rename(). This could be redone
369 $self->{engine}->close_fh( $self );
372 if (!rename $self->_root->{file} . '.tmp', $self->_root->{file}) {
373 unlink $self->_root->{file} . '.tmp';
375 $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
379 $self->{engine}->close_fh( $self );
380 $self->{engine}->setup_fh( $self );
387 # Make copy of object and return
389 my $self = $_[0]->_get_self;
391 return DBM::Deep->new(
392 type => $self->_type,
393 base_offset => $self->_base_offset,
399 my %is_legal_filter = map {
402 store_key store_value
403 fetch_key fetch_value
408 # Setup filter function for storing or fetching the key or value
410 my $self = $_[0]->_get_self;
412 my $func = $_[2] ? $_[2] : undef;
414 if ( $is_legal_filter{$type} ) {
415 $self->_root->{"filter_$type"} = $func;
429 # Get access to the root structure
431 my $self = $_[0]->_get_self;
432 return $self->{root};
437 # Get access to the raw fh
439 my $self = $_[0]->_get_self;
440 return $self->_root->{fh};
445 # Get type of current node (TYPE_HASH or TYPE_ARRAY)
447 my $self = $_[0]->_get_self;
448 return $self->{type};
453 # Get base_offset of current node (TYPE_HASH or TYPE_ARRAY)
455 my $self = $_[0]->_get_self;
456 return $self->{base_offset};
464 die "DBM::Deep: $_[1]\n";
469 (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
474 # (O_RDONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush = 0);
479 # Store single hash key/value or array element in database.
481 my $self = shift->_get_self;
482 my ($key, $value) = @_;
484 unless ( _is_writable( $self->_fh ) ) {
485 $self->_throw_error( 'Cannot write to a readonly filehandle' );
489 # Request exclusive lock for writing
491 $self->lock( LOCK_EX );
493 my $md5 = $self->{engine}{digest}->($key);
495 my $tag = $self->{engine}->find_bucket_list( $self, $md5, { create => 1 } );
497 # User may be storing a hash, in which case we do not want it run
498 # through the filtering system
499 if ( !ref($value) && $self->_root->{filter_store_value} ) {
500 $value = $self->_root->{filter_store_value}->( $value );
504 # Add key/value to bucket list
506 my $result = $self->{engine}->add_bucket( $self, $tag, $md5, $key, $value );
515 # Fetch single value or element given plain key or array index
517 my $self = shift->_get_self;
520 my $md5 = $self->{engine}{digest}->($key);
523 # Request shared lock for reading
525 $self->lock( LOCK_SH );
527 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
534 # Get value from bucket list
536 my $result = $self->{engine}->get_bucket_value( $self, $tag, $md5 );
540 # Filters only apply to scalar values, so the ref check is making
541 # sure the fetched bucket is a scalar, not a child hash or array.
542 return ($result && !ref($result) && $self->_root->{filter_fetch_value})
543 ? $self->_root->{filter_fetch_value}->($result)
549 # Delete single key/value pair or element given plain key or array index
551 my $self = $_[0]->_get_self;
554 unless ( _is_writable( $self->_fh ) ) {
555 $self->_throw_error( 'Cannot write to a readonly filehandle' );
559 # Request exclusive lock for writing
561 $self->lock( LOCK_EX );
563 my $md5 = $self->{engine}{digest}->($key);
565 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
574 my $value = $self->{engine}->get_bucket_value($self, $tag, $md5 );
576 if (defined $value && !ref($value) && $self->_root->{filter_fetch_value}) {
577 $value = $self->_root->{filter_fetch_value}->($value);
580 my $result = $self->{engine}->delete_bucket( $self, $tag, $md5 );
583 # If this object is an array and the key deleted was on the end of the stack,
584 # decrement the length variable.
594 # Check if a single key or element exists given plain key or array index
596 my $self = $_[0]->_get_self;
599 my $md5 = $self->{engine}{digest}->($key);
602 # Request shared lock for reading
604 $self->lock( LOCK_SH );
606 my $tag = $self->{engine}->find_bucket_list( $self, $md5 );
611 # For some reason, the built-in exists() function returns '' for false
617 # Check if bucket exists and return 1 or ''
619 my $result = $self->{engine}->bucket_exists( $self, $tag, $md5 ) || '';
628 # Clear all keys from hash, or all elements from array.
630 my $self = $_[0]->_get_self;
632 unless ( _is_writable( $self->_fh ) ) {
633 $self->_throw_error( 'Cannot write to a readonly filehandle' );
637 # Request exclusive lock for writing
639 $self->lock( LOCK_EX );
643 seek($fh, $self->_base_offset + $self->_root->{file_offset}, SEEK_SET);
649 $self->{engine}->create_tag($self, $self->_base_offset, $self->_type, chr(0) x $self->{engine}{index_size});
657 # Public method aliases
659 sub put { (shift)->STORE( @_ ) }
660 sub store { (shift)->STORE( @_ ) }
661 sub get { (shift)->FETCH( @_ ) }
662 sub fetch { (shift)->FETCH( @_ ) }
663 sub delete { (shift)->DELETE( @_ ) }
664 sub exists { (shift)->EXISTS( @_ ) }
665 sub clear { (shift)->CLEAR( @_ ) }
667 package DBM::Deep::_::Root;
676 #XXX It should be this in order to work with the initial create_tag(),
677 #XXX but it's not ... it works out because of the stat() in setup_fh(),
678 #XXX but that's not good.
679 end => 0, #length(DBM::Deep->SIG_FILE),
685 filter_store_key => undef,
686 filter_store_value => undef,
687 filter_fetch_key => undef,
688 filter_fetch_value => undef,
692 if ( $self->{fh} && !$self->{file_offset} ) {
693 $self->{file_offset} = tell( $self->{fh} );
703 close $self->{fh} if $self->{fh};
714 DBM::Deep - A pure perl multi-level hash/array DBM
719 my $db = DBM::Deep->new( "foo.db" );
721 $db->{key} = 'value'; # tie() style
724 $db->put('key' => 'value'); # OO style
725 print $db->get('key');
727 # true multi-level support
728 $db->{my_complex} = [
729 'hello', { perl => 'rules' },
735 A unique flat-file database module, written in pure perl. True
736 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
737 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
738 handle millions of keys and unlimited hash levels without significant
739 slow-down. Written from the ground-up in pure perl -- this is NOT a
740 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
741 Mac OS X and Windows.
745 Hopefully you are using Perl's excellent CPAN module, which will download
746 and install the module for you. If not, get the tarball, and run these
758 Construction can be done OO-style (which is the recommended way), or using
759 Perl's tie() function. Both are examined here.
761 =head2 OO CONSTRUCTION
763 The recommended way to construct a DBM::Deep object is to use the new()
764 method, which gets you a blessed, tied hash or array reference.
766 my $db = DBM::Deep->new( "foo.db" );
768 This opens a new database handle, mapped to the file "foo.db". If this
769 file does not exist, it will automatically be created. DB files are
770 opened in "r+" (read/write) mode, and the type of object returned is a
771 hash, unless otherwise specified (see L<OPTIONS> below).
773 You can pass a number of options to the constructor to specify things like
774 locking, autoflush, etc. This is done by passing an inline hash:
776 my $db = DBM::Deep->new(
782 Notice that the filename is now specified I<inside> the hash with
783 the "file" parameter, as opposed to being the sole argument to the
784 constructor. This is required if any options are specified.
785 See L<OPTIONS> below for the complete list.
789 You can also start with an array instead of a hash. For this, you must
790 specify the C<type> parameter:
792 my $db = DBM::Deep->new(
794 type => DBM::Deep->TYPE_ARRAY
797 B<Note:> Specifing the C<type> parameter only takes effect when beginning
798 a new DB file. If you create a DBM::Deep object with an existing file, the
799 C<type> will be loaded from the file header, and an error will be thrown if
800 the wrong type is passed in.
802 =head2 TIE CONSTRUCTION
804 Alternately, you can create a DBM::Deep handle by using Perl's built-in
805 tie() function. The object returned from tie() can be used to call methods,
806 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
807 file (as expected with most tie'd objects).
810 my $db = tie %hash, "DBM::Deep", "foo.db";
813 my $db = tie @array, "DBM::Deep", "bar.db";
815 As with the OO constructor, you can replace the DB filename parameter with
816 a hash containing one or more options (see L<OPTIONS> just below for the
819 tie %hash, "DBM::Deep", {
827 There are a number of options that can be passed in when constructing your
828 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
834 Filename of the DB file to link the handle to. You can pass a full absolute
835 filesystem path, partial path, or a plain filename if the file is in the
836 current working directory. This is a required parameter (though q.v. fh).
840 If you want, you can pass in the fh instead of the file. This is most useful for doing
843 my $db = DBM::Deep->new( { fh => \*DATA } );
845 You are responsible for making sure that the fh has been opened appropriately for your
846 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
847 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
848 needs to read from the fh.
852 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
853 not need to set this. However, it's there if you want it.
855 If you pass in fh and do not set this, it will be set appropriately.
859 This parameter specifies what type of object to create, a hash or array. Use
860 one of these two constants: C<DBM::Deep-E<gt>TYPE_HASH> or C<DBM::Deep-E<gt>TYPE_ARRAY>.
861 This only takes effect when beginning a new file. This is an optional
862 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
866 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
867 function to lock the database in exclusive mode for writes, and shared mode for
868 reads. Pass any true value to enable. This affects the base DB handle I<and
869 any child hashes or arrays> that use the same DB file. This is an optional
870 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
874 Specifies whether autoflush is to be enabled on the underlying filehandle.
875 This obviously slows down write operations, but is required if you may have
876 multiple processes accessing the same DB file (also consider enable I<locking>).
877 Pass any true value to enable. This is an optional parameter, and defaults to 0
882 If I<autobless> mode is enabled, DBM::Deep will preserve blessed hashes, and
883 restore them when fetched. This is an B<experimental> feature, and does have
884 side-effects. Basically, when hashes are re-blessed into their original
885 classes, they are no longer blessed into the DBM::Deep class! So you won't be
886 able to call any DBM::Deep methods on them. You have been warned.
887 This is an optional parameter, and defaults to 0 (disabled).
891 See L<FILTERS> below.
897 With DBM::Deep you can access your databases using Perl's standard hash/array
898 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
899 treat them as such. DBM::Deep will intercept all reads/writes and direct them
900 to the right place -- the DB file. This has nothing to do with the
901 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
902 using regular hashes and arrays, rather than calling functions like C<get()>
903 and C<put()> (although those work too). It is entirely up to you how to want
904 to access your databases.
908 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
909 or even nested hashes (or arrays) using standard Perl syntax:
911 my $db = DBM::Deep->new( "foo.db" );
913 $db->{mykey} = "myvalue";
915 $db->{myhash}->{subkey} = "subvalue";
917 print $db->{myhash}->{subkey} . "\n";
919 You can even step through hash keys using the normal Perl C<keys()> function:
921 foreach my $key (keys %$db) {
922 print "$key: " . $db->{$key} . "\n";
925 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
926 pushes them onto an array, all before the loop even begins. If you have an
927 extra large hash, this may exhaust Perl's memory. Instead, consider using
928 Perl's C<each()> function, which pulls keys/values one at a time, using very
931 while (my ($key, $value) = each %$db) {
932 print "$key: $value\n";
935 Please note that when using C<each()>, you should always pass a direct
936 hash reference, not a lookup. Meaning, you should B<never> do this:
939 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
941 This causes an infinite loop, because for each iteration, Perl is calling
942 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
943 it effectively keeps returning the first key over and over again. Instead,
944 assign a temporary variable to C<$db->{foo}>, then pass that to each().
948 As with hashes, you can treat any DBM::Deep object like a normal Perl array
949 reference. This includes inserting, removing and manipulating elements,
950 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
951 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
952 or simply be a nested array reference inside a hash. Example:
954 my $db = DBM::Deep->new(
955 file => "foo-array.db",
956 type => DBM::Deep->TYPE_ARRAY
960 push @$db, "bar", "baz";
963 my $last_elem = pop @$db; # baz
964 my $first_elem = shift @$db; # bah
965 my $second_elem = $db->[1]; # bar
967 my $num_elements = scalar @$db;
971 In addition to the I<tie()> interface, you can also use a standard OO interface
972 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
973 array) has its own methods, but both types share the following common methods:
974 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
978 =item * new() / clone()
980 These are the constructor and copy-functions.
982 =item * put() / store()
984 Stores a new hash key/value pair, or sets an array element value. Takes two
985 arguments, the hash key or array index, and the new value. The value can be
986 a scalar, hash ref or array ref. Returns true on success, false on failure.
988 $db->put("foo", "bar"); # for hashes
989 $db->put(1, "bar"); # for arrays
991 =item * get() / fetch()
993 Fetches the value of a hash key or array element. Takes one argument: the hash
994 key or array index. Returns a scalar, hash ref or array ref, depending on the
997 my $value = $db->get("foo"); # for hashes
998 my $value = $db->get(1); # for arrays
1002 Checks if a hash key or array index exists. Takes one argument: the hash key
1003 or array index. Returns true if it exists, false if not.
1005 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
1006 if ($db->exists(1)) { print "yay!\n"; } # for arrays
1010 Deletes one hash key/value pair or array element. Takes one argument: the hash
1011 key or array index. Returns true on success, false if not found. For arrays,
1012 the remaining elements located after the deleted element are NOT moved over.
1013 The deleted element is essentially just undefined, which is exactly how Perl's
1014 internal arrays work. Please note that the space occupied by the deleted
1015 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
1016 below for details and workarounds.
1018 $db->delete("foo"); # for hashes
1019 $db->delete(1); # for arrays
1023 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1024 value. Please note that the space occupied by the deleted keys/values or
1025 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1026 details and workarounds.
1028 $db->clear(); # hashes or arrays
1030 =item * lock() / unlock()
1036 Recover lost disk space.
1038 =item * import() / export()
1040 Data going in and out.
1042 =item * set_digest() / set_pack() / set_filter()
1044 q.v. adjusting the interal parameters.
1050 For hashes, DBM::Deep supports all the common methods described above, and the
1051 following additional methods: C<first_key()> and C<next_key()>.
1057 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1058 fetched in an undefined order (which appears random). Takes no arguments,
1059 returns the key as a scalar value.
1061 my $key = $db->first_key();
1065 Returns the "next" key in the hash, given the previous one as the sole argument.
1066 Returns undef if there are no more keys to be fetched.
1068 $key = $db->next_key($key);
1072 Here are some examples of using hashes:
1074 my $db = DBM::Deep->new( "foo.db" );
1076 $db->put("foo", "bar");
1077 print "foo: " . $db->get("foo") . "\n";
1079 $db->put("baz", {}); # new child hash ref
1080 $db->get("baz")->put("buz", "biz");
1081 print "buz: " . $db->get("baz")->get("buz") . "\n";
1083 my $key = $db->first_key();
1085 print "$key: " . $db->get($key) . "\n";
1086 $key = $db->next_key($key);
1089 if ($db->exists("foo")) { $db->delete("foo"); }
1093 For arrays, DBM::Deep supports all the common methods described above, and the
1094 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1095 C<unshift()> and C<splice()>.
1101 Returns the number of elements in the array. Takes no arguments.
1103 my $len = $db->length();
1107 Adds one or more elements onto the end of the array. Accepts scalars, hash
1108 refs or array refs. No return value.
1110 $db->push("foo", "bar", {});
1114 Fetches the last element in the array, and deletes it. Takes no arguments.
1115 Returns undef if array is empty. Returns the element value.
1117 my $elem = $db->pop();
1121 Fetches the first element in the array, deletes it, then shifts all the
1122 remaining elements over to take up the space. Returns the element value. This
1123 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1126 my $elem = $db->shift();
1130 Inserts one or more elements onto the beginning of the array, shifting all
1131 existing elements over to make room. Accepts scalars, hash refs or array refs.
1132 No return value. This method is not recommended with large arrays -- see
1133 <LARGE ARRAYS> below for details.
1135 $db->unshift("foo", "bar", {});
1139 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1140 -f splice> for usage -- it is too complicated to document here. This method is
1141 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1145 Here are some examples of using arrays:
1147 my $db = DBM::Deep->new(
1149 type => DBM::Deep->TYPE_ARRAY
1152 $db->push("bar", "baz");
1153 $db->unshift("foo");
1156 my $len = $db->length();
1157 print "length: $len\n"; # 4
1159 for (my $k=0; $k<$len; $k++) {
1160 print "$k: " . $db->get($k) . "\n";
1163 $db->splice(1, 2, "biz", "baf");
1165 while (my $elem = shift @$db) {
1166 print "shifted: $elem\n";
1171 Enable automatic file locking by passing a true value to the C<locking>
1172 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1174 my $db = DBM::Deep->new(
1179 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1180 mode for writes, and shared mode for reads. This is required if you have
1181 multiple processes accessing the same database file, to avoid file corruption.
1182 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1183 NFS> below for more.
1185 =head2 EXPLICIT LOCKING
1187 You can explicitly lock a database, so it remains locked for multiple
1188 transactions. This is done by calling the C<lock()> method, and passing an
1189 optional lock mode argument (defaults to exclusive mode). This is particularly
1190 useful for things like counters, where the current value needs to be fetched,
1191 then incremented, then stored again.
1194 my $counter = $db->get("counter");
1196 $db->put("counter", $counter);
1205 You can pass C<lock()> an optional argument, which specifies which mode to use
1206 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1207 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1208 same as the constants defined in Perl's C<Fcntl> module.
1210 $db->lock( DBM::Deep->LOCK_SH );
1214 =head1 IMPORTING/EXPORTING
1216 You can import existing complex structures by calling the C<import()> method,
1217 and export an entire database into an in-memory structure using the C<export()>
1218 method. Both are examined here.
1222 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1223 walking the structure and adding keys/elements to the database as you go,
1224 simply pass a reference to the C<import()> method. This recursively adds
1225 everything to an existing DBM::Deep object for you. Here is an example:
1230 array1 => [ "elem0", "elem1", "elem2" ],
1232 subkey1 => "subvalue1",
1233 subkey2 => "subvalue2"
1237 my $db = DBM::Deep->new( "foo.db" );
1238 $db->import( $struct );
1240 print $db->{key1} . "\n"; # prints "value1"
1242 This recursively imports the entire C<$struct> object into C<$db>, including
1243 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1244 keys are merged with the existing ones, replacing if they already exist.
1245 The C<import()> method can be called on any database level (not just the base
1246 level), and works with both hash and array DB types.
1248 B<Note:> Make sure your existing structure has no circular references in it.
1249 These will cause an infinite loop when importing.
1253 Calling the C<export()> method on an existing DBM::Deep object will return
1254 a reference to a new in-memory copy of the database. The export is done
1255 recursively, so all nested hashes/arrays are all exported to standard Perl
1256 objects. Here is an example:
1258 my $db = DBM::Deep->new( "foo.db" );
1260 $db->{key1} = "value1";
1261 $db->{key2} = "value2";
1263 $db->{hash1}->{subkey1} = "subvalue1";
1264 $db->{hash1}->{subkey2} = "subvalue2";
1266 my $struct = $db->export();
1268 print $struct->{key1} . "\n"; # prints "value1"
1270 This makes a complete copy of the database in memory, and returns a reference
1271 to it. The C<export()> method can be called on any database level (not just
1272 the base level), and works with both hash and array DB types. Be careful of
1273 large databases -- you can store a lot more data in a DBM::Deep object than an
1274 in-memory Perl structure.
1276 B<Note:> Make sure your database has no circular references in it.
1277 These will cause an infinite loop when exporting.
1281 DBM::Deep has a number of hooks where you can specify your own Perl function
1282 to perform filtering on incoming or outgoing data. This is a perfect
1283 way to extend the engine, and implement things like real-time compression or
1284 encryption. Filtering applies to the base DB level, and all child hashes /
1285 arrays. Filter hooks can be specified when your DBM::Deep object is first
1286 constructed, or by calling the C<set_filter()> method at any time. There are
1287 four available filter hooks, described below:
1291 =item * filter_store_key
1293 This filter is called whenever a hash key is stored. It
1294 is passed the incoming key, and expected to return a transformed key.
1296 =item * filter_store_value
1298 This filter is called whenever a hash key or array element is stored. It
1299 is passed the incoming value, and expected to return a transformed value.
1301 =item * filter_fetch_key
1303 This filter is called whenever a hash key is fetched (i.e. via
1304 C<first_key()> or C<next_key()>). It is passed the transformed key,
1305 and expected to return the plain key.
1307 =item * filter_fetch_value
1309 This filter is called whenever a hash key or array element is fetched.
1310 It is passed the transformed value, and expected to return the plain value.
1314 Here are the two ways to setup a filter hook:
1316 my $db = DBM::Deep->new(
1318 filter_store_value => \&my_filter_store,
1319 filter_fetch_value => \&my_filter_fetch
1324 $db->set_filter( "filter_store_value", \&my_filter_store );
1325 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1327 Your filter function will be called only when dealing with SCALAR keys or
1328 values. When nested hashes and arrays are being stored/fetched, filtering
1329 is bypassed. Filters are called as static functions, passed a single SCALAR
1330 argument, and expected to return a single SCALAR value. If you want to
1331 remove a filter, set the function reference to C<undef>:
1333 $db->set_filter( "filter_store_value", undef );
1335 =head2 REAL-TIME ENCRYPTION EXAMPLE
1337 Here is a working example that uses the I<Crypt::Blowfish> module to
1338 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1339 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1340 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1343 use Crypt::Blowfish;
1346 my $cipher = Crypt::CBC->new({
1347 'key' => 'my secret key',
1348 'cipher' => 'Blowfish',
1350 'regenerate_key' => 0,
1351 'padding' => 'space',
1355 my $db = DBM::Deep->new(
1356 file => "foo-encrypt.db",
1357 filter_store_key => \&my_encrypt,
1358 filter_store_value => \&my_encrypt,
1359 filter_fetch_key => \&my_decrypt,
1360 filter_fetch_value => \&my_decrypt,
1363 $db->{key1} = "value1";
1364 $db->{key2} = "value2";
1365 print "key1: " . $db->{key1} . "\n";
1366 print "key2: " . $db->{key2} . "\n";
1372 return $cipher->encrypt( $_[0] );
1375 return $cipher->decrypt( $_[0] );
1378 =head2 REAL-TIME COMPRESSION EXAMPLE
1380 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1381 compression / decompression of keys & values with DBM::Deep Filters.
1382 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1383 more on I<Compress::Zlib>.
1388 my $db = DBM::Deep->new(
1389 file => "foo-compress.db",
1390 filter_store_key => \&my_compress,
1391 filter_store_value => \&my_compress,
1392 filter_fetch_key => \&my_decompress,
1393 filter_fetch_value => \&my_decompress,
1396 $db->{key1} = "value1";
1397 $db->{key2} = "value2";
1398 print "key1: " . $db->{key1} . "\n";
1399 print "key2: " . $db->{key2} . "\n";
1405 return Compress::Zlib::memGzip( $_[0] ) ;
1408 return Compress::Zlib::memGunzip( $_[0] ) ;
1411 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1412 actually numerical index numbers, and are not filtered.
1414 =head1 ERROR HANDLING
1416 Most DBM::Deep methods return a true value for success, and call die() on
1417 failure. You can wrap calls in an eval block to catch the die.
1419 my $db = DBM::Deep->new( "foo.db" ); # create hash
1420 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1422 print $@; # prints error message
1424 =head1 LARGEFILE SUPPORT
1426 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1427 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1428 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1429 by calling the static C<set_pack()> method before you do anything else.
1431 DBM::Deep::set_pack(8, 'Q');
1433 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1434 instead of 32-bit longs. After setting these values your DB files have a
1435 theoretical maximum size of 16 XB (exabytes).
1437 B<Note:> Changing these values will B<NOT> work for existing database files.
1438 Only change this for new files, and make sure it stays set consistently
1439 throughout the file's life. If you do set these values, you can no longer
1440 access 32-bit DB files. You can, however, call C<set_pack(4, 'N')> to change
1441 back to 32-bit mode.
1443 B<Note:> I have not personally tested files > 2 GB -- all my systems have
1444 only a 32-bit Perl. However, I have received user reports that this does
1447 =head1 LOW-LEVEL ACCESS
1449 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1450 you can call the C<_fh()> method, which returns the handle:
1452 my $fh = $db->_fh();
1454 This method can be called on the root level of the datbase, or any child
1455 hashes or arrays. All levels share a I<root> structure, which contains things
1456 like the filehandle, a reference counter, and all the options specified
1457 when you created the object. You can get access to this root structure by
1458 calling the C<root()> method.
1460 my $root = $db->_root();
1462 This is useful for changing options after the object has already been created,
1463 such as enabling/disabling locking. You can also store your own temporary user
1464 data in this structure (be wary of name collision), which is then accessible from
1465 any child hash or array.
1467 =head1 CUSTOM DIGEST ALGORITHM
1469 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1470 keys. However you can override this, and use another algorithm (such as SHA-256)
1471 or even write your own. But please note that DBM::Deep currently expects zero
1472 collisions, so your algorithm has to be I<perfect>, so to speak.
1473 Collision detection may be introduced in a later version.
1477 You can specify a custom digest algorithm by calling the static C<set_digest()>
1478 function, passing a reference to a subroutine, and the length of the algorithm's
1479 hashes (in bytes). This is a global static function, which affects ALL DBM::Deep
1480 objects. Here is a working example that uses a 256-bit hash from the
1481 I<Digest::SHA256> module. Please see
1482 L<http://search.cpan.org/search?module=Digest::SHA256> for more.
1487 my $context = Digest::SHA256::new(256);
1489 DBM::Deep::set_digest( \&my_digest, 32 );
1491 my $db = DBM::Deep->new( "foo-sha.db" );
1493 $db->{key1} = "value1";
1494 $db->{key2} = "value2";
1495 print "key1: " . $db->{key1} . "\n";
1496 print "key2: " . $db->{key2} . "\n";
1502 return substr( $context->hash($_[0]), 0, 32 );
1505 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1506 of bytes you specify in the C<set_digest()> function (in this case 32).
1508 =head1 CIRCULAR REFERENCES
1510 DBM::Deep has B<experimental> support for circular references. Meaning you
1511 can have a nested hash key or array element that points to a parent object.
1512 This relationship is stored in the DB file, and is preserved between sessions.
1515 my $db = DBM::Deep->new( "foo.db" );
1518 $db->{circle} = $db; # ref to self
1520 print $db->{foo} . "\n"; # prints "foo"
1521 print $db->{circle}->{foo} . "\n"; # prints "foo" again
1523 One catch is, passing the object to a function that recursively walks the
1524 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1525 C<export()> methods) will result in an infinite loop. The other catch is,
1526 if you fetch the I<key> of a circular reference (i.e. using the C<first_key()>
1527 or C<next_key()> methods), you will get the I<target object's key>, not the
1528 ref's key. This gets even more interesting with the above example, where
1529 the I<circle> key points to the base DB object, which technically doesn't
1530 have a key. So I made DBM::Deep return "[base]" as the key name in that
1533 =head1 CAVEATS / ISSUES / BUGS
1535 This section describes all the known issues with DBM::Deep. It you have found
1536 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1538 =head2 UNUSED SPACE RECOVERY
1540 One major caveat with DBM::Deep is that space occupied by existing keys and
1541 values is not recovered when they are deleted. Meaning if you keep deleting
1542 and adding new keys, your file will continuously grow. I am working on this,
1543 but in the meantime you can call the built-in C<optimize()> method from time to
1544 time (perhaps in a crontab or something) to recover all your unused space.
1546 $db->optimize(); # returns true on success
1548 This rebuilds the ENTIRE database into a new file, then moves it on top of
1549 the original. The new file will have no unused space, thus it will take up as
1550 little disk space as possible. Please note that this operation can take
1551 a long time for large files, and you need enough disk space to temporarily hold
1552 2 copies of your DB file. The temporary file is created in the same directory
1553 as the original, named with a ".tmp" extension, and is deleted when the
1554 operation completes. Oh, and if locking is enabled, the DB is automatically
1555 locked for the entire duration of the copy.
1557 B<WARNING:> Only call optimize() on the top-level node of the database, and
1558 make sure there are no child references lying around. DBM::Deep keeps a reference
1559 counter, and if it is greater than 1, optimize() will abort and return undef.
1561 =head2 AUTOVIVIFICATION
1563 Unfortunately, autovivification doesn't work with tied hashes. This appears to
1564 be a bug in Perl's tie() system, as I<Jakob Schmidt> encountered the very same
1565 issue with his I<DWH_FIle> module (see L<http://search.cpan.org/search?module=DWH_File>),
1566 and it is also mentioned in the BUGS section for the I<MLDBM> module <see
1567 L<http://search.cpan.org/search?module=MLDBM>). Basically, on a new db file,
1570 $db->{foo}->{bar} = "hello";
1572 Since "foo" doesn't exist, you cannot add "bar" to it. You end up with "foo"
1573 being an empty hash. Try this instead, which works fine:
1575 $db->{foo} = { bar => "hello" };
1577 As of Perl 5.8.7, this bug still exists. I have walked very carefully through
1578 the execution path, and Perl indeed passes an empty hash to the STORE() method.
1579 Probably a bug in Perl.
1581 =head2 FILE CORRUPTION
1583 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1584 for a 32-bit signature when opened, but other corruption in files can cause
1585 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1586 stuck in an infinite loop depending on the level of corruption. File write
1587 operations are not checked for failure (for speed), so if you happen to run
1588 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1589 be addressed in a later version of DBM::Deep.
1593 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1594 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1595 about setting up your NFS server with a locking daemon, then using lockf() to
1596 lock your files, but your mileage may vary there as well. From what I
1597 understand, there is no real way to do it. However, if you need access to the
1598 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1599 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1601 =head2 COPYING OBJECTS
1603 Beware of copying tied objects in Perl. Very strange things can happen.
1604 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1605 returns a new, blessed, tied hash or array to the same level in the DB.
1607 my $copy = $db->clone();
1609 B<Note>: Since clone() here is cloning the object, not the database location, any
1610 modifications to either $db or $copy will be visible in both.
1614 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1615 These functions cause every element in the array to move, which can be murder
1616 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1617 a different location. This will be addressed in the forthcoming version 1.00.
1619 =head2 WRITEONLY FILES
1621 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1622 writeonly mode. STORE will verify that the filehandle is writable. However, there
1623 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1624 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1628 This section discusses DBM::Deep's speed and memory usage.
1632 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1633 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1634 multi-level hash/array support, and cross-platform FTPable files. Even so,
1635 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1636 with huge databases. Here is some test data:
1638 Adding 1,000,000 keys to new DB file...
1640 At 100 keys, avg. speed is 2,703 keys/sec
1641 At 200 keys, avg. speed is 2,642 keys/sec
1642 At 300 keys, avg. speed is 2,598 keys/sec
1643 At 400 keys, avg. speed is 2,578 keys/sec
1644 At 500 keys, avg. speed is 2,722 keys/sec
1645 At 600 keys, avg. speed is 2,628 keys/sec
1646 At 700 keys, avg. speed is 2,700 keys/sec
1647 At 800 keys, avg. speed is 2,607 keys/sec
1648 At 900 keys, avg. speed is 2,190 keys/sec
1649 At 1,000 keys, avg. speed is 2,570 keys/sec
1650 At 2,000 keys, avg. speed is 2,417 keys/sec
1651 At 3,000 keys, avg. speed is 1,982 keys/sec
1652 At 4,000 keys, avg. speed is 1,568 keys/sec
1653 At 5,000 keys, avg. speed is 1,533 keys/sec
1654 At 6,000 keys, avg. speed is 1,787 keys/sec
1655 At 7,000 keys, avg. speed is 1,977 keys/sec
1656 At 8,000 keys, avg. speed is 2,028 keys/sec
1657 At 9,000 keys, avg. speed is 2,077 keys/sec
1658 At 10,000 keys, avg. speed is 2,031 keys/sec
1659 At 20,000 keys, avg. speed is 1,970 keys/sec
1660 At 30,000 keys, avg. speed is 2,050 keys/sec
1661 At 40,000 keys, avg. speed is 2,073 keys/sec
1662 At 50,000 keys, avg. speed is 1,973 keys/sec
1663 At 60,000 keys, avg. speed is 1,914 keys/sec
1664 At 70,000 keys, avg. speed is 2,091 keys/sec
1665 At 80,000 keys, avg. speed is 2,103 keys/sec
1666 At 90,000 keys, avg. speed is 1,886 keys/sec
1667 At 100,000 keys, avg. speed is 1,970 keys/sec
1668 At 200,000 keys, avg. speed is 2,053 keys/sec
1669 At 300,000 keys, avg. speed is 1,697 keys/sec
1670 At 400,000 keys, avg. speed is 1,838 keys/sec
1671 At 500,000 keys, avg. speed is 1,941 keys/sec
1672 At 600,000 keys, avg. speed is 1,930 keys/sec
1673 At 700,000 keys, avg. speed is 1,735 keys/sec
1674 At 800,000 keys, avg. speed is 1,795 keys/sec
1675 At 900,000 keys, avg. speed is 1,221 keys/sec
1676 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1678 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1679 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1680 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1681 Run time was 12 min 3 sec.
1685 One of the great things about DBM::Deep is that it uses very little memory.
1686 Even with huge databases (1,000,000+ keys) you will not see much increased
1687 memory on your process. DBM::Deep relies solely on the filesystem for storing
1688 and fetching data. Here is output from I</usr/bin/top> before even opening a
1691 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1692 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1694 Basically the process is taking 2,716K of memory. And here is the same
1695 process after storing and fetching 1,000,000 keys:
1697 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1698 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1700 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1701 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1703 =head1 DB FILE FORMAT
1705 In case you were interested in the underlying DB file format, it is documented
1706 here in this section. You don't need to know this to use the module, it's just
1707 included for reference.
1711 DBM::Deep files always start with a 32-bit signature to identify the file type.
1712 This is at offset 0. The signature is "DPDB" in network byte order. This is
1713 checked for when the file is opened and an error will be thrown if it's not found.
1717 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1718 has a standard header containing the type of data, the length of data, and then
1719 the data itself. The type is a single character (1 byte), the length is a
1720 32-bit unsigned long in network byte order, and the data is, well, the data.
1721 Here is how it unfolds:
1725 Immediately after the 32-bit file signature is the I<Master Index> record.
1726 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1727 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1728 depending on how the DBM::Deep object was constructed.
1730 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1731 number). The first 8-bit char of the MD5 signature is the offset into the
1732 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1733 index element is a file offset of the next tag for the key/element in question,
1734 which is usually a I<Bucket List> tag (see below).
1736 The next tag I<could> be another index, depending on how many keys/elements
1737 exist. See L<RE-INDEXING> below for details.
1741 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1742 file offsets to where the actual data is stored. It starts with a standard
1743 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1744 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1745 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1746 When the list fills up, a I<Re-Index> operation is performed (See
1747 L<RE-INDEXING> below).
1751 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1752 index/value pair (in array mode). It starts with a standard tag header with
1753 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1754 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1755 header. The size reported in the tag header is only for the value, but then,
1756 just after the value is another size (32-bit unsigned long) and then the plain
1757 key itself. Since the value is likely to be fetched more often than the plain
1758 key, I figured it would be I<slightly> faster to store the value first.
1760 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1761 record for the nested structure, where the process begins all over again.
1765 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1766 exhausted. Then, when another key/element comes in, the list is converted to a
1767 new index record. However, this index will look at the next char in the MD5
1768 hash, and arrange new Bucket List pointers accordingly. This process is called
1769 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1770 17 (16 + new one) keys/elements are removed from the old Bucket List and
1771 inserted into the new index. Several new Bucket Lists are created in the
1772 process, as a new MD5 char from the key is being examined (it is unlikely that
1773 the keys will all share the same next char of their MD5s).
1775 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1776 when the Bucket Lists will turn into indexes, but the first round tends to
1777 happen right around 4,000 keys. You will see a I<slight> decrease in
1778 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1779 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1780 right around 900,000 keys. This process can continue nearly indefinitely --
1781 right up until the point the I<MD5> signatures start colliding with each other,
1782 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1783 getting struck by lightning while you are walking to cash in your tickets.
1784 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1785 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1786 this is 340 unodecillion, but don't quote me).
1790 When a new key/element is stored, the key (or index number) is first run through
1791 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1792 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1793 for the first char of the signature (in this case I<b0>). If it does not exist,
1794 a new I<Bucket List> is created for our key (and the next 15 future keys that
1795 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1796 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1797 this point, unless we are replacing an existing I<Bucket>), where the actual
1798 data will be stored.
1802 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1803 (or index number), then walking along the indexes. If there are enough
1804 keys/elements in this DB level, there might be nested indexes, each linked to
1805 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1806 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1807 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1808 plain key are stored.
1810 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1811 methods. In this process the indexes are walked systematically, and each key
1812 fetched in increasing MD5 order (which is why it appears random). Once the
1813 I<Bucket> is found, the value is skipped and the plain key returned instead.
1814 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1815 alphabetically sorted. This only happens on an index-level -- as soon as the
1816 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1817 so it's pretty much undefined how the keys will come out -- just like Perl's
1820 =head1 CODE COVERAGE
1822 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1823 B<Devel::Cover> report on this module's test suite.
1825 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1826 File stmt bran cond sub pod time total
1827 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1828 blib/lib/DBM/Deep.pm 95.1 81.6 70.3 100.0 100.0 33.4 91.0
1829 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 n/a 27.8 98.0
1830 blib/lib/DBM/Deep/Engine.pm 97.8 85.6 75.0 100.0 0.0 25.8 90.8
1831 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 n/a 13.0 97.2
1832 Total 97.5 85.4 76.6 100.0 46.9 100.0 92.5
1833 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1835 =head1 MORE INFORMATION
1837 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1838 or send email to L<DBM-Deep@googlegroups.com>.
1842 Joseph Huckaby, L<jhuckaby@cpan.org>
1844 Rob Kinyon, L<rkinyon@cpan.org>
1846 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1850 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1851 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1855 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1856 This is free software, you may use it and distribute it under the
1857 same terms as Perl itself.