7 # Multi-level database module for storing hash trees, arrays and simple
8 # key/value pairs into FTP-able, cross-platform binary database files.
10 # Type `perldoc DBM::Deep` for complete documentation.
14 # tie %db, 'DBM::Deep', 'my_database.db'; # standard tie() method
16 # my $db = new DBM::Deep( 'my_database.db' ); # preferred OO method
18 # $db->{my_scalar} = 'hello world';
19 # $db->{my_hash} = { larry => 'genius', hashes => 'fast' };
20 # $db->{my_array} = [ 1, 2, 3, time() ];
21 # $db->{my_complex} = [ 'hello', { perl => 'rules' }, 42, 99 ];
22 # push @{$db->{my_array}}, 'another value';
23 # my @key_list = keys %{$db->{my_hash}};
24 # print "This module " . $db->{my_complex}->[1]->{perl} . "!\n";
27 # (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
28 # This program is free software; you can redistribute it and/or
29 # modify it under the same terms as Perl itself.
34 use Fcntl qw( :DEFAULT :flock :seek );
38 use DBM::Deep::Engine;
40 use vars qw( $VERSION );
41 $VERSION = q(0.99_01);
45 # Setup file and tag signatures. These should never change.
47 sub SIG_FILE () { 'DPDB' }
48 sub SIG_HASH () { 'H' }
49 sub SIG_ARRAY () { 'A' }
50 sub SIG_SCALAR () { 'S' }
51 sub SIG_NULL () { 'N' }
52 sub SIG_DATA () { 'D' }
53 sub SIG_INDEX () { 'I' }
54 sub SIG_BLIST () { 'B' }
58 # Setup constants for users to pass to new()
60 sub TYPE_HASH () { SIG_HASH }
61 sub TYPE_ARRAY () { SIG_ARRAY }
62 sub TYPE_SCALAR () { SIG_SCALAR }
70 $proto->_throw_error( "Odd number of parameters to " . (caller(1))[2] );
75 unless ( eval { local $SIG{'__DIE__'}; %{$_[0]} || 1 } ) {
76 $proto->_throw_error( "Not a hashref in args to " . (caller(1))[2] );
81 $args = { file => shift };
89 # Class constructor method for Perl OO interface.
90 # Calls tie() and returns blessed reference to tied hash or array,
91 # providing a hybrid OO/tie interface.
94 my $args = $class->_get_args( @_ );
97 # Check if we want a tied hash or array.
100 if (defined($args->{type}) && $args->{type} eq TYPE_ARRAY) {
101 $class = 'DBM::Deep::Array';
102 require DBM::Deep::Array;
103 tie @$self, $class, %$args;
106 $class = 'DBM::Deep::Hash';
107 require DBM::Deep::Hash;
108 tie %$self, $class, %$args;
111 return bless $self, $class;
116 # Setup $self and bless into this class.
121 # These are the defaults to be optionally overridden below
124 base_offset => length(SIG_FILE),
125 engine => DBM::Deep::Engine->new,
128 foreach my $param ( keys %$self ) {
129 next unless exists $args->{$param};
130 $self->{$param} = delete $args->{$param}
133 # locking implicitly enables autoflush
134 if ($args->{locking}) { $args->{autoflush} = 1; }
136 $self->{root} = exists $args->{root}
138 : DBM::Deep::_::Root->new( $args );
140 $self->{engine}->setup_fh( $self );
147 require DBM::Deep::Hash;
148 return DBM::Deep::Hash->TIEHASH( @_ );
153 require DBM::Deep::Array;
154 return DBM::Deep::Array->TIEARRAY( @_ );
157 #XXX Unneeded now ...
163 # If db locking is set, flock() the db file. If called multiple
164 # times before unlock(), then the same number of unlocks() must
165 # be called before the lock is released.
167 my $self = $_[0]->_get_self;
169 $type = LOCK_EX unless defined $type;
171 if (!defined($self->_fh)) { return; }
173 if ($self->_root->{locking}) {
174 if (!$self->_root->{locked}) {
175 flock($self->_fh, $type);
177 # refresh end counter in case file has changed size
178 my @stats = stat($self->_root->{file});
179 $self->_root->{end} = $stats[7];
181 # double-check file inode, in case another process
182 # has optimize()d our file while we were waiting.
183 if ($stats[1] != $self->_root->{inode}) {
184 $self->{engine}->close_fh( $self );
185 $self->{engine}->setup_fh( $self );
186 flock($self->_fh, $type); # re-lock
188 # This may not be necessary after re-opening
189 $self->_root->{end} = (stat($self->_fh))[7]; # re-end
192 $self->_root->{locked}++;
202 # If db locking is set, unlock the db file. See note in lock()
203 # regarding calling lock() multiple times.
205 my $self = $_[0]->_get_self;
207 if (!defined($self->_fh)) { return; }
209 if ($self->_root->{locking} && $self->_root->{locked} > 0) {
210 $self->_root->{locked}--;
211 if (!$self->_root->{locked}) { flock($self->_fh, LOCK_UN); }
220 my $self = shift->_get_self;
221 my ($spot, $value) = @_;
226 elsif ( eval { local $SIG{__DIE__}; $value->isa( 'DBM::Deep' ) } ) {
227 my $type = $value->_type;
228 ${$spot} = $type eq TYPE_HASH ? {} : [];
229 $value->_copy_node( ${$spot} );
232 my $r = Scalar::Util::reftype( $value );
233 my $c = Scalar::Util::blessed( $value );
234 if ( $r eq 'ARRAY' ) {
235 ${$spot} = [ @{$value} ];
238 ${$spot} = { %{$value} };
240 ${$spot} = bless ${$spot}, $c
249 # Copy single level of keys or elements to new DB handle.
250 # Recurse for nested structures
252 my $self = shift->_get_self;
255 if ($self->_type eq TYPE_HASH) {
256 my $key = $self->first_key();
258 my $value = $self->get($key);
259 $self->_copy_value( \$db_temp->{$key}, $value );
260 $key = $self->next_key($key);
264 my $length = $self->length();
265 for (my $index = 0; $index < $length; $index++) {
266 my $value = $self->get($index);
267 $self->_copy_value( \$db_temp->[$index], $value );
276 # Recursively export into standard Perl hashes and arrays.
278 my $self = $_[0]->_get_self;
281 if ($self->_type eq TYPE_HASH) { $temp = {}; }
282 elsif ($self->_type eq TYPE_ARRAY) { $temp = []; }
285 $self->_copy_node( $temp );
293 # Recursively import Perl hash/array structure
295 #XXX This use of ref() seems to be ok
296 if (!ref($_[0])) { return; } # Perl calls import() on use -- ignore
298 my $self = $_[0]->_get_self;
301 #XXX This use of ref() seems to be ok
304 # struct is not a reference, so just import based on our type
308 if ($self->_type eq TYPE_HASH) { $struct = {@_}; }
309 elsif ($self->_type eq TYPE_ARRAY) { $struct = [@_]; }
312 my $r = Scalar::Util::reftype($struct) || '';
313 if ($r eq "HASH" && $self->_type eq TYPE_HASH) {
314 foreach my $key (keys %$struct) { $self->put($key, $struct->{$key}); }
316 elsif ($r eq "ARRAY" && $self->_type eq TYPE_ARRAY) {
317 $self->push( @$struct );
320 return $self->_throw_error("Cannot import: type mismatch");
328 # Rebuild entire database into new file, then move
329 # it back on top of original.
331 my $self = $_[0]->_get_self;
333 #XXX Need to create a new test for this
334 # if ($self->_root->{links} > 1) {
335 # return $self->_throw_error("Cannot optimize: reference count is greater than 1");
338 my $db_temp = DBM::Deep->new(
339 file => $self->_root->{file} . '.tmp',
343 return $self->_throw_error("Cannot optimize: failed to open temp file: $!");
347 $self->_copy_node( $db_temp );
351 # Attempt to copy user, group and permissions over to new file
353 my @stats = stat($self->_fh);
354 my $perms = $stats[2] & 07777;
357 chown( $uid, $gid, $self->_root->{file} . '.tmp' );
358 chmod( $perms, $self->_root->{file} . '.tmp' );
360 # q.v. perlport for more information on this variable
361 if ( $^O eq 'MSWin32' || $^O eq 'cygwin' ) {
363 # Potential race condition when optmizing on Win32 with locking.
364 # The Windows filesystem requires that the filehandle be closed
365 # before it is overwritten with rename(). This could be redone
369 $self->{engine}->close_fh( $self );
372 if (!rename $self->_root->{file} . '.tmp', $self->_root->{file}) {
373 unlink $self->_root->{file} . '.tmp';
375 return $self->_throw_error("Optimize failed: Cannot copy temp file over original: $!");
379 $self->{engine}->close_fh( $self );
380 $self->{engine}->setup_fh( $self );
387 # Make copy of object and return
389 my $self = $_[0]->_get_self;
391 return DBM::Deep->new(
392 type => $self->_type,
393 base_offset => $self->_base_offset,
399 my %is_legal_filter = map {
402 store_key store_value
403 fetch_key fetch_value
408 # Setup filter function for storing or fetching the key or value
410 my $self = $_[0]->_get_self;
412 my $func = $_[2] ? $_[2] : undef;
414 if ( $is_legal_filter{$type} ) {
415 $self->_root->{"filter_$type"} = $func;
429 # Get access to the root structure
431 my $self = $_[0]->_get_self;
432 return $self->{root};
437 # Get access to the raw fh
439 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;
682 filter_store_key => undef,
683 filter_store_value => undef,
684 filter_fetch_key => undef,
685 filter_fetch_value => undef,
689 if ( $self->{fh} && !$self->{file_offset} ) {
690 $self->{file_offset} = tell( $self->{fh} );
700 close $self->{fh} if $self->{fh};
711 DBM::Deep - A pure perl multi-level hash/array DBM
716 my $db = DBM::Deep->new( "foo.db" );
718 $db->{key} = 'value'; # tie() style
721 $db->put('key' => 'value'); # OO style
722 print $db->get('key');
724 # true multi-level support
725 $db->{my_complex} = [
726 'hello', { perl => 'rules' },
732 A unique flat-file database module, written in pure perl. True
733 multi-level hash/array support (unlike MLDBM, which is faked), hybrid
734 OO / tie() interface, cross-platform FTPable files, and quite fast. Can
735 handle millions of keys and unlimited hash levels without significant
736 slow-down. Written from the ground-up in pure perl -- this is NOT a
737 wrapper around a C-based DBM. Out-of-the-box compatibility with Unix,
738 Mac OS X and Windows.
742 Hopefully you are using Perl's excellent CPAN module, which will download
743 and install the module for you. If not, get the tarball, and run these
755 Construction can be done OO-style (which is the recommended way), or using
756 Perl's tie() function. Both are examined here.
758 =head2 OO CONSTRUCTION
760 The recommended way to construct a DBM::Deep object is to use the new()
761 method, which gets you a blessed, tied hash or array reference.
763 my $db = DBM::Deep->new( "foo.db" );
765 This opens a new database handle, mapped to the file "foo.db". If this
766 file does not exist, it will automatically be created. DB files are
767 opened in "r+" (read/write) mode, and the type of object returned is a
768 hash, unless otherwise specified (see L<OPTIONS> below).
770 You can pass a number of options to the constructor to specify things like
771 locking, autoflush, etc. This is done by passing an inline hash:
773 my $db = DBM::Deep->new(
779 Notice that the filename is now specified I<inside> the hash with
780 the "file" parameter, as opposed to being the sole argument to the
781 constructor. This is required if any options are specified.
782 See L<OPTIONS> below for the complete list.
786 You can also start with an array instead of a hash. For this, you must
787 specify the C<type> parameter:
789 my $db = DBM::Deep->new(
791 type => DBM::Deep->TYPE_ARRAY
794 B<Note:> Specifing the C<type> parameter only takes effect when beginning
795 a new DB file. If you create a DBM::Deep object with an existing file, the
796 C<type> will be loaded from the file header, and an error will be thrown if
797 the wrong type is passed in.
799 =head2 TIE CONSTRUCTION
801 Alternately, you can create a DBM::Deep handle by using Perl's built-in
802 tie() function. The object returned from tie() can be used to call methods,
803 such as lock() and unlock(), but cannot be used to assign to the DBM::Deep
804 file (as expected with most tie'd objects).
807 my $db = tie %hash, "DBM::Deep", "foo.db";
810 my $db = tie @array, "DBM::Deep", "bar.db";
812 As with the OO constructor, you can replace the DB filename parameter with
813 a hash containing one or more options (see L<OPTIONS> just below for the
816 tie %hash, "DBM::Deep", {
824 There are a number of options that can be passed in when constructing your
825 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
831 Filename of the DB file to link the handle to. You can pass a full absolute
832 filesystem path, partial path, or a plain filename if the file is in the
833 current working directory. This is a required parameter (though q.v. fh).
837 If you want, you can pass in the fh instead of the file. This is most useful for doing
840 my $db = DBM::Deep->new( { fh => \*DATA } );
842 You are responsible for making sure that the fh has been opened appropriately for your
843 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
844 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
845 needs to read from the fh.
849 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
850 not need to set this. However, it's there if you want it.
852 If you pass in fh and do not set this, it will be set appropriately.
856 This parameter specifies what type of object to create, a hash or array. Use
857 one of these two constants: C<DBM::Deep-E<gt>TYPE_HASH> or C<DBM::Deep-E<gt>TYPE_ARRAY>.
858 This only takes effect when beginning a new file. This is an optional
859 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
863 Specifies whether locking is to be enabled. DBM::Deep uses Perl's Fnctl flock()
864 function to lock the database in exclusive mode for writes, and shared mode for
865 reads. Pass any true value to enable. This affects the base DB handle I<and
866 any child hashes or arrays> that use the same DB file. This is an optional
867 parameter, and defaults to 0 (disabled). See L<LOCKING> below for more.
871 Specifies whether autoflush is to be enabled on the underlying filehandle.
872 This obviously slows down write operations, but is required if you may have
873 multiple processes accessing the same DB file (also consider enable I<locking>).
874 Pass any true value to enable. This is an optional parameter, and defaults to 0
879 If I<autobless> mode is enabled, DBM::Deep will preserve blessed hashes, and
880 restore them when fetched. This is an B<experimental> feature, and does have
881 side-effects. Basically, when hashes are re-blessed into their original
882 classes, they are no longer blessed into the DBM::Deep class! So you won't be
883 able to call any DBM::Deep methods on them. You have been warned.
884 This is an optional parameter, and defaults to 0 (disabled).
888 See L<FILTERS> below.
894 With DBM::Deep you can access your databases using Perl's standard hash/array
895 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
896 treat them as such. DBM::Deep will intercept all reads/writes and direct them
897 to the right place -- the DB file. This has nothing to do with the
898 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
899 using regular hashes and arrays, rather than calling functions like C<get()>
900 and C<put()> (although those work too). It is entirely up to you how to want
901 to access your databases.
905 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
906 or even nested hashes (or arrays) using standard Perl syntax:
908 my $db = DBM::Deep->new( "foo.db" );
910 $db->{mykey} = "myvalue";
912 $db->{myhash}->{subkey} = "subvalue";
914 print $db->{myhash}->{subkey} . "\n";
916 You can even step through hash keys using the normal Perl C<keys()> function:
918 foreach my $key (keys %$db) {
919 print "$key: " . $db->{$key} . "\n";
922 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
923 pushes them onto an array, all before the loop even begins. If you have an
924 extra large hash, this may exhaust Perl's memory. Instead, consider using
925 Perl's C<each()> function, which pulls keys/values one at a time, using very
928 while (my ($key, $value) = each %$db) {
929 print "$key: $value\n";
932 Please note that when using C<each()>, you should always pass a direct
933 hash reference, not a lookup. Meaning, you should B<never> do this:
936 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
938 This causes an infinite loop, because for each iteration, Perl is calling
939 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
940 it effectively keeps returning the first key over and over again. Instead,
941 assign a temporary variable to C<$db->{foo}>, then pass that to each().
945 As with hashes, you can treat any DBM::Deep object like a normal Perl array
946 reference. This includes inserting, removing and manipulating elements,
947 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
948 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
949 or simply be a nested array reference inside a hash. Example:
951 my $db = DBM::Deep->new(
952 file => "foo-array.db",
953 type => DBM::Deep->TYPE_ARRAY
957 push @$db, "bar", "baz";
960 my $last_elem = pop @$db; # baz
961 my $first_elem = shift @$db; # bah
962 my $second_elem = $db->[1]; # bar
964 my $num_elements = scalar @$db;
968 In addition to the I<tie()> interface, you can also use a standard OO interface
969 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
970 array) has its own methods, but both types share the following common methods:
971 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>.
975 =item * new() / clone()
977 These are the constructor and copy-functions.
979 =item * put() / store()
981 Stores a new hash key/value pair, or sets an array element value. Takes two
982 arguments, the hash key or array index, and the new value. The value can be
983 a scalar, hash ref or array ref. Returns true on success, false on failure.
985 $db->put("foo", "bar"); # for hashes
986 $db->put(1, "bar"); # for arrays
988 =item * get() / fetch()
990 Fetches the value of a hash key or array element. Takes one argument: the hash
991 key or array index. Returns a scalar, hash ref or array ref, depending on the
994 my $value = $db->get("foo"); # for hashes
995 my $value = $db->get(1); # for arrays
999 Checks if a hash key or array index exists. Takes one argument: the hash key
1000 or array index. Returns true if it exists, false if not.
1002 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
1003 if ($db->exists(1)) { print "yay!\n"; } # for arrays
1007 Deletes one hash key/value pair or array element. Takes one argument: the hash
1008 key or array index. Returns true on success, false if not found. For arrays,
1009 the remaining elements located after the deleted element are NOT moved over.
1010 The deleted element is essentially just undefined, which is exactly how Perl's
1011 internal arrays work. Please note that the space occupied by the deleted
1012 key/value or element is B<not> reused again -- see L<UNUSED SPACE RECOVERY>
1013 below for details and workarounds.
1015 $db->delete("foo"); # for hashes
1016 $db->delete(1); # for arrays
1020 Deletes B<all> hash keys or array elements. Takes no arguments. No return
1021 value. Please note that the space occupied by the deleted keys/values or
1022 elements is B<not> reused again -- see L<UNUSED SPACE RECOVERY> below for
1023 details and workarounds.
1025 $db->clear(); # hashes or arrays
1027 =item * lock() / unlock()
1033 Recover lost disk space.
1035 =item * import() / export()
1037 Data going in and out.
1039 =item * set_digest() / set_pack() / set_filter()
1041 q.v. adjusting the interal parameters.
1047 For hashes, DBM::Deep supports all the common methods described above, and the
1048 following additional methods: C<first_key()> and C<next_key()>.
1054 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
1055 fetched in an undefined order (which appears random). Takes no arguments,
1056 returns the key as a scalar value.
1058 my $key = $db->first_key();
1062 Returns the "next" key in the hash, given the previous one as the sole argument.
1063 Returns undef if there are no more keys to be fetched.
1065 $key = $db->next_key($key);
1069 Here are some examples of using hashes:
1071 my $db = DBM::Deep->new( "foo.db" );
1073 $db->put("foo", "bar");
1074 print "foo: " . $db->get("foo") . "\n";
1076 $db->put("baz", {}); # new child hash ref
1077 $db->get("baz")->put("buz", "biz");
1078 print "buz: " . $db->get("baz")->get("buz") . "\n";
1080 my $key = $db->first_key();
1082 print "$key: " . $db->get($key) . "\n";
1083 $key = $db->next_key($key);
1086 if ($db->exists("foo")) { $db->delete("foo"); }
1090 For arrays, DBM::Deep supports all the common methods described above, and the
1091 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
1092 C<unshift()> and C<splice()>.
1098 Returns the number of elements in the array. Takes no arguments.
1100 my $len = $db->length();
1104 Adds one or more elements onto the end of the array. Accepts scalars, hash
1105 refs or array refs. No return value.
1107 $db->push("foo", "bar", {});
1111 Fetches the last element in the array, and deletes it. Takes no arguments.
1112 Returns undef if array is empty. Returns the element value.
1114 my $elem = $db->pop();
1118 Fetches the first element in the array, deletes it, then shifts all the
1119 remaining elements over to take up the space. Returns the element value. This
1120 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
1123 my $elem = $db->shift();
1127 Inserts one or more elements onto the beginning of the array, shifting all
1128 existing elements over to make room. Accepts scalars, hash refs or array refs.
1129 No return value. This method is not recommended with large arrays -- see
1130 <LARGE ARRAYS> below for details.
1132 $db->unshift("foo", "bar", {});
1136 Performs exactly like Perl's built-in function of the same name. See L<perldoc
1137 -f splice> for usage -- it is too complicated to document here. This method is
1138 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
1142 Here are some examples of using arrays:
1144 my $db = DBM::Deep->new(
1146 type => DBM::Deep->TYPE_ARRAY
1149 $db->push("bar", "baz");
1150 $db->unshift("foo");
1153 my $len = $db->length();
1154 print "length: $len\n"; # 4
1156 for (my $k=0; $k<$len; $k++) {
1157 print "$k: " . $db->get($k) . "\n";
1160 $db->splice(1, 2, "biz", "baf");
1162 while (my $elem = shift @$db) {
1163 print "shifted: $elem\n";
1168 Enable automatic file locking by passing a true value to the C<locking>
1169 parameter when constructing your DBM::Deep object (see L<SETUP> above).
1171 my $db = DBM::Deep->new(
1176 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
1177 mode for writes, and shared mode for reads. This is required if you have
1178 multiple processes accessing the same database file, to avoid file corruption.
1179 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
1180 NFS> below for more.
1182 =head2 EXPLICIT LOCKING
1184 You can explicitly lock a database, so it remains locked for multiple
1185 transactions. This is done by calling the C<lock()> method, and passing an
1186 optional lock mode argument (defaults to exclusive mode). This is particularly
1187 useful for things like counters, where the current value needs to be fetched,
1188 then incremented, then stored again.
1191 my $counter = $db->get("counter");
1193 $db->put("counter", $counter);
1202 You can pass C<lock()> an optional argument, which specifies which mode to use
1203 (exclusive or shared). Use one of these two constants: C<DBM::Deep-E<gt>LOCK_EX>
1204 or C<DBM::Deep-E<gt>LOCK_SH>. These are passed directly to C<flock()>, and are the
1205 same as the constants defined in Perl's C<Fcntl> module.
1207 $db->lock( DBM::Deep->LOCK_SH );
1211 =head1 IMPORTING/EXPORTING
1213 You can import existing complex structures by calling the C<import()> method,
1214 and export an entire database into an in-memory structure using the C<export()>
1215 method. Both are examined here.
1219 Say you have an existing hash with nested hashes/arrays inside it. Instead of
1220 walking the structure and adding keys/elements to the database as you go,
1221 simply pass a reference to the C<import()> method. This recursively adds
1222 everything to an existing DBM::Deep object for you. Here is an example:
1227 array1 => [ "elem0", "elem1", "elem2" ],
1229 subkey1 => "subvalue1",
1230 subkey2 => "subvalue2"
1234 my $db = DBM::Deep->new( "foo.db" );
1235 $db->import( $struct );
1237 print $db->{key1} . "\n"; # prints "value1"
1239 This recursively imports the entire C<$struct> object into C<$db>, including
1240 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
1241 keys are merged with the existing ones, replacing if they already exist.
1242 The C<import()> method can be called on any database level (not just the base
1243 level), and works with both hash and array DB types.
1245 B<Note:> Make sure your existing structure has no circular references in it.
1246 These will cause an infinite loop when importing.
1250 Calling the C<export()> method on an existing DBM::Deep object will return
1251 a reference to a new in-memory copy of the database. The export is done
1252 recursively, so all nested hashes/arrays are all exported to standard Perl
1253 objects. Here is an example:
1255 my $db = DBM::Deep->new( "foo.db" );
1257 $db->{key1} = "value1";
1258 $db->{key2} = "value2";
1260 $db->{hash1}->{subkey1} = "subvalue1";
1261 $db->{hash1}->{subkey2} = "subvalue2";
1263 my $struct = $db->export();
1265 print $struct->{key1} . "\n"; # prints "value1"
1267 This makes a complete copy of the database in memory, and returns a reference
1268 to it. The C<export()> method can be called on any database level (not just
1269 the base level), and works with both hash and array DB types. Be careful of
1270 large databases -- you can store a lot more data in a DBM::Deep object than an
1271 in-memory Perl structure.
1273 B<Note:> Make sure your database has no circular references in it.
1274 These will cause an infinite loop when exporting.
1278 DBM::Deep has a number of hooks where you can specify your own Perl function
1279 to perform filtering on incoming or outgoing data. This is a perfect
1280 way to extend the engine, and implement things like real-time compression or
1281 encryption. Filtering applies to the base DB level, and all child hashes /
1282 arrays. Filter hooks can be specified when your DBM::Deep object is first
1283 constructed, or by calling the C<set_filter()> method at any time. There are
1284 four available filter hooks, described below:
1288 =item * filter_store_key
1290 This filter is called whenever a hash key is stored. It
1291 is passed the incoming key, and expected to return a transformed key.
1293 =item * filter_store_value
1295 This filter is called whenever a hash key or array element is stored. It
1296 is passed the incoming value, and expected to return a transformed value.
1298 =item * filter_fetch_key
1300 This filter is called whenever a hash key is fetched (i.e. via
1301 C<first_key()> or C<next_key()>). It is passed the transformed key,
1302 and expected to return the plain key.
1304 =item * filter_fetch_value
1306 This filter is called whenever a hash key or array element is fetched.
1307 It is passed the transformed value, and expected to return the plain value.
1311 Here are the two ways to setup a filter hook:
1313 my $db = DBM::Deep->new(
1315 filter_store_value => \&my_filter_store,
1316 filter_fetch_value => \&my_filter_fetch
1321 $db->set_filter( "filter_store_value", \&my_filter_store );
1322 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
1324 Your filter function will be called only when dealing with SCALAR keys or
1325 values. When nested hashes and arrays are being stored/fetched, filtering
1326 is bypassed. Filters are called as static functions, passed a single SCALAR
1327 argument, and expected to return a single SCALAR value. If you want to
1328 remove a filter, set the function reference to C<undef>:
1330 $db->set_filter( "filter_store_value", undef );
1332 =head2 REAL-TIME ENCRYPTION EXAMPLE
1334 Here is a working example that uses the I<Crypt::Blowfish> module to
1335 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
1336 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
1337 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
1340 use Crypt::Blowfish;
1343 my $cipher = Crypt::CBC->new({
1344 'key' => 'my secret key',
1345 'cipher' => 'Blowfish',
1347 'regenerate_key' => 0,
1348 'padding' => 'space',
1352 my $db = DBM::Deep->new(
1353 file => "foo-encrypt.db",
1354 filter_store_key => \&my_encrypt,
1355 filter_store_value => \&my_encrypt,
1356 filter_fetch_key => \&my_decrypt,
1357 filter_fetch_value => \&my_decrypt,
1360 $db->{key1} = "value1";
1361 $db->{key2} = "value2";
1362 print "key1: " . $db->{key1} . "\n";
1363 print "key2: " . $db->{key2} . "\n";
1369 return $cipher->encrypt( $_[0] );
1372 return $cipher->decrypt( $_[0] );
1375 =head2 REAL-TIME COMPRESSION EXAMPLE
1377 Here is a working example that uses the I<Compress::Zlib> module to do real-time
1378 compression / decompression of keys & values with DBM::Deep Filters.
1379 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
1380 more on I<Compress::Zlib>.
1385 my $db = DBM::Deep->new(
1386 file => "foo-compress.db",
1387 filter_store_key => \&my_compress,
1388 filter_store_value => \&my_compress,
1389 filter_fetch_key => \&my_decompress,
1390 filter_fetch_value => \&my_decompress,
1393 $db->{key1} = "value1";
1394 $db->{key2} = "value2";
1395 print "key1: " . $db->{key1} . "\n";
1396 print "key2: " . $db->{key2} . "\n";
1402 return Compress::Zlib::memGzip( $_[0] ) ;
1405 return Compress::Zlib::memGunzip( $_[0] ) ;
1408 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
1409 actually numerical index numbers, and are not filtered.
1411 =head1 ERROR HANDLING
1413 Most DBM::Deep methods return a true value for success, and call die() on
1414 failure. You can wrap calls in an eval block to catch the die.
1416 my $db = DBM::Deep->new( "foo.db" ); # create hash
1417 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
1419 print $@; # prints error message
1421 =head1 LARGEFILE SUPPORT
1423 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
1424 and 64-bit support, you I<may> be able to create databases larger than 2 GB.
1425 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
1426 by calling the static C<set_pack()> method before you do anything else.
1428 DBM::Deep::set_pack(8, 'Q');
1430 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
1431 instead of 32-bit longs. After setting these values your DB files have a
1432 theoretical maximum size of 16 XB (exabytes).
1434 B<Note:> Changing these values will B<NOT> work for existing database files.
1435 Only change this for new files, and make sure it stays set consistently
1436 throughout the file's life. If you do set these values, you can no longer
1437 access 32-bit DB files. You can, however, call C<set_pack(4, 'N')> to change
1438 back to 32-bit mode.
1440 B<Note:> I have not personally tested files > 2 GB -- all my systems have
1441 only a 32-bit Perl. However, I have received user reports that this does
1444 =head1 LOW-LEVEL ACCESS
1446 If you require low-level access to the underlying filehandle that DBM::Deep uses,
1447 you can call the C<_fh()> method, which returns the handle:
1449 my $fh = $db->_fh();
1451 This method can be called on the root level of the datbase, or any child
1452 hashes or arrays. All levels share a I<root> structure, which contains things
1453 like the filehandle, a reference counter, and all the options specified
1454 when you created the object. You can get access to this root structure by
1455 calling the C<root()> method.
1457 my $root = $db->_root();
1459 This is useful for changing options after the object has already been created,
1460 such as enabling/disabling locking. You can also store your own temporary user
1461 data in this structure (be wary of name collision), which is then accessible from
1462 any child hash or array.
1464 =head1 CUSTOM DIGEST ALGORITHM
1466 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
1467 keys. However you can override this, and use another algorithm (such as SHA-256)
1468 or even write your own. But please note that DBM::Deep currently expects zero
1469 collisions, so your algorithm has to be I<perfect>, so to speak.
1470 Collision detection may be introduced in a later version.
1474 You can specify a custom digest algorithm by calling the static C<set_digest()>
1475 function, passing a reference to a subroutine, and the length of the algorithm's
1476 hashes (in bytes). This is a global static function, which affects ALL DBM::Deep
1477 objects. Here is a working example that uses a 256-bit hash from the
1478 I<Digest::SHA256> module. Please see
1479 L<http://search.cpan.org/search?module=Digest::SHA256> for more.
1484 my $context = Digest::SHA256::new(256);
1486 DBM::Deep::set_digest( \&my_digest, 32 );
1488 my $db = DBM::Deep->new( "foo-sha.db" );
1490 $db->{key1} = "value1";
1491 $db->{key2} = "value2";
1492 print "key1: " . $db->{key1} . "\n";
1493 print "key2: " . $db->{key2} . "\n";
1499 return substr( $context->hash($_[0]), 0, 32 );
1502 B<Note:> Your returned digest strings must be B<EXACTLY> the number
1503 of bytes you specify in the C<set_digest()> function (in this case 32).
1505 =head1 CIRCULAR REFERENCES
1507 DBM::Deep has B<experimental> support for circular references. Meaning you
1508 can have a nested hash key or array element that points to a parent object.
1509 This relationship is stored in the DB file, and is preserved between sessions.
1512 my $db = DBM::Deep->new( "foo.db" );
1515 $db->{circle} = $db; # ref to self
1517 print $db->{foo} . "\n"; # prints "foo"
1518 print $db->{circle}->{foo} . "\n"; # prints "foo" again
1520 One catch is, passing the object to a function that recursively walks the
1521 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
1522 C<export()> methods) will result in an infinite loop. The other catch is,
1523 if you fetch the I<key> of a circular reference (i.e. using the C<first_key()>
1524 or C<next_key()> methods), you will get the I<target object's key>, not the
1525 ref's key. This gets even more interesting with the above example, where
1526 the I<circle> key points to the base DB object, which technically doesn't
1527 have a key. So I made DBM::Deep return "[base]" as the key name in that
1530 =head1 CAVEATS / ISSUES / BUGS
1532 This section describes all the known issues with DBM::Deep. It you have found
1533 something that is not listed here, please send e-mail to L<jhuckaby@cpan.org>.
1535 =head2 UNUSED SPACE RECOVERY
1537 One major caveat with DBM::Deep is that space occupied by existing keys and
1538 values is not recovered when they are deleted. Meaning if you keep deleting
1539 and adding new keys, your file will continuously grow. I am working on this,
1540 but in the meantime you can call the built-in C<optimize()> method from time to
1541 time (perhaps in a crontab or something) to recover all your unused space.
1543 $db->optimize(); # returns true on success
1545 This rebuilds the ENTIRE database into a new file, then moves it on top of
1546 the original. The new file will have no unused space, thus it will take up as
1547 little disk space as possible. Please note that this operation can take
1548 a long time for large files, and you need enough disk space to temporarily hold
1549 2 copies of your DB file. The temporary file is created in the same directory
1550 as the original, named with a ".tmp" extension, and is deleted when the
1551 operation completes. Oh, and if locking is enabled, the DB is automatically
1552 locked for the entire duration of the copy.
1554 B<WARNING:> Only call optimize() on the top-level node of the database, and
1555 make sure there are no child references lying around. DBM::Deep keeps a reference
1556 counter, and if it is greater than 1, optimize() will abort and return undef.
1558 =head2 AUTOVIVIFICATION
1560 Unfortunately, autovivification doesn't work with tied hashes. This appears to
1561 be a bug in Perl's tie() system, as I<Jakob Schmidt> encountered the very same
1562 issue with his I<DWH_FIle> module (see L<http://search.cpan.org/search?module=DWH_File>),
1563 and it is also mentioned in the BUGS section for the I<MLDBM> module <see
1564 L<http://search.cpan.org/search?module=MLDBM>). Basically, on a new db file,
1567 $db->{foo}->{bar} = "hello";
1569 Since "foo" doesn't exist, you cannot add "bar" to it. You end up with "foo"
1570 being an empty hash. Try this instead, which works fine:
1572 $db->{foo} = { bar => "hello" };
1574 As of Perl 5.8.7, this bug still exists. I have walked very carefully through
1575 the execution path, and Perl indeed passes an empty hash to the STORE() method.
1576 Probably a bug in Perl.
1578 =head2 FILE CORRUPTION
1580 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1581 for a 32-bit signature when opened, but other corruption in files can cause
1582 segmentation faults. DBM::Deep may try to seek() past the end of a file, or get
1583 stuck in an infinite loop depending on the level of corruption. File write
1584 operations are not checked for failure (for speed), so if you happen to run
1585 out of disk space, DBM::Deep will probably fail in a bad way. These things will
1586 be addressed in a later version of DBM::Deep.
1590 Beware of using DB files over NFS. DBM::Deep uses flock(), which works well on local
1591 filesystems, but will NOT protect you from file corruption over NFS. I've heard
1592 about setting up your NFS server with a locking daemon, then using lockf() to
1593 lock your files, but your mileage may vary there as well. From what I
1594 understand, there is no real way to do it. However, if you need access to the
1595 underlying filehandle in DBM::Deep for using some other kind of locking scheme like
1596 lockf(), see the L<LOW-LEVEL ACCESS> section above.
1598 =head2 COPYING OBJECTS
1600 Beware of copying tied objects in Perl. Very strange things can happen.
1601 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1602 returns a new, blessed, tied hash or array to the same level in the DB.
1604 my $copy = $db->clone();
1606 B<Note>: Since clone() here is cloning the object, not the database location, any
1607 modifications to either $db or $copy will be visible in both.
1611 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1612 These functions cause every element in the array to move, which can be murder
1613 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1614 a different location. This will be addressed in the forthcoming version 1.00.
1616 =head2 WRITEONLY FILES
1618 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1619 writeonly mode. STORE will verify that the filehandle is writable. However, there
1620 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1621 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1625 This section discusses DBM::Deep's speed and memory usage.
1629 Obviously, DBM::Deep isn't going to be as fast as some C-based DBMs, such as
1630 the almighty I<BerkeleyDB>. But it makes up for it in features like true
1631 multi-level hash/array support, and cross-platform FTPable files. Even so,
1632 DBM::Deep is still pretty fast, and the speed stays fairly consistent, even
1633 with huge databases. Here is some test data:
1635 Adding 1,000,000 keys to new DB file...
1637 At 100 keys, avg. speed is 2,703 keys/sec
1638 At 200 keys, avg. speed is 2,642 keys/sec
1639 At 300 keys, avg. speed is 2,598 keys/sec
1640 At 400 keys, avg. speed is 2,578 keys/sec
1641 At 500 keys, avg. speed is 2,722 keys/sec
1642 At 600 keys, avg. speed is 2,628 keys/sec
1643 At 700 keys, avg. speed is 2,700 keys/sec
1644 At 800 keys, avg. speed is 2,607 keys/sec
1645 At 900 keys, avg. speed is 2,190 keys/sec
1646 At 1,000 keys, avg. speed is 2,570 keys/sec
1647 At 2,000 keys, avg. speed is 2,417 keys/sec
1648 At 3,000 keys, avg. speed is 1,982 keys/sec
1649 At 4,000 keys, avg. speed is 1,568 keys/sec
1650 At 5,000 keys, avg. speed is 1,533 keys/sec
1651 At 6,000 keys, avg. speed is 1,787 keys/sec
1652 At 7,000 keys, avg. speed is 1,977 keys/sec
1653 At 8,000 keys, avg. speed is 2,028 keys/sec
1654 At 9,000 keys, avg. speed is 2,077 keys/sec
1655 At 10,000 keys, avg. speed is 2,031 keys/sec
1656 At 20,000 keys, avg. speed is 1,970 keys/sec
1657 At 30,000 keys, avg. speed is 2,050 keys/sec
1658 At 40,000 keys, avg. speed is 2,073 keys/sec
1659 At 50,000 keys, avg. speed is 1,973 keys/sec
1660 At 60,000 keys, avg. speed is 1,914 keys/sec
1661 At 70,000 keys, avg. speed is 2,091 keys/sec
1662 At 80,000 keys, avg. speed is 2,103 keys/sec
1663 At 90,000 keys, avg. speed is 1,886 keys/sec
1664 At 100,000 keys, avg. speed is 1,970 keys/sec
1665 At 200,000 keys, avg. speed is 2,053 keys/sec
1666 At 300,000 keys, avg. speed is 1,697 keys/sec
1667 At 400,000 keys, avg. speed is 1,838 keys/sec
1668 At 500,000 keys, avg. speed is 1,941 keys/sec
1669 At 600,000 keys, avg. speed is 1,930 keys/sec
1670 At 700,000 keys, avg. speed is 1,735 keys/sec
1671 At 800,000 keys, avg. speed is 1,795 keys/sec
1672 At 900,000 keys, avg. speed is 1,221 keys/sec
1673 At 1,000,000 keys, avg. speed is 1,077 keys/sec
1675 This test was performed on a PowerMac G4 1gHz running Mac OS X 10.3.2 & Perl
1676 5.8.1, with an 80GB Ultra ATA/100 HD spinning at 7200RPM. The hash keys and
1677 values were between 6 - 12 chars in length. The DB file ended up at 210MB.
1678 Run time was 12 min 3 sec.
1682 One of the great things about DBM::Deep is that it uses very little memory.
1683 Even with huge databases (1,000,000+ keys) you will not see much increased
1684 memory on your process. DBM::Deep relies solely on the filesystem for storing
1685 and fetching data. Here is output from I</usr/bin/top> before even opening a
1688 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1689 22831 root 11 0 2716 2716 1296 R 0.0 0.2 0:07 perl
1691 Basically the process is taking 2,716K of memory. And here is the same
1692 process after storing and fetching 1,000,000 keys:
1694 PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1695 22831 root 14 0 2772 2772 1328 R 0.0 0.2 13:32 perl
1697 Notice the memory usage increased by only 56K. Test was performed on a 700mHz
1698 x86 box running Linux RedHat 7.2 & Perl 5.6.1.
1700 =head1 DB FILE FORMAT
1702 In case you were interested in the underlying DB file format, it is documented
1703 here in this section. You don't need to know this to use the module, it's just
1704 included for reference.
1708 DBM::Deep files always start with a 32-bit signature to identify the file type.
1709 This is at offset 0. The signature is "DPDB" in network byte order. This is
1710 checked for when the file is opened and an error will be thrown if it's not found.
1714 The DBM::Deep file is in a I<tagged format>, meaning each section of the file
1715 has a standard header containing the type of data, the length of data, and then
1716 the data itself. The type is a single character (1 byte), the length is a
1717 32-bit unsigned long in network byte order, and the data is, well, the data.
1718 Here is how it unfolds:
1722 Immediately after the 32-bit file signature is the I<Master Index> record.
1723 This is a standard tag header followed by 1024 bytes (in 32-bit mode) or 2048
1724 bytes (in 64-bit mode) of data. The type is I<H> for hash or I<A> for array,
1725 depending on how the DBM::Deep object was constructed.
1727 The index works by looking at a I<MD5 Hash> of the hash key (or array index
1728 number). The first 8-bit char of the MD5 signature is the offset into the
1729 index, multipled by 4 in 32-bit mode, or 8 in 64-bit mode. The value of the
1730 index element is a file offset of the next tag for the key/element in question,
1731 which is usually a I<Bucket List> tag (see below).
1733 The next tag I<could> be another index, depending on how many keys/elements
1734 exist. See L<RE-INDEXING> below for details.
1738 A I<Bucket List> is a collection of 16 MD5 hashes for keys/elements, plus
1739 file offsets to where the actual data is stored. It starts with a standard
1740 tag header, with type I<B>, and a data size of 320 bytes in 32-bit mode, or
1741 384 bytes in 64-bit mode. Each MD5 hash is stored in full (16 bytes), plus
1742 the 32-bit or 64-bit file offset for the I<Bucket> containing the actual data.
1743 When the list fills up, a I<Re-Index> operation is performed (See
1744 L<RE-INDEXING> below).
1748 A I<Bucket> is a tag containing a key/value pair (in hash mode), or a
1749 index/value pair (in array mode). It starts with a standard tag header with
1750 type I<D> for scalar data (string, binary, etc.), or it could be a nested
1751 hash (type I<H>) or array (type I<A>). The value comes just after the tag
1752 header. The size reported in the tag header is only for the value, but then,
1753 just after the value is another size (32-bit unsigned long) and then the plain
1754 key itself. Since the value is likely to be fetched more often than the plain
1755 key, I figured it would be I<slightly> faster to store the value first.
1757 If the type is I<H> (hash) or I<A> (array), the value is another I<Master Index>
1758 record for the nested structure, where the process begins all over again.
1762 After a I<Bucket List> grows to 16 records, its allocated space in the file is
1763 exhausted. Then, when another key/element comes in, the list is converted to a
1764 new index record. However, this index will look at the next char in the MD5
1765 hash, and arrange new Bucket List pointers accordingly. This process is called
1766 I<Re-Indexing>. Basically, a new index tag is created at the file EOF, and all
1767 17 (16 + new one) keys/elements are removed from the old Bucket List and
1768 inserted into the new index. Several new Bucket Lists are created in the
1769 process, as a new MD5 char from the key is being examined (it is unlikely that
1770 the keys will all share the same next char of their MD5s).
1772 Because of the way the I<MD5> algorithm works, it is impossible to tell exactly
1773 when the Bucket Lists will turn into indexes, but the first round tends to
1774 happen right around 4,000 keys. You will see a I<slight> decrease in
1775 performance here, but it picks back up pretty quick (see L<SPEED> above). Then
1776 it takes B<a lot> more keys to exhaust the next level of Bucket Lists. It's
1777 right around 900,000 keys. This process can continue nearly indefinitely --
1778 right up until the point the I<MD5> signatures start colliding with each other,
1779 and this is B<EXTREMELY> rare -- like winning the lottery 5 times in a row AND
1780 getting struck by lightning while you are walking to cash in your tickets.
1781 Theoretically, since I<MD5> hashes are 128-bit values, you I<could> have up to
1782 340,282,366,921,000,000,000,000,000,000,000,000,000 keys/elements (I believe
1783 this is 340 unodecillion, but don't quote me).
1787 When a new key/element is stored, the key (or index number) is first run through
1788 I<Digest::MD5> to get a 128-bit signature (example, in hex:
1789 b05783b0773d894396d475ced9d2f4f6). Then, the I<Master Index> record is checked
1790 for the first char of the signature (in this case I<b0>). If it does not exist,
1791 a new I<Bucket List> is created for our key (and the next 15 future keys that
1792 happen to also have I<b> as their first MD5 char). The entire MD5 is written
1793 to the I<Bucket List> along with the offset of the new I<Bucket> record (EOF at
1794 this point, unless we are replacing an existing I<Bucket>), where the actual
1795 data will be stored.
1799 Fetching an existing key/element involves getting a I<Digest::MD5> of the key
1800 (or index number), then walking along the indexes. If there are enough
1801 keys/elements in this DB level, there might be nested indexes, each linked to
1802 a particular char of the MD5. Finally, a I<Bucket List> is pointed to, which
1803 contains up to 16 full MD5 hashes. Each is checked for equality to the key in
1804 question. If we found a match, the I<Bucket> tag is loaded, where the value and
1805 plain key are stored.
1807 Fetching the plain key occurs when calling the I<first_key()> and I<next_key()>
1808 methods. In this process the indexes are walked systematically, and each key
1809 fetched in increasing MD5 order (which is why it appears random). Once the
1810 I<Bucket> is found, the value is skipped and the plain key returned instead.
1811 B<Note:> Do not count on keys being fetched as if the MD5 hashes were
1812 alphabetically sorted. This only happens on an index-level -- as soon as the
1813 I<Bucket Lists> are hit, the keys will come out in the order they went in --
1814 so it's pretty much undefined how the keys will come out -- just like Perl's
1817 =head1 CODE COVERAGE
1819 We use B<Devel::Cover> to test the code coverage of our tests, below is the
1820 B<Devel::Cover> report on this module's test suite.
1822 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1823 File stmt bran cond sub pod time total
1824 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1825 blib/lib/DBM/Deep.pm 95.1 81.6 70.3 100.0 100.0 33.4 91.0
1826 blib/lib/DBM/Deep/Array.pm 100.0 91.1 100.0 100.0 n/a 27.8 98.0
1827 blib/lib/DBM/Deep/Engine.pm 97.8 85.6 75.0 100.0 0.0 25.8 90.8
1828 blib/lib/DBM/Deep/Hash.pm 100.0 87.5 100.0 100.0 n/a 13.0 97.2
1829 Total 97.5 85.4 76.6 100.0 46.9 100.0 92.5
1830 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1832 =head1 MORE INFORMATION
1834 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1835 or send email to L<DBM-Deep@googlegroups.com>.
1839 Joseph Huckaby, L<jhuckaby@cpan.org>
1841 Rob Kinyon, L<rkinyon@cpan.org>
1843 Special thanks to Adam Sah and Rich Gaushell! You know why :-)
1847 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1848 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1852 Copyright (c) 2002-2006 Joseph Huckaby. All Rights Reserved.
1853 This is free software, you may use it and distribute it under the
1854 same terms as Perl itself.