3 DBM::Deep - A pure perl multi-level hash/array DBM that supports transactions
8 my $db = DBM::Deep->new( "foo.db" );
13 $db->put('key' => 'value');
14 print $db->get('key');
16 # true multi-level support
18 'hello', { perl => 'rules' },
29 tie my %db, 'DBM::Deep', 'foo.db';
33 tied(%db)->put('key' => 'value');
34 print tied(%db)->get('key');
38 A unique flat-file database module, written in pure perl. True multi-level
39 hash/array support (unlike MLDBM, which is faked), hybrid OO / tie()
40 interface, cross-platform FTPable files, ACID transactions, and is quite fast.
41 Can handle millions of keys and unlimited levels without significant
42 slow-down. Written from the ground-up in pure perl -- this is NOT a wrapper
43 around a C-based DBM. Out-of-the-box compatibility with Unix, Mac OS X and
46 =head1 VERSION DIFFERENCES
48 B<NOTE>: 0.99_03 has significant file format differences from prior versions.
49 THere will be a backwards-compatibility layer in 1.00, but that is slated for
50 a later 0.99_x release. This version is B<NOT> backwards compatible with any
51 other release of DBM::Deep.
53 B<NOTE>: 0.99_01 and above have significant file format differences from 0.983 and
54 before. There will be a backwards-compatibility layer in 1.00, but that is
55 slated for a later 0.99_x release. This version is B<NOT> backwards compatible
56 with 0.983 and before.
60 Construction can be done OO-style (which is the recommended way), or using
61 Perl's tie() function. Both are examined here.
63 =head2 OO Construction
65 The recommended way to construct a DBM::Deep object is to use the new()
66 method, which gets you a blessed I<and> tied hash (or array) reference.
68 my $db = DBM::Deep->new( "foo.db" );
70 This opens a new database handle, mapped to the file "foo.db". If this
71 file does not exist, it will automatically be created. DB files are
72 opened in "r+" (read/write) mode, and the type of object returned is a
73 hash, unless otherwise specified (see L<OPTIONS> below).
75 You can pass a number of options to the constructor to specify things like
76 locking, autoflush, etc. This is done by passing an inline hash (or hashref):
78 my $db = DBM::Deep->new(
84 Notice that the filename is now specified I<inside> the hash with
85 the "file" parameter, as opposed to being the sole argument to the
86 constructor. This is required if any options are specified.
87 See L<OPTIONS> below for the complete list.
89 You can also start with an array instead of a hash. For this, you must
90 specify the C<type> parameter:
92 my $db = DBM::Deep->new(
94 type => DBM::Deep->TYPE_ARRAY
97 B<Note:> Specifing the C<type> parameter only takes effect when beginning
98 a new DB file. If you create a DBM::Deep object with an existing file, the
99 C<type> will be loaded from the file header, and an error will be thrown if
100 the wrong type is passed in.
102 =head2 Tie Construction
104 Alternately, you can create a DBM::Deep handle by using Perl's built-in
105 tie() function. The object returned from tie() can be used to call methods,
106 such as lock() and unlock(). (That object can be retrieved from the tied
107 variable at any time using tied() - please see L<perltie/> for more info.
110 my $db = tie %hash, "DBM::Deep", "foo.db";
113 my $db = tie @array, "DBM::Deep", "bar.db";
115 As with the OO constructor, you can replace the DB filename parameter with
116 a hash containing one or more options (see L<OPTIONS> just below for the
119 tie %hash, "DBM::Deep", {
127 There are a number of options that can be passed in when constructing your
128 DBM::Deep objects. These apply to both the OO- and tie- based approaches.
134 Filename of the DB file to link the handle to. You can pass a full absolute
135 filesystem path, partial path, or a plain filename if the file is in the
136 current working directory. This is a required parameter (though q.v. fh).
140 If you want, you can pass in the fh instead of the file. This is most useful for doing
143 my $db = DBM::Deep->new( { fh => \*DATA } );
145 You are responsible for making sure that the fh has been opened appropriately for your
146 needs. If you open it read-only and attempt to write, an exception will be thrown. If you
147 open it write-only or append-only, an exception will be thrown immediately as DBM::Deep
148 needs to read from the fh.
152 This is the offset within the file that the DBM::Deep db starts. Most of the time, you will
153 not need to set this. However, it's there if you want it.
155 If you pass in fh and do not set this, it will be set appropriately.
159 This parameter specifies what type of object to create, a hash or array. Use
160 one of these two constants:
164 =item * C<DBM::Deep-E<gt>TYPE_HASH>
166 =item * C<DBM::Deep-E<gt>TYPE_ARRAY>.
170 This only takes effect when beginning a new file. This is an optional
171 parameter, and defaults to C<DBM::Deep-E<gt>TYPE_HASH>.
175 Specifies whether locking is to be enabled. DBM::Deep uses Perl's flock()
176 function to lock the database in exclusive mode for writes, and shared mode
177 for reads. Pass any true value to enable. This affects the base DB handle
178 I<and any child hashes or arrays> that use the same DB file. This is an
179 optional parameter, and defaults to 1 (enabled). See L<LOCKING> below for
184 Specifies whether autoflush is to be enabled on the underlying filehandle.
185 This obviously slows down write operations, but is required if you may have
186 multiple processes accessing the same DB file (also consider enable I<locking>).
187 Pass any true value to enable. This is an optional parameter, and defaults to 1
192 See L</FILTERS> below.
196 The following parameters may be specified in the constructor the first time the
197 datafile is created. However, they will be stored in the header of the file and
198 cannot be overridden by subsequent openings of the file - the values will be set
199 from the values stored in the datafile's header.
205 This is the number of transactions that can be running at one time. The
206 default is one - the HEAD. The minimum is one and the maximum is 255. The more
207 transactions, the larger and quicker the datafile grows.
209 See L</TRANSACTIONS> below.
213 This is the number of entries that can be added before a reindexing. The larger
214 this number is made, the larger a file gets, but the better performance you will
215 have. The default and minimum number this can be is 16. The maximum is 256, but
216 more than 64 isn't recommended.
218 =item * data_sector_size
220 This is the size in bytes of a given data sector. Data sectors will chain, so
221 a value of any size can be stored. However, chaining is expensive in terms of
222 time. Setting this value to something close to the expected common length of
223 your scalars will improve your performance. If it is too small, your file will
224 have a lot of chaining. If it is too large, your file will have a lot of dead
227 The default for this is 64 bytes. The minimum value is 32 and the maximum is
230 B<Note:> There are between 6 and 10 bytes taken up in each data sector for
231 bookkeeping. (It's 4 + the number of bytes in your L</pack_size>.) This is
232 included within the data_sector_size, thus the effective value is 6-10 bytes
233 less than what you specified.
237 This is the size of the file pointer used throughout the file. The valid values
244 This uses 2-byte offsets, allowing for a maximum file size of 65 KB.
246 =item * medium (default)
248 This uses 4-byte offsets, allowing for a maximum file size of 4 GB.
252 This uses 8-byte offsets, allowing for a maximum file size of 16 XB
253 (exabytes). This can only be enabled if your Perl is compiled for 64-bit.
257 See L</LARGEFILE SUPPORT> for more information.
263 With DBM::Deep you can access your databases using Perl's standard hash/array
264 syntax. Because all DBM::Deep objects are I<tied> to hashes or arrays, you can
265 treat them as such. DBM::Deep will intercept all reads/writes and direct them
266 to the right place -- the DB file. This has nothing to do with the
267 L<TIE CONSTRUCTION> section above. This simply tells you how to use DBM::Deep
268 using regular hashes and arrays, rather than calling functions like C<get()>
269 and C<put()> (although those work too). It is entirely up to you how to want
270 to access your databases.
274 You can treat any DBM::Deep object like a normal Perl hash reference. Add keys,
275 or even nested hashes (or arrays) using standard Perl syntax:
277 my $db = DBM::Deep->new( "foo.db" );
279 $db->{mykey} = "myvalue";
281 $db->{myhash}->{subkey} = "subvalue";
283 print $db->{myhash}->{subkey} . "\n";
285 You can even step through hash keys using the normal Perl C<keys()> function:
287 foreach my $key (keys %$db) {
288 print "$key: " . $db->{$key} . "\n";
291 Remember that Perl's C<keys()> function extracts I<every> key from the hash and
292 pushes them onto an array, all before the loop even begins. If you have an
293 extremely large hash, this may exhaust Perl's memory. Instead, consider using
294 Perl's C<each()> function, which pulls keys/values one at a time, using very
297 while (my ($key, $value) = each %$db) {
298 print "$key: $value\n";
301 Please note that when using C<each()>, you should always pass a direct
302 hash reference, not a lookup. Meaning, you should B<never> do this:
305 while (my ($key, $value) = each %{$db->{foo}}) { # BAD
307 This causes an infinite loop, because for each iteration, Perl is calling
308 FETCH() on the $db handle, resulting in a "new" hash for foo every time, so
309 it effectively keeps returning the first key over and over again. Instead,
310 assign a temporary variable to C<$db->{foo}>, then pass that to each().
314 As with hashes, you can treat any DBM::Deep object like a normal Perl array
315 reference. This includes inserting, removing and manipulating elements,
316 and the C<push()>, C<pop()>, C<shift()>, C<unshift()> and C<splice()> functions.
317 The object must have first been created using type C<DBM::Deep-E<gt>TYPE_ARRAY>,
318 or simply be a nested array reference inside a hash. Example:
320 my $db = DBM::Deep->new(
321 file => "foo-array.db",
322 type => DBM::Deep->TYPE_ARRAY
326 push @$db, "bar", "baz";
329 my $last_elem = pop @$db; # baz
330 my $first_elem = shift @$db; # bah
331 my $second_elem = $db->[1]; # bar
333 my $num_elements = scalar @$db;
337 In addition to the I<tie()> interface, you can also use a standard OO interface
338 to manipulate all aspects of DBM::Deep databases. Each type of object (hash or
339 array) has its own methods, but both types share the following common methods:
340 C<put()>, C<get()>, C<exists()>, C<delete()> and C<clear()>. C<fetch()> and
341 C<store(> are aliases to C<put()> and C<get()>, respectively.
345 =item * new() / clone()
347 These are the constructor and copy-functions.
349 =item * put() / store()
351 Stores a new hash key/value pair, or sets an array element value. Takes two
352 arguments, the hash key or array index, and the new value. The value can be
353 a scalar, hash ref or array ref. Returns true on success, false on failure.
355 $db->put("foo", "bar"); # for hashes
356 $db->put(1, "bar"); # for arrays
358 =item * get() / fetch()
360 Fetches the value of a hash key or array element. Takes one argument: the hash
361 key or array index. Returns a scalar, hash ref or array ref, depending on the
364 my $value = $db->get("foo"); # for hashes
365 my $value = $db->get(1); # for arrays
369 Checks if a hash key or array index exists. Takes one argument: the hash key
370 or array index. Returns true if it exists, false if not.
372 if ($db->exists("foo")) { print "yay!\n"; } # for hashes
373 if ($db->exists(1)) { print "yay!\n"; } # for arrays
377 Deletes one hash key/value pair or array element. Takes one argument: the hash
378 key or array index. Returns true on success, false if not found. For arrays,
379 the remaining elements located after the deleted element are NOT moved over.
380 The deleted element is essentially just undefined, which is exactly how Perl's
381 internal arrays work.
383 $db->delete("foo"); # for hashes
384 $db->delete(1); # for arrays
388 Deletes B<all> hash keys or array elements. Takes no arguments. No return
391 $db->clear(); # hashes or arrays
393 =item * lock() / unlock()
399 Recover lost disk space. This is important to do, especially if you use
402 =item * import() / export()
404 Data going in and out.
406 =item * begin_work() / commit() / rollback()
408 These are the transactional functions. L</TRANSACTIONS> for more information.
414 For hashes, DBM::Deep supports all the common methods described above, and the
415 following additional methods: C<first_key()> and C<next_key()>.
421 Returns the "first" key in the hash. As with built-in Perl hashes, keys are
422 fetched in an undefined order (which appears random). Takes no arguments,
423 returns the key as a scalar value.
425 my $key = $db->first_key();
429 Returns the "next" key in the hash, given the previous one as the sole argument.
430 Returns undef if there are no more keys to be fetched.
432 $key = $db->next_key($key);
436 Here are some examples of using hashes:
438 my $db = DBM::Deep->new( "foo.db" );
440 $db->put("foo", "bar");
441 print "foo: " . $db->get("foo") . "\n";
443 $db->put("baz", {}); # new child hash ref
444 $db->get("baz")->put("buz", "biz");
445 print "buz: " . $db->get("baz")->get("buz") . "\n";
447 my $key = $db->first_key();
449 print "$key: " . $db->get($key) . "\n";
450 $key = $db->next_key($key);
453 if ($db->exists("foo")) { $db->delete("foo"); }
457 For arrays, DBM::Deep supports all the common methods described above, and the
458 following additional methods: C<length()>, C<push()>, C<pop()>, C<shift()>,
459 C<unshift()> and C<splice()>.
465 Returns the number of elements in the array. Takes no arguments.
467 my $len = $db->length();
471 Adds one or more elements onto the end of the array. Accepts scalars, hash
472 refs or array refs. No return value.
474 $db->push("foo", "bar", {});
478 Fetches the last element in the array, and deletes it. Takes no arguments.
479 Returns undef if array is empty. Returns the element value.
481 my $elem = $db->pop();
485 Fetches the first element in the array, deletes it, then shifts all the
486 remaining elements over to take up the space. Returns the element value. This
487 method is not recommended with large arrays -- see L<LARGE ARRAYS> below for
490 my $elem = $db->shift();
494 Inserts one or more elements onto the beginning of the array, shifting all
495 existing elements over to make room. Accepts scalars, hash refs or array refs.
496 No return value. This method is not recommended with large arrays -- see
497 <LARGE ARRAYS> below for details.
499 $db->unshift("foo", "bar", {});
503 Performs exactly like Perl's built-in function of the same name. See L<perldoc
504 -f splice> for usage -- it is too complicated to document here. This method is
505 not recommended with large arrays -- see L<LARGE ARRAYS> below for details.
509 Here are some examples of using arrays:
511 my $db = DBM::Deep->new(
513 type => DBM::Deep->TYPE_ARRAY
516 $db->push("bar", "baz");
520 my $len = $db->length();
521 print "length: $len\n"; # 4
523 for (my $k=0; $k<$len; $k++) {
524 print "$k: " . $db->get($k) . "\n";
527 $db->splice(1, 2, "biz", "baf");
529 while (my $elem = shift @$db) {
530 print "shifted: $elem\n";
535 Enable or disable automatic file locking by passing a boolean value to the
536 C<locking> parameter when constructing your DBM::Deep object (see L<SETUP>
539 my $db = DBM::Deep->new(
544 This causes DBM::Deep to C<flock()> the underlying filehandle with exclusive
545 mode for writes, and shared mode for reads. This is required if you have
546 multiple processes accessing the same database file, to avoid file corruption.
547 Please note that C<flock()> does NOT work for files over NFS. See L<DB OVER
550 =head2 Explicit Locking
552 You can explicitly lock a database, so it remains locked for multiple
553 actions. This is done by calling the C<lock()> method, and passing an
554 optional lock mode argument (defaults to exclusive mode). This is particularly
555 useful for things like counters, where the current value needs to be fetched,
556 then incremented, then stored again.
559 my $counter = $db->get("counter");
561 $db->put("counter", $counter);
570 You can pass C<lock()> an optional argument, which specifies which mode to use
571 (exclusive or shared). Use one of these two constants:
572 C<DBM::Deep-E<gt>LOCK_EX> or C<DBM::Deep-E<gt>LOCK_SH>. These are passed
573 directly to C<flock()>, and are the same as the constants defined in Perl's
576 $db->lock( $db->LOCK_SH );
580 =head1 IMPORTING/EXPORTING
582 You can import existing complex structures by calling the C<import()> method,
583 and export an entire database into an in-memory structure using the C<export()>
584 method. Both are examined here.
588 Say you have an existing hash with nested hashes/arrays inside it. Instead of
589 walking the structure and adding keys/elements to the database as you go,
590 simply pass a reference to the C<import()> method. This recursively adds
591 everything to an existing DBM::Deep object for you. Here is an example:
596 array1 => [ "elem0", "elem1", "elem2" ],
598 subkey1 => "subvalue1",
599 subkey2 => "subvalue2"
603 my $db = DBM::Deep->new( "foo.db" );
604 $db->import( $struct );
606 print $db->{key1} . "\n"; # prints "value1"
608 This recursively imports the entire C<$struct> object into C<$db>, including
609 all nested hashes and arrays. If the DBM::Deep object contains exsiting data,
610 keys are merged with the existing ones, replacing if they already exist.
611 The C<import()> method can be called on any database level (not just the base
612 level), and works with both hash and array DB types.
614 B<Note:> Make sure your existing structure has no circular references in it.
615 These will cause an infinite loop when importing. There are plans to fix this
620 Calling the C<export()> method on an existing DBM::Deep object will return
621 a reference to a new in-memory copy of the database. The export is done
622 recursively, so all nested hashes/arrays are all exported to standard Perl
623 objects. Here is an example:
625 my $db = DBM::Deep->new( "foo.db" );
627 $db->{key1} = "value1";
628 $db->{key2} = "value2";
630 $db->{hash1}->{subkey1} = "subvalue1";
631 $db->{hash1}->{subkey2} = "subvalue2";
633 my $struct = $db->export();
635 print $struct->{key1} . "\n"; # prints "value1"
637 This makes a complete copy of the database in memory, and returns a reference
638 to it. The C<export()> method can be called on any database level (not just
639 the base level), and works with both hash and array DB types. Be careful of
640 large databases -- you can store a lot more data in a DBM::Deep object than an
641 in-memory Perl structure.
643 B<Note:> Make sure your database has no circular references in it.
644 These will cause an infinite loop when exporting. There are plans to fix this
649 DBM::Deep has a number of hooks where you can specify your own Perl function
650 to perform filtering on incoming or outgoing data. This is a perfect
651 way to extend the engine, and implement things like real-time compression or
652 encryption. Filtering applies to the base DB level, and all child hashes /
653 arrays. Filter hooks can be specified when your DBM::Deep object is first
654 constructed, or by calling the C<set_filter()> method at any time. There are
655 four available filter hooks, described below:
659 =item * filter_store_key
661 This filter is called whenever a hash key is stored. It
662 is passed the incoming key, and expected to return a transformed key.
664 =item * filter_store_value
666 This filter is called whenever a hash key or array element is stored. It
667 is passed the incoming value, and expected to return a transformed value.
669 =item * filter_fetch_key
671 This filter is called whenever a hash key is fetched (i.e. via
672 C<first_key()> or C<next_key()>). It is passed the transformed key,
673 and expected to return the plain key.
675 =item * filter_fetch_value
677 This filter is called whenever a hash key or array element is fetched.
678 It is passed the transformed value, and expected to return the plain value.
682 Here are the two ways to setup a filter hook:
684 my $db = DBM::Deep->new(
686 filter_store_value => \&my_filter_store,
687 filter_fetch_value => \&my_filter_fetch
692 $db->set_filter( "filter_store_value", \&my_filter_store );
693 $db->set_filter( "filter_fetch_value", \&my_filter_fetch );
695 Your filter function will be called only when dealing with SCALAR keys or
696 values. When nested hashes and arrays are being stored/fetched, filtering
697 is bypassed. Filters are called as static functions, passed a single SCALAR
698 argument, and expected to return a single SCALAR value. If you want to
699 remove a filter, set the function reference to C<undef>:
701 $db->set_filter( "filter_store_value", undef );
703 =head2 Real-time Encryption Example
705 Here is a working example that uses the I<Crypt::Blowfish> module to
706 do real-time encryption / decryption of keys & values with DBM::Deep Filters.
707 Please visit L<http://search.cpan.org/search?module=Crypt::Blowfish> for more
708 on I<Crypt::Blowfish>. You'll also need the I<Crypt::CBC> module.
714 my $cipher = Crypt::CBC->new({
715 'key' => 'my secret key',
716 'cipher' => 'Blowfish',
718 'regenerate_key' => 0,
719 'padding' => 'space',
723 my $db = DBM::Deep->new(
724 file => "foo-encrypt.db",
725 filter_store_key => \&my_encrypt,
726 filter_store_value => \&my_encrypt,
727 filter_fetch_key => \&my_decrypt,
728 filter_fetch_value => \&my_decrypt,
731 $db->{key1} = "value1";
732 $db->{key2} = "value2";
733 print "key1: " . $db->{key1} . "\n";
734 print "key2: " . $db->{key2} . "\n";
740 return $cipher->encrypt( $_[0] );
743 return $cipher->decrypt( $_[0] );
746 =head2 Real-time Compression Example
748 Here is a working example that uses the I<Compress::Zlib> module to do real-time
749 compression / decompression of keys & values with DBM::Deep Filters.
750 Please visit L<http://search.cpan.org/search?module=Compress::Zlib> for
751 more on I<Compress::Zlib>.
756 my $db = DBM::Deep->new(
757 file => "foo-compress.db",
758 filter_store_key => \&my_compress,
759 filter_store_value => \&my_compress,
760 filter_fetch_key => \&my_decompress,
761 filter_fetch_value => \&my_decompress,
764 $db->{key1} = "value1";
765 $db->{key2} = "value2";
766 print "key1: " . $db->{key1} . "\n";
767 print "key2: " . $db->{key2} . "\n";
773 return Compress::Zlib::memGzip( $_[0] ) ;
776 return Compress::Zlib::memGunzip( $_[0] ) ;
779 B<Note:> Filtering of keys only applies to hashes. Array "keys" are
780 actually numerical index numbers, and are not filtered.
782 =head1 ERROR HANDLING
784 Most DBM::Deep methods return a true value for success, and call die() on
785 failure. You can wrap calls in an eval block to catch the die.
787 my $db = DBM::Deep->new( "foo.db" ); # create hash
788 eval { $db->push("foo"); }; # ILLEGAL -- push is array-only call
790 print $@; # prints error message
792 =head1 LARGEFILE SUPPORT
794 If you have a 64-bit system, and your Perl is compiled with both LARGEFILE
795 and 64-bit support, you I<may> be able to create databases larger than 4 GB.
796 DBM::Deep by default uses 32-bit file offset tags, but these can be changed
797 by specifying the 'pack_size' parameter when constructing the file.
800 filename => $filename,
801 pack_size => 'large',
804 This tells DBM::Deep to pack all file offsets with 8-byte (64-bit) quad words
805 instead of 32-bit longs. After setting these values your DB files have a
806 theoretical maximum size of 16 XB (exabytes).
808 You can also use C<pack_size =E<gt> 'small'> in order to use 16-bit file
811 B<Note:> Changing these values will B<NOT> work for existing database files.
812 Only change this for new files. Once the value has been set, it is stored in
813 the file's header and cannot be changed for the life of the file. These
814 parameters are per-file, meaning you can access 32-bit and 64-bit files, as
817 B<Note:> We have not personally tested files larger than 4 GB -- all my
818 systems have only a 32-bit Perl. However, I have received user reports that
819 this does indeed work.
821 =head1 LOW-LEVEL ACCESS
823 If you require low-level access to the underlying filehandle that DBM::Deep uses,
824 you can call the C<_fh()> method, which returns the handle:
828 This method can be called on the root level of the datbase, or any child
829 hashes or arrays. All levels share a I<root> structure, which contains things
830 like the filehandle, a reference counter, and all the options specified
831 when you created the object. You can get access to this file object by
832 calling the C<_storage()> method.
834 my $file_obj = $db->_storage();
836 This is useful for changing options after the object has already been created,
837 such as enabling/disabling locking. You can also store your own temporary user
838 data in this structure (be wary of name collision), which is then accessible from
839 any child hash or array.
841 =head1 CUSTOM DIGEST ALGORITHM
843 DBM::Deep by default uses the I<Message Digest 5> (MD5) algorithm for hashing
844 keys. However you can override this, and use another algorithm (such as SHA-256)
845 or even write your own. But please note that DBM::Deep currently expects zero
846 collisions, so your algorithm has to be I<perfect>, so to speak. Collision
847 detection may be introduced in a later version.
849 You can specify a custom digest algorithm by passing it into the parameter
850 list for new(), passing a reference to a subroutine as the 'digest' parameter,
851 and the length of the algorithm's hashes (in bytes) as the 'hash_size'
852 parameter. Here is a working example that uses a 256-bit hash from the
853 I<Digest::SHA256> module. Please see
854 L<http://search.cpan.org/search?module=Digest::SHA256> for more information.
859 my $context = Digest::SHA256::new(256);
861 my $db = DBM::Deep->new(
862 filename => "foo-sha.db",
863 digest => \&my_digest,
867 $db->{key1} = "value1";
868 $db->{key2} = "value2";
869 print "key1: " . $db->{key1} . "\n";
870 print "key2: " . $db->{key2} . "\n";
876 return substr( $context->hash($_[0]), 0, 32 );
879 B<Note:> Your returned digest strings must be B<EXACTLY> the number
880 of bytes you specify in the hash_size parameter (in this case 32).
882 B<Note:> If you do choose to use a custom digest algorithm, you must set it
883 every time you access this file. Otherwise, the default (MD5) will be used.
885 =head1 CIRCULAR REFERENCES
887 B<NOTE>: DBM::Deep 0.99_03 has turned off circular references pending
888 evaluation of some edge cases. I hope to be able to re-enable circular
889 references in a future version after 1.00. This means that circular references
890 are B<NO LONGER> available.
892 DBM::Deep has B<experimental> support for circular references. Meaning you
893 can have a nested hash key or array element that points to a parent object.
894 This relationship is stored in the DB file, and is preserved between sessions.
897 my $db = DBM::Deep->new( "foo.db" );
900 $db->{circle} = $db; # ref to self
902 print $db->{foo} . "\n"; # prints "bar"
903 print $db->{circle}->{foo} . "\n"; # prints "bar" again
905 B<Note>: Passing the object to a function that recursively walks the
906 object tree (such as I<Data::Dumper> or even the built-in C<optimize()> or
907 C<export()> methods) will result in an infinite loop. This will be fixed in
912 New in 0.99_01 is ACID transactions. Every DBM::Deep object is completely
913 transaction-ready - it is not an option you have to turn on. You do have to
914 specify how many transactions may run simultaneously (q.v. L</num_txns>).
916 Three new methods have been added to support them. They are:
922 This starts a transaction.
926 This applies the changes done within the transaction to the mainline and ends
931 This discards the changes done within the transaction to the mainline and ends
936 Transactions in DBM::Deep are done using a variant of the MVCC method, the
937 same method used by the InnoDB MySQL engine.
941 Because DBM::Deep is a conncurrent datastore, every change is flushed to disk
942 immediately and every read goes to disk. This means that DBM::Deep functions
943 at the speed of disk (generally 10-20ms) vs. the speed of RAM (generally
944 50-70ns), or at least 150-200x slower than the comparable in-memory
945 datastructure in Perl.
947 There are several techniques you can use to speed up how DBM::Deep functions.
951 =item * Put it on a ramdisk
953 The easiest and quickest mechanism to making DBM::Deep run faster is to create
954 a ramdisk and locate the DBM::Deep file there. Doing this as an option may
955 become a feature of DBM::Deep, assuming there is a good ramdisk wrapper on CPAN.
957 =item * Work at the tightest level possible
959 It is much faster to assign the level of your db that you are working with to
960 an intermediate variable than to re-look it up every time. Thus
963 while ( my ($k, $v) = each %{$db->{foo}{bar}{baz}} ) {
968 my $x = $db->{foo}{bar}{baz};
969 while ( my ($k, $v) = each %$x ) {
973 =item * Make your file as tight as possible
975 If you know that you are not going to use more than 65K in your database,
976 consider using the C<pack_size =E<gt> 'small'> option. This will instruct
977 DBM::Deep to use 16bit addresses, meaning that the seek times will be less.
983 As of 1.0000, the file format has changed. Furthermore, DBM::Deep is now
984 designed to potentially change file format between point-releases, if needed to
985 support a requested feature. To aid in this, a migration script is provided
986 within the CPAN distribution called C<utils/upgrade_db.pl>.
988 B<NOTE:> This script is not installed onto your system because it carries a copy
989 of every version prior to the current version.
993 The following are items that are planned to be added in future releases. These
994 are separate from the L<CAVEATS, ISSUES & BUGS> below.
996 =head2 Sub-Transactions
998 Right now, you cannot run a transaction within a transaction. Removing this
999 restriction is technically straightforward, but the combinatorial explosion of
1000 possible usecases hurts my head. If this is something you want to see
1001 immediately, please submit many testcases.
1005 If a user is willing to assert upon opening the file that this process will be
1006 the only consumer of that datafile, then there are a number of caching
1007 possibilities that can be taken advantage of. This does, however, mean that
1008 DBM::Deep is more vulnerable to losing data due to unflushed changes. It also
1009 means a much larger in-memory footprint. As such, it's not clear exactly how
1010 this should be done. Suggestions are welcome.
1014 The techniques used in DBM::Deep simply require a seekable contiguous
1015 datastore. This could just as easily be a large string as a file. By using
1016 substr, the STM capabilities of DBM::Deep could be used within a
1017 single-process. I have no idea how I'd specify this, though. Suggestions are
1020 =head2 Importing using Data::Walker
1022 Right now, importing is done using C<Clone::clone()> to make a complete copy
1023 in memory, then tying that copy. It would be much better to use
1024 L<Data::Walker/> to walk the data structure instead, particularly in the case
1025 of large datastructures.
1027 =head2 Different contention resolution mechanisms
1029 Currently, the only contention resolution mechanism is last-write-wins. This
1030 is the mechanism used by most RDBMSes and should be good enough for most uses.
1031 For advanced uses of STM, other contention mechanisms will be needed. If you
1032 have an idea of how you'd like to see contention resolution in DBM::Deep,
1035 =head1 CAVEATS, ISSUES & BUGS
1037 This section describes all the known issues with DBM::Deep. These are issues
1038 that are either intractable or depend on some feature within Perl working
1039 exactly right. It you have found something that is not listed below, please
1040 send an e-mail to L<rkinyon@cpan.org>. Likewise, if you think you know of a
1041 way around one of these issues, please let me know.
1045 (The following assumes a high level of Perl understanding, specifically of
1046 references. Most users can safely skip this section.)
1048 Currently, the only references supported are HASH and ARRAY. The other reference
1049 types (SCALAR, CODE, GLOB, and REF) cannot be supported for various reasons.
1055 These are things like filehandles and other sockets. They can't be supported
1056 because it's completely unclear how DBM::Deep should serialize them.
1058 =item * SCALAR / REF
1060 The discussion here refers to the following type of example:
1067 # In some other process ...
1069 my $val = ${ $db->{key1} };
1071 is( $val, 50, "What actually gets stored in the DB file?" );
1073 The problem is one of synchronization. When the variable being referred to
1074 changes value, the reference isn't notified, which is kind of the point of
1075 references. This means that the new value won't be stored in the datafile for
1076 other processes to read. There is no TIEREF.
1078 It is theoretically possible to store references to values already within a
1079 DBM::Deep object because everything already is synchronized, but the change to
1080 the internals would be quite large. Specifically, DBM::Deep would have to tie
1081 every single value that is stored. This would bloat the RAM footprint of
1082 DBM::Deep at least twofold (if not more) and be a significant performance drain,
1083 all to support a feature that has never been requested.
1087 L<Data::Dump::Streamer/> provides a mechanism for serializing coderefs,
1088 including saving off all closure state. This would allow for DBM::Deep to
1089 store the code for a subroutine. Then, whenever the subroutine is read, the
1090 code could be C<eval()>'ed into being. However, just as for SCALAR and REF,
1091 that closure state may change without notifying the DBM::Deep object storing
1092 the reference. Again, this would generally be considered a feature.
1096 =head2 File corruption
1098 The current level of error handling in DBM::Deep is minimal. Files I<are> checked
1099 for a 32-bit signature when opened, but any other form of corruption in the
1100 datafile can cause segmentation faults. DBM::Deep may try to C<seek()> past
1101 the end of a file, or get stuck in an infinite loop depending on the level and
1102 type of corruption. File write operations are not checked for failure (for
1103 speed), so if you happen to run out of disk space, DBM::Deep will probably fail in
1104 a bad way. These things will be addressed in a later version of DBM::Deep.
1108 Beware of using DBM::Deep files over NFS. DBM::Deep uses flock(), which works
1109 well on local filesystems, but will NOT protect you from file corruption over
1110 NFS. I've heard about setting up your NFS server with a locking daemon, then
1111 using C<lockf()> to lock your files, but your mileage may vary there as well.
1112 From what I understand, there is no real way to do it. However, if you need
1113 access to the underlying filehandle in DBM::Deep for using some other kind of
1114 locking scheme like C<lockf()>, see the L<LOW-LEVEL ACCESS> section above.
1116 =head2 Copying Objects
1118 Beware of copying tied objects in Perl. Very strange things can happen.
1119 Instead, use DBM::Deep's C<clone()> method which safely copies the object and
1120 returns a new, blessed and tied hash or array to the same level in the DB.
1122 my $copy = $db->clone();
1124 B<Note>: Since clone() here is cloning the object, not the database location, any
1125 modifications to either $db or $copy will be visible to both.
1129 Beware of using C<shift()>, C<unshift()> or C<splice()> with large arrays.
1130 These functions cause every element in the array to move, which can be murder
1131 on DBM::Deep, as every element has to be fetched from disk, then stored again in
1132 a different location. This will be addressed in a future version.
1134 =head2 Writeonly Files
1136 If you pass in a filehandle to new(), you may have opened it in either a readonly or
1137 writeonly mode. STORE will verify that the filehandle is writable. However, there
1138 doesn't seem to be a good way to determine if a filehandle is readable. And, if the
1139 filehandle isn't readable, it's not clear what will happen. So, don't do that.
1141 =head2 Assignments Within Transactions
1143 The following will I<not> work as one might expect:
1151 is( $x->{a}, 1 ); # This will fail!
1153 The problem is that the moment a reference used as the rvalue to a DBM::Deep
1154 object's lvalue, it becomes tied itself. This is so that future changes to
1155 C<$x> can be tracked within the DBM::Deep file and is considered to be a
1156 feature. By the time the rollback occurs, there is no knowledge that there had
1157 been an C<$x> or what memory location to assign an C<export()> to.
1159 B<NOTE:> This does not affect importing because imports do a walk over the
1160 reference to be imported in order to explicitly leave it untied.
1162 =head1 CODE COVERAGE
1164 B<Devel::Cover> is used to test the code coverage of the tests. Below is the
1165 B<Devel::Cover> report on this distribution's test suite.
1167 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1168 File stmt bran cond sub pod time total
1169 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1170 blib/lib/DBM/Deep.pm 96.8 87.9 90.5 100.0 89.5 4.5 95.2
1171 blib/lib/DBM/Deep/Array.pm 100.0 94.3 100.0 100.0 100.0 4.8 98.7
1172 blib/lib/DBM/Deep/Engine.pm 97.2 86.4 86.0 100.0 0.0 56.8 91.0
1173 blib/lib/DBM/Deep/File.pm 98.1 83.3 66.7 100.0 0.0 31.4 88.0
1174 blib/lib/DBM/Deep/Hash.pm 100.0 100.0 100.0 100.0 100.0 2.5 100.0
1175 Total 97.7 88.1 86.6 100.0 31.6 100.0 93.0
1176 ---------------------------- ------ ------ ------ ------ ------ ------ ------
1178 =head1 MORE INFORMATION
1180 Check out the DBM::Deep Google Group at L<http://groups.google.com/group/DBM-Deep>
1181 or send email to L<DBM-Deep@googlegroups.com>. You can also visit #dbm-deep on
1184 The source code repository is at L<http://svn.perl.org/modules/DBM-Deep>
1188 Rob Kinyon, L<rkinyon@cpan.org>
1190 Originally written by Joseph Huckaby, L<jhuckaby@cpan.org>
1194 Stonehenge Consulting (L<http://www.stonehenge.com/>) sponsored the
1195 developement of transactions and freespace management, leading to the 1.0000
1196 release. A great debt of gratitude goes out to them for their continuing
1197 leadership in and support of the Perl community.
1201 The following have contributed greatly to make DBM::Deep what it is today:
1205 =item * Adam Sah and Rich Gaushell for innumerable contributions early on.
1207 =item * Dan Golden and others at YAPC::NA 2006 for helping me design through transactions.
1213 perltie(1), Tie::Hash(3), Digest::MD5(3), Fcntl(3), flock(2), lockf(3), nfs(5),
1214 Digest::SHA256(3), Crypt::Blowfish(3), Compress::Zlib(3)
1218 Copyright (c) 2007 Rob Kinyon. All Rights Reserved.
1219 This is free software, you may use it and distribute it under the same terms