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1 | package DBM::Deep::Engine; |
2 | |
3 | use 5.006_000; |
4 | |
5 | use strict; |
6 | use warnings FATAL => 'all'; |
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7 | no warnings 'recursion'; |
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8 | |
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9 | use DBM::Deep::Iterator (); |
10 | |
11 | # File-wide notes: |
12 | # * Every method in here assumes that the storage has been appropriately |
13 | # safeguarded. This can be anything from flock() to some sort of manual |
14 | # mutex. But, it's the caller's responsability to make sure that this has |
15 | # been done. |
16 | |
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17 | sub SIG_HASH () { 'H' } |
18 | sub SIG_ARRAY () { 'A' } |
19 | |
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20 | =head1 NAME |
21 | |
22 | DBM::Deep::Engine |
23 | |
24 | =head1 PURPOSE |
25 | |
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26 | This is an internal-use-only object for L<DBM::Deep>. It mediates the low-level |
27 | mapping between the L<DBM::Deep> objects and the storage medium. |
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28 | |
29 | The purpose of this documentation is to provide low-level documentation for |
30 | developers. It is B<not> intended to be used by the general public. This |
31 | documentation and what it documents can and will change without notice. |
32 | |
33 | =head1 OVERVIEW |
34 | |
35 | The engine exposes an API to the DBM::Deep objects (DBM::Deep, DBM::Deep::Array, |
36 | and DBM::Deep::Hash) for their use to access the actual stored values. This API |
37 | is the following: |
38 | |
39 | =over 4 |
40 | |
41 | =item * new |
42 | |
43 | =item * read_value |
44 | |
45 | =item * get_classname |
46 | |
47 | =item * make_reference |
48 | |
49 | =item * key_exists |
50 | |
51 | =item * delete_key |
52 | |
53 | =item * write_value |
54 | |
55 | =item * get_next_key |
56 | |
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57 | =item * setup |
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58 | |
59 | =item * begin_work |
60 | |
61 | =item * commit |
62 | |
63 | =item * rollback |
64 | |
65 | =item * lock_exclusive |
66 | |
67 | =item * lock_shared |
68 | |
69 | =item * unlock |
70 | |
71 | =back |
72 | |
73 | They are explained in their own sections below. These methods, in turn, may |
74 | provide some bounds-checking, but primarily act to instantiate objects in the |
75 | Engine::Sector::* hierarchy and dispatch to them. |
76 | |
77 | =head1 TRANSACTIONS |
78 | |
79 | Transactions in DBM::Deep are implemented using a variant of MVCC. This attempts |
80 | to keep the amount of actual work done against the file low while stil providing |
81 | Atomicity, Consistency, and Isolation. Durability, unfortunately, cannot be done |
82 | with only one file. |
83 | |
84 | =head2 STALENESS |
85 | |
86 | If another process uses a transaction slot and writes stuff to it, then |
87 | terminates, the data that process wrote it still within the file. In order to |
88 | address this, there is also a transaction staleness counter associated within |
89 | every write. Each time a transaction is started, that process increments that |
90 | transaction's staleness counter. If, when it reads a value, the staleness |
91 | counters aren't identical, DBM::Deep will consider the value on disk to be stale |
92 | and discard it. |
93 | |
94 | =head2 DURABILITY |
95 | |
96 | The fourth leg of ACID is Durability, the guarantee that when a commit returns, |
97 | the data will be there the next time you read from it. This should be regardless |
98 | of any crashes or powerdowns in between the commit and subsequent read. |
99 | DBM::Deep does provide that guarantee; once the commit returns, all of the data |
100 | has been transferred from the transaction shadow to the HEAD. The issue arises |
101 | with partial commits - a commit that is interrupted in some fashion. In keeping |
102 | with DBM::Deep's "tradition" of very light error-checking and non-existent |
103 | error-handling, there is no way to recover from a partial commit. (This is |
104 | probably a failure in Consistency as well as Durability.) |
105 | |
106 | Other DBMSes use transaction logs (a separate file, generally) to achieve |
107 | Durability. As DBM::Deep is a single-file, we would have to do something |
108 | similar to what SQLite and BDB do in terms of committing using synchonized |
109 | writes. To do this, we would have to use a much higher RAM footprint and some |
110 | serious programming that make my head hurts just to think about it. |
111 | |
112 | =cut |
113 | |
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114 | =head1 METHODS |
115 | |
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116 | =head2 read_value( $obj, $key ) |
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117 | |
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118 | This takes an object that provides _base_offset() and a string. It returns the |
119 | value stored in the corresponding Sector::Value's data section. |
120 | |
121 | =cut |
122 | |
123 | sub read_value { die "read_value must be implemented in a child class" } |
124 | |
125 | =head2 get_classname( $obj ) |
126 | |
127 | This takes an object that provides _base_offset() and returns the classname (if |
128 | any) associated with it. |
129 | |
130 | It delegates to Sector::Reference::get_classname() for the heavy lifting. |
131 | |
132 | It performs a staleness check. |
133 | |
134 | =cut |
135 | |
136 | sub get_classname { die "get_classname must be implemented in a child class" } |
137 | |
138 | =head2 make_reference( $obj, $old_key, $new_key ) |
139 | |
140 | This takes an object that provides _base_offset() and two strings. The |
141 | strings correspond to the old key and new key, respectively. This operation |
142 | is equivalent to (given C<< $db->{foo} = []; >>) C<< $db->{bar} = $db->{foo} >>. |
143 | |
144 | This returns nothing. |
145 | |
146 | =cut |
147 | |
148 | sub make_reference { die "make_reference must be implemented in a child class" } |
149 | |
150 | =head2 key_exists( $obj, $key ) |
151 | |
152 | This takes an object that provides _base_offset() and a string for |
153 | the key to be checked. This returns 1 for true and "" for false. |
154 | |
155 | =cut |
156 | |
157 | sub key_exists { die "key_exists must be implemented in a child class" } |
158 | |
159 | =head2 delete_key( $obj, $key ) |
160 | |
161 | This takes an object that provides _base_offset() and a string for |
162 | the key to be deleted. This returns the result of the Sector::Reference |
163 | delete_key() method. |
164 | |
165 | =cut |
166 | |
167 | sub delete_key { die "delete_key must be implemented in a child class" } |
168 | |
169 | =head2 write_value( $obj, $key, $value ) |
170 | |
171 | This takes an object that provides _base_offset(), a string for the |
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172 | key, and a value. This value can be anything storable within L<DBM::Deep>. |
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173 | |
174 | This returns 1 upon success. |
175 | |
176 | =cut |
177 | |
178 | sub write_value { die "write_value must be implemented in a child class" } |
179 | |
180 | =head2 setup( $obj ) |
181 | |
182 | This takes an object that provides _base_offset(). It will do everything needed |
183 | in order to properly initialize all values for necessary functioning. If this is |
184 | called upon an already initialized object, this will also reset the inode. |
185 | |
186 | This returns 1. |
187 | |
188 | =cut |
189 | |
190 | sub setup { die "setup must be implemented in a child class" } |
191 | |
192 | =head2 begin_work( $obj ) |
193 | |
194 | This takes an object that provides _base_offset(). It will set up all necessary |
195 | bookkeeping in order to run all work within a transaction. |
196 | |
197 | If $obj is already within a transaction, an error wiill be thrown. If there are |
198 | no more available transactions, an error will be thrown. |
199 | |
200 | This returns undef. |
201 | |
202 | =cut |
203 | |
204 | sub begin_work { die "begin_work must be implemented in a child class" } |
205 | |
206 | =head2 rollback( $obj ) |
207 | |
208 | This takes an object that provides _base_offset(). It will revert all |
209 | actions taken within the running transaction. |
210 | |
211 | If $obj is not within a transaction, an error will be thrown. |
212 | |
213 | This returns 1. |
214 | |
215 | =cut |
216 | |
217 | sub rollback { die "rollback must be implemented in a child class" } |
218 | |
219 | =head2 commit( $obj ) |
220 | |
221 | This takes an object that provides _base_offset(). It will apply all |
222 | actions taken within the transaction to the HEAD. |
223 | |
224 | If $obj is not within a transaction, an error will be thrown. |
225 | |
226 | This returns 1. |
227 | |
228 | =cut |
229 | |
230 | sub commit { die "commit must be implemented in a child class" } |
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231 | |
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232 | =head2 get_next_key( $obj, $prev_key ) |
233 | |
234 | This takes an object that provides _base_offset() and an optional string |
235 | representing the prior key returned via a prior invocation of this method. |
236 | |
237 | This method delegates to C<< DBM::Deep::Iterator->get_next_key() >>. |
238 | |
239 | =cut |
240 | |
241 | # XXX Add staleness here |
242 | sub get_next_key { |
243 | my $self = shift; |
244 | my ($obj, $prev_key) = @_; |
245 | |
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246 | # XXX Need to add logic about resetting the iterator if any key in the |
247 | # reference has changed |
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248 | unless ( defined $prev_key ) { |
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249 | eval "use " . $self->iterator_class; die $@ if $@; |
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250 | $obj->{iterator} = $self->iterator_class->new({ |
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251 | base_offset => $obj->_base_offset, |
252 | engine => $self, |
253 | }); |
254 | } |
255 | |
256 | return $obj->{iterator}->get_next_key( $obj ); |
257 | } |
258 | |
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259 | =head2 lock_exclusive() |
260 | |
261 | This takes an object that provides _base_offset(). It will guarantee that |
262 | the storage has taken precautions to be safe for a write. |
263 | |
264 | This returns nothing. |
265 | |
266 | =cut |
267 | |
268 | sub lock_exclusive { |
269 | my $self = shift; |
270 | my ($obj) = @_; |
271 | return $self->storage->lock_exclusive( $obj ); |
272 | } |
273 | |
274 | =head2 lock_shared() |
275 | |
276 | This takes an object that provides _base_offset(). It will guarantee that |
277 | the storage has taken precautions to be safe for a read. |
278 | |
279 | This returns nothing. |
280 | |
281 | =cut |
282 | |
283 | sub lock_shared { |
284 | my $self = shift; |
285 | my ($obj) = @_; |
286 | return $self->storage->lock_shared( $obj ); |
287 | } |
288 | |
289 | =head2 unlock() |
290 | |
291 | This takes an object that provides _base_offset(). It will guarantee that |
292 | the storage has released the most recently-taken lock. |
293 | |
294 | This returns nothing. |
295 | |
296 | =cut |
297 | |
298 | sub unlock { |
299 | my $self = shift; |
300 | my ($obj) = @_; |
301 | |
302 | my $rv = $self->storage->unlock( $obj ); |
303 | |
304 | $self->flush if $rv; |
305 | |
306 | return $rv; |
307 | } |
308 | |
309 | =head1 INTERNAL METHODS |
310 | |
311 | The following methods are internal-use-only to DBM::Deep::Engine and its |
312 | child classes. |
313 | |
314 | =cut |
315 | |
316 | =head2 flush() |
317 | |
318 | This takes no arguments. It will do everything necessary to flush all things to |
319 | disk. This is usually called during unlock() and setup(). |
320 | |
321 | This returns nothing. |
322 | |
323 | =cut |
324 | |
325 | sub flush { |
326 | my $self = shift; |
327 | |
328 | # Why do we need to have the storage flush? Shouldn't autoflush take care of |
329 | # things? -RobK, 2008-06-26 |
330 | $self->storage->flush; |
331 | |
332 | return; |
333 | } |
334 | |
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335 | =head2 load_sector( $loc ) |
336 | |
337 | This takes an id/location/offset and loads the sector based on the engine's |
338 | defined sector type. |
339 | |
340 | =cut |
341 | |
342 | sub load_sector { $_[0]->sector_type->load( @_ ) } |
343 | |
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344 | =head2 clear( $obj ) |
345 | |
346 | This takes an object that provides _base_offset() and deletes all its |
347 | elements, returning nothing. |
348 | |
349 | =cut |
350 | |
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351 | sub clear { die "clear must be implemented in a child class" } |
352 | |
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353 | =head2 cache / clear_cache |
354 | |
355 | This is the cache of loaded Reference sectors. |
356 | |
357 | =cut |
358 | |
359 | sub cache { $_[0]{cache} ||= {} } |
360 | sub clear_cache { %{$_[0]->cache} = () } |
361 | |
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362 | =head2 supports( $option ) |
363 | |
364 | This returns a boolean depending on if this instance of DBM::Dep supports |
365 | that feature. C<$option> can be one of: |
366 | |
367 | =over 4 |
368 | |
369 | =item * transactions |
370 | |
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371 | =item * singletons |
372 | |
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373 | =back |
374 | |
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375 | Any other value will return false. |
376 | |
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377 | =cut |
378 | |
379 | sub supports { die "supports must be implemented in a child class" } |
380 | |
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381 | =head1 ACCESSORS |
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382 | |
383 | The following are readonly attributes. |
384 | |
385 | =over 4 |
386 | |
387 | =item * storage |
388 | |
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389 | =item * sector_type |
390 | |
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391 | =item * iterator_class |
392 | |
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393 | =back |
394 | |
395 | =cut |
396 | |
397 | sub storage { $_[0]{storage} } |
398 | |
399 | sub sector_type { die "sector_type must be implemented in a child class" } |
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400 | sub iterator_class { die "iterator_class must be implemented in a child class" } |
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401 | |
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402 | # This code is to make sure we write all the values in the $value to the |
403 | # disk and to make sure all changes to $value after the assignment are |
404 | # reflected on disk. This may be counter-intuitive at first, but it is |
405 | # correct dwimmery. |
406 | # NOTE - simply tying $value won't perform a STORE on each value. Hence, |
407 | # the copy to a temp value. |
408 | sub _descend { |
409 | my $self = shift; |
410 | my ($value, $value_sector) = @_; |
411 | my $r = Scalar::Util::reftype( $value ) || ''; |
412 | |
413 | if ( $r eq 'ARRAY' ) { |
414 | my @temp = @$value; |
415 | tie @$value, 'DBM::Deep', { |
416 | base_offset => $value_sector->offset, |
417 | staleness => $value_sector->staleness, |
418 | storage => $self->storage, |
419 | engine => $self, |
420 | }; |
421 | @$value = @temp; |
422 | bless $value, 'DBM::Deep::Array' unless Scalar::Util::blessed( $value ); |
423 | } |
424 | elsif ( $r eq 'HASH' ) { |
425 | my %temp = %$value; |
426 | tie %$value, 'DBM::Deep', { |
427 | base_offset => $value_sector->offset, |
428 | staleness => $value_sector->staleness, |
429 | storage => $self->storage, |
430 | engine => $self, |
431 | }; |
432 | %$value = %temp; |
433 | bless $value, 'DBM::Deep::Hash' unless Scalar::Util::blessed( $value ); |
434 | } |
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435 | |
436 | return; |
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437 | } |
438 | |
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439 | 1; |
440 | __END__ |