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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 | =head2 read_value( $obj, $key ) |
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115 | |
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116 | This takes an object that provides _base_offset() and a string. It returns the |
117 | value stored in the corresponding Sector::Value's data section. |
118 | |
119 | =cut |
120 | |
121 | sub read_value { die "read_value must be implemented in a child class" } |
122 | |
123 | =head2 get_classname( $obj ) |
124 | |
125 | This takes an object that provides _base_offset() and returns the classname (if |
126 | any) associated with it. |
127 | |
128 | It delegates to Sector::Reference::get_classname() for the heavy lifting. |
129 | |
130 | It performs a staleness check. |
131 | |
132 | =cut |
133 | |
134 | sub get_classname { die "get_classname must be implemented in a child class" } |
135 | |
136 | =head2 make_reference( $obj, $old_key, $new_key ) |
137 | |
138 | This takes an object that provides _base_offset() and two strings. The |
139 | strings correspond to the old key and new key, respectively. This operation |
140 | is equivalent to (given C<< $db->{foo} = []; >>) C<< $db->{bar} = $db->{foo} >>. |
141 | |
142 | This returns nothing. |
143 | |
144 | =cut |
145 | |
146 | sub make_reference { die "make_reference must be implemented in a child class" } |
147 | |
148 | =head2 key_exists( $obj, $key ) |
149 | |
150 | This takes an object that provides _base_offset() and a string for |
151 | the key to be checked. This returns 1 for true and "" for false. |
152 | |
153 | =cut |
154 | |
155 | sub key_exists { die "key_exists must be implemented in a child class" } |
156 | |
157 | =head2 delete_key( $obj, $key ) |
158 | |
159 | This takes an object that provides _base_offset() and a string for |
160 | the key to be deleted. This returns the result of the Sector::Reference |
161 | delete_key() method. |
162 | |
163 | =cut |
164 | |
165 | sub delete_key { die "delete_key must be implemented in a child class" } |
166 | |
167 | =head2 write_value( $obj, $key, $value ) |
168 | |
169 | This takes an object that provides _base_offset(), a string for the |
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170 | key, and a value. This value can be anything storable within L<DBM::Deep>. |
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171 | |
172 | This returns 1 upon success. |
173 | |
174 | =cut |
175 | |
176 | sub write_value { die "write_value must be implemented in a child class" } |
177 | |
178 | =head2 setup( $obj ) |
179 | |
180 | This takes an object that provides _base_offset(). It will do everything needed |
181 | in order to properly initialize all values for necessary functioning. If this is |
182 | called upon an already initialized object, this will also reset the inode. |
183 | |
184 | This returns 1. |
185 | |
186 | =cut |
187 | |
188 | sub setup { die "setup must be implemented in a child class" } |
189 | |
190 | =head2 begin_work( $obj ) |
191 | |
192 | This takes an object that provides _base_offset(). It will set up all necessary |
193 | bookkeeping in order to run all work within a transaction. |
194 | |
195 | If $obj is already within a transaction, an error wiill be thrown. If there are |
196 | no more available transactions, an error will be thrown. |
197 | |
198 | This returns undef. |
199 | |
200 | =cut |
201 | |
202 | sub begin_work { die "begin_work must be implemented in a child class" } |
203 | |
204 | =head2 rollback( $obj ) |
205 | |
206 | This takes an object that provides _base_offset(). It will revert all |
207 | actions taken within the running transaction. |
208 | |
209 | If $obj is not within a transaction, an error will be thrown. |
210 | |
211 | This returns 1. |
212 | |
213 | =cut |
214 | |
215 | sub rollback { die "rollback must be implemented in a child class" } |
216 | |
217 | =head2 commit( $obj ) |
218 | |
219 | This takes an object that provides _base_offset(). It will apply all |
220 | actions taken within the transaction to the HEAD. |
221 | |
222 | If $obj is not within a transaction, an error will be thrown. |
223 | |
224 | This returns 1. |
225 | |
226 | =cut |
227 | |
228 | sub commit { die "commit must be implemented in a child class" } |
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229 | |
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230 | =head2 get_next_key( $obj, $prev_key ) |
231 | |
232 | This takes an object that provides _base_offset() and an optional string |
233 | representing the prior key returned via a prior invocation of this method. |
234 | |
235 | This method delegates to C<< DBM::Deep::Iterator->get_next_key() >>. |
236 | |
237 | =cut |
238 | |
239 | # XXX Add staleness here |
240 | sub get_next_key { |
241 | my $self = shift; |
242 | my ($obj, $prev_key) = @_; |
243 | |
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244 | # XXX Need to add logic about resetting the iterator if any key in the |
245 | # reference has changed |
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246 | unless ( defined $prev_key ) { |
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247 | $obj->{iterator} = $self->iterator_class->new({ |
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248 | base_offset => $obj->_base_offset, |
249 | engine => $self, |
250 | }); |
251 | } |
252 | |
253 | return $obj->{iterator}->get_next_key( $obj ); |
254 | } |
255 | |
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256 | =head2 lock_exclusive() |
257 | |
258 | This takes an object that provides _base_offset(). It will guarantee that |
259 | the storage has taken precautions to be safe for a write. |
260 | |
261 | This returns nothing. |
262 | |
263 | =cut |
264 | |
265 | sub lock_exclusive { |
266 | my $self = shift; |
267 | my ($obj) = @_; |
268 | return $self->storage->lock_exclusive( $obj ); |
269 | } |
270 | |
271 | =head2 lock_shared() |
272 | |
273 | This takes an object that provides _base_offset(). It will guarantee that |
274 | the storage has taken precautions to be safe for a read. |
275 | |
276 | This returns nothing. |
277 | |
278 | =cut |
279 | |
280 | sub lock_shared { |
281 | my $self = shift; |
282 | my ($obj) = @_; |
283 | return $self->storage->lock_shared( $obj ); |
284 | } |
285 | |
286 | =head2 unlock() |
287 | |
288 | This takes an object that provides _base_offset(). It will guarantee that |
289 | the storage has released the most recently-taken lock. |
290 | |
291 | This returns nothing. |
292 | |
293 | =cut |
294 | |
295 | sub unlock { |
296 | my $self = shift; |
297 | my ($obj) = @_; |
298 | |
299 | my $rv = $self->storage->unlock( $obj ); |
300 | |
301 | $self->flush if $rv; |
302 | |
303 | return $rv; |
304 | } |
305 | |
306 | =head1 INTERNAL METHODS |
307 | |
308 | The following methods are internal-use-only to DBM::Deep::Engine and its |
309 | child classes. |
310 | |
311 | =cut |
312 | |
313 | =head2 flush() |
314 | |
315 | This takes no arguments. It will do everything necessary to flush all things to |
316 | disk. This is usually called during unlock() and setup(). |
317 | |
318 | This returns nothing. |
319 | |
320 | =cut |
321 | |
322 | sub flush { |
323 | my $self = shift; |
324 | |
325 | # Why do we need to have the storage flush? Shouldn't autoflush take care of |
326 | # things? -RobK, 2008-06-26 |
327 | $self->storage->flush; |
328 | |
329 | return; |
330 | } |
331 | |
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332 | =head2 load_sector( $loc ) |
333 | |
334 | This takes an id/location/offset and loads the sector based on the engine's |
335 | defined sector type. |
336 | |
337 | =cut |
338 | |
339 | sub load_sector { $_[0]->sector_type->load( @_ ) } |
340 | |
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341 | =head2 clear |
342 | |
343 | =cut |
344 | |
345 | =head2 clear( $obj ) |
346 | |
347 | This takes an object that provides _base_offset() and deletes all its |
348 | elements, returning nothing. |
349 | |
350 | =cut |
351 | |
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352 | =head2 cache / clear_cache |
353 | |
354 | This is the cache of loaded Reference sectors. |
355 | |
356 | =cut |
357 | |
358 | sub cache { $_[0]{cache} ||= {} } |
359 | sub clear_cache { %{$_[0]->cache} = () } |
360 | |
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361 | =head2 supports( $option ) |
362 | |
363 | This returns a boolean depending on if this instance of DBM::Dep supports |
364 | that feature. C<$option> can be one of: |
365 | |
366 | =over 4 |
367 | |
368 | =item * transactions |
369 | |
370 | =back |
371 | |
372 | =cut |
373 | |
374 | sub supports { die "supports must be implemented in a child class" } |
375 | |
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376 | =head2 ACCESSORS |
377 | |
378 | The following are readonly attributes. |
379 | |
380 | =over 4 |
381 | |
382 | =item * storage |
383 | |
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384 | =item * sector_type |
385 | |
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386 | =back |
387 | |
388 | =cut |
389 | |
390 | sub storage { $_[0]{storage} } |
391 | |
392 | sub sector_type { die "sector_type must be implemented in a child class" } |
393 | |
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394 | # This code is to make sure we write all the values in the $value to the |
395 | # disk and to make sure all changes to $value after the assignment are |
396 | # reflected on disk. This may be counter-intuitive at first, but it is |
397 | # correct dwimmery. |
398 | # NOTE - simply tying $value won't perform a STORE on each value. Hence, |
399 | # the copy to a temp value. |
400 | sub _descend { |
401 | my $self = shift; |
402 | my ($value, $value_sector) = @_; |
403 | my $r = Scalar::Util::reftype( $value ) || ''; |
404 | |
405 | if ( $r eq 'ARRAY' ) { |
406 | my @temp = @$value; |
407 | tie @$value, 'DBM::Deep', { |
408 | base_offset => $value_sector->offset, |
409 | staleness => $value_sector->staleness, |
410 | storage => $self->storage, |
411 | engine => $self, |
412 | }; |
413 | @$value = @temp; |
414 | bless $value, 'DBM::Deep::Array' unless Scalar::Util::blessed( $value ); |
415 | } |
416 | elsif ( $r eq 'HASH' ) { |
417 | my %temp = %$value; |
418 | tie %$value, 'DBM::Deep', { |
419 | base_offset => $value_sector->offset, |
420 | staleness => $value_sector->staleness, |
421 | storage => $self->storage, |
422 | engine => $self, |
423 | }; |
424 | %$value = %temp; |
425 | bless $value, 'DBM::Deep::Hash' unless Scalar::Util::blessed( $value ); |
426 | } |
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427 | |
428 | return; |
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429 | } |
430 | |
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431 | 1; |
432 | __END__ |