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