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