1 package DBM::Deep::Engine;
6 use warnings FATAL => 'all';
7 no warnings 'recursion';
9 use DBM::Deep::Iterator ();
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
17 sub SIG_HASH () { 'H' }
18 sub SIG_ARRAY () { 'A' }
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.
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.
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
47 =item * make_reference
65 =item * lock_exclusive
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.
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
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
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.)
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.
114 =head2 read_value( $obj, $key )
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.
121 sub read_value { die "read_value must be implemented in a child class" }
123 =head2 get_classname( $obj )
125 This takes an object that provides _base_offset() and returns the classname (if
126 any) associated with it.
128 It delegates to Sector::Reference::get_classname() for the heavy lifting.
130 It performs a staleness check.
134 sub get_classname { die "get_classname must be implemented in a child class" }
136 =head2 make_reference( $obj, $old_key, $new_key )
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} >>.
142 This returns nothing.
146 sub make_reference { die "make_reference must be implemented in a child class" }
148 =head2 key_exists( $obj, $key )
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.
155 sub key_exists { die "key_exists must be implemented in a child class" }
157 =head2 delete_key( $obj, $key )
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
165 sub delete_key { die "delete_key must be implemented in a child class" }
167 =head2 write_value( $obj, $key, $value )
169 This takes an object that provides _base_offset(), a string for the
170 key, and a value. This value can be anything storable within L<DBM::Deep>.
172 This returns 1 upon success.
176 sub write_value { die "write_value must be implemented in a child class" }
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.
188 sub setup { die "setup must be implemented in a child class" }
190 =head2 begin_work( $obj )
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.
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.
202 sub begin_work { die "begin_work must be implemented in a child class" }
204 =head2 rollback( $obj )
206 This takes an object that provides _base_offset(). It will revert all
207 actions taken within the running transaction.
209 If $obj is not within a transaction, an error will be thrown.
215 sub rollback { die "rollback must be implemented in a child class" }
217 =head2 commit( $obj )
219 This takes an object that provides _base_offset(). It will apply all
220 actions taken within the transaction to the HEAD.
222 If $obj is not within a transaction, an error will be thrown.
228 sub commit { die "commit must be implemented in a child class" }
230 =head2 get_next_key( $obj, $prev_key )
232 This takes an object that provides _base_offset() and an optional string
233 representing the prior key returned via a prior invocation of this method.
235 This method delegates to C<< DBM::Deep::Iterator->get_next_key() >>.
239 # XXX Add staleness here
242 my ($obj, $prev_key) = @_;
244 # XXX Need to add logic about resetting the iterator if any key in the
245 # reference has changed
246 unless ( defined $prev_key ) {
247 $obj->{iterator} = $self->iterator_class->new({
248 base_offset => $obj->_base_offset,
253 return $obj->{iterator}->get_next_key( $obj );
256 =head2 lock_exclusive()
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.
261 This returns nothing.
268 return $self->storage->lock_exclusive( $obj );
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.
276 This returns nothing.
283 return $self->storage->lock_shared( $obj );
288 This takes an object that provides _base_offset(). It will guarantee that
289 the storage has released the most recently-taken lock.
291 This returns nothing.
299 my $rv = $self->storage->unlock( $obj );
306 =head1 INTERNAL METHODS
308 The following methods are internal-use-only to DBM::Deep::Engine and its
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().
318 This returns nothing.
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;
332 =head2 load_sector( $loc )
334 This takes an id/location/offset and loads the sector based on the engine's
339 sub load_sector { $_[0]->sector_type->load( @_ ) }
347 This takes an object that provides _base_offset() and deletes all its
348 elements, returning nothing.
352 =head2 cache / clear_cache
354 This is the cache of loaded Reference sectors.
358 sub cache { $_[0]{cache} ||= {} }
359 sub clear_cache { %{$_[0]->cache} = () }
361 =head2 supports( $option )
363 This returns a boolean depending on if this instance of DBM::Dep supports
364 that feature. C<$option> can be one of:
374 sub supports { die "supports must be implemented in a child class" }
378 The following are readonly attributes.
390 sub storage { $_[0]{storage} }
392 sub sector_type { die "sector_type must be implemented in a child class" }
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
398 # NOTE - simply tying $value won't perform a STORE on each value. Hence,
399 # the copy to a temp value.
402 my ($value, $value_sector) = @_;
403 my $r = Scalar::Util::reftype( $value ) || '';
405 if ( $r eq 'ARRAY' ) {
407 tie @$value, 'DBM::Deep', {
408 base_offset => $value_sector->offset,
409 staleness => $value_sector->staleness,
410 storage => $self->storage,
414 bless $value, 'DBM::Deep::Array' unless Scalar::Util::blessed( $value );
416 elsif ( $r eq 'HASH' ) {
418 tie %$value, 'DBM::Deep', {
419 base_offset => $value_sector->offset,
420 staleness => $value_sector->staleness,
421 storage => $self->storage,
425 bless $value, 'DBM::Deep::Hash' unless Scalar::Util::blessed( $value );