# mutex. But, it's the caller's responsability to make sure that this has
# been done.
-# Setup file and tag signatures. These should never change.
-sub SIG_FILE () { 'DPDB' }
-sub SIG_HEADER () { 'h' }
-sub SIG_HASH () { 'H' }
-sub SIG_ARRAY () { 'A' }
-sub SIG_NULL () { 'N' }
-sub SIG_DATA () { 'D' }
-sub SIG_INDEX () { 'I' }
-sub SIG_BLIST () { 'B' }
-sub SIG_FREE () { 'F' }
-sub SIG_SIZE () { 1 }
+sub SIG_HASH () { 'H' }
+sub SIG_ARRAY () { 'A' }
+
+=head1 NAME
+
+DBM::Deep::Engine
+
+=head1 PURPOSE
+
+This is an internal-use-only object for L<DBM::Deep/>. It mediates the low-level
+mapping between the L<DBM::Deep/> objects and the storage medium.
+
+The purpose of this documentation is to provide low-level documentation for
+developers. It is B<not> intended to be used by the general public. This
+documentation and what it documents can and will change without notice.
+
+=head1 OVERVIEW
+
+The engine exposes an API to the DBM::Deep objects (DBM::Deep, DBM::Deep::Array,
+and DBM::Deep::Hash) for their use to access the actual stored values. This API
+is the following:
+
+=over 4
+
+=item * new
+
+=item * read_value
+
+=item * get_classname
+
+=item * make_reference
+
+=item * key_exists
+
+=item * delete_key
+
+=item * write_value
+
+=item * get_next_key
+
+=item * setup
+
+=item * begin_work
+
+=item * commit
+
+=item * rollback
+
+=item * lock_exclusive
+
+=item * lock_shared
+
+=item * unlock
+
+=back
+
+They are explained in their own sections below. These methods, in turn, may
+provide some bounds-checking, but primarily act to instantiate objects in the
+Engine::Sector::* hierarchy and dispatch to them.
+
+=head1 TRANSACTIONS
+
+Transactions in DBM::Deep are implemented using a variant of MVCC. This attempts
+to keep the amount of actual work done against the file low while stil providing
+Atomicity, Consistency, and Isolation. Durability, unfortunately, cannot be done
+with only one file.
+
+=head2 STALENESS
+
+If another process uses a transaction slot and writes stuff to it, then
+terminates, the data that process wrote it still within the file. In order to
+address this, there is also a transaction staleness counter associated within
+every write. Each time a transaction is started, that process increments that
+transaction's staleness counter. If, when it reads a value, the staleness
+counters aren't identical, DBM::Deep will consider the value on disk to be stale
+and discard it.
+
+=head2 DURABILITY
+
+The fourth leg of ACID is Durability, the guarantee that when a commit returns,
+the data will be there the next time you read from it. This should be regardless
+of any crashes or powerdowns in between the commit and subsequent read.
+DBM::Deep does provide that guarantee; once the commit returns, all of the data
+has been transferred from the transaction shadow to the HEAD. The issue arises
+with partial commits - a commit that is interrupted in some fashion. In keeping
+with DBM::Deep's "tradition" of very light error-checking and non-existent
+error-handling, there is no way to recover from a partial commit. (This is
+probably a failure in Consistency as well as Durability.)
+
+Other DBMSes use transaction logs (a separate file, generally) to achieve
+Durability. As DBM::Deep is a single-file, we would have to do something
+similar to what SQLite and BDB do in terms of committing using synchonized
+writes. To do this, we would have to use a much higher RAM footprint and some
+serious programming that make my head hurts just to think about it.
+
+=cut
+
+=head2 read_value( $obj, $key )
+
+This takes an object that provides _base_offset() and a string. It returns the
+value stored in the corresponding Sector::Value's data section.
+
+=cut
+
+sub read_value { die "read_value must be implemented in a child class" }
+
+=head2 get_classname( $obj )
+
+This takes an object that provides _base_offset() and returns the classname (if
+any) associated with it.
+
+It delegates to Sector::Reference::get_classname() for the heavy lifting.
+
+It performs a staleness check.
+
+=cut
+
+sub get_classname { die "get_classname must be implemented in a child class" }
+
+=head2 make_reference( $obj, $old_key, $new_key )
+
+This takes an object that provides _base_offset() and two strings. The
+strings correspond to the old key and new key, respectively. This operation
+is equivalent to (given C<< $db->{foo} = []; >>) C<< $db->{bar} = $db->{foo} >>.
+
+This returns nothing.
+
+=cut
+
+sub make_reference { die "make_reference must be implemented in a child class" }
+
+=head2 key_exists( $obj, $key )
+
+This takes an object that provides _base_offset() and a string for
+the key to be checked. This returns 1 for true and "" for false.
+
+=cut
+
+sub key_exists { die "key_exists must be implemented in a child class" }
+
+=head2 delete_key( $obj, $key )
+
+This takes an object that provides _base_offset() and a string for
+the key to be deleted. This returns the result of the Sector::Reference
+delete_key() method.
+
+=cut
+
+sub delete_key { die "delete_key must be implemented in a child class" }
+
+=head2 write_value( $obj, $key, $value )
+
+This takes an object that provides _base_offset(), a string for the
+key, and a value. This value can be anything storable within L<DBM::Deep/>.
+
+This returns 1 upon success.
+
+=cut
+
+sub write_value { die "write_value must be implemented in a child class" }
+
+=head2 setup( $obj )
+
+This takes an object that provides _base_offset(). It will do everything needed
+in order to properly initialize all values for necessary functioning. If this is
+called upon an already initialized object, this will also reset the inode.
+
+This returns 1.
+
+=cut
+
+sub setup { die "setup must be implemented in a child class" }
+
+=head2 begin_work( $obj )
+
+This takes an object that provides _base_offset(). It will set up all necessary
+bookkeeping in order to run all work within a transaction.
+
+If $obj is already within a transaction, an error wiill be thrown. If there are
+no more available transactions, an error will be thrown.
+
+This returns undef.
+
+=cut
+
+sub begin_work { die "begin_work must be implemented in a child class" }
+
+=head2 rollback( $obj )
+
+This takes an object that provides _base_offset(). It will revert all
+actions taken within the running transaction.
+
+If $obj is not within a transaction, an error will be thrown.
+
+This returns 1.
+
+=cut
+
+sub rollback { die "rollback must be implemented in a child class" }
+
+=head2 commit( $obj )
+
+This takes an object that provides _base_offset(). It will apply all
+actions taken within the transaction to the HEAD.
+
+If $obj is not within a transaction, an error will be thrown.
+
+This returns 1.
+
+=cut
+
+sub commit { die "commit must be implemented in a child class" }
=head2 get_next_key( $obj, $prev_key )
my $self = shift;
my ($obj, $prev_key) = @_;
- # XXX Need to add logic about resetting the iterator if any key in the reference has changed
+ # XXX Need to add logic about resetting the iterator if any key in the
+ # reference has changed
unless ( $prev_key ) {
$obj->{iterator} = DBM::Deep::Iterator->new({
base_offset => $obj->_base_offset,
return $obj->{iterator}->get_next_key( $obj );
}
+=head2 lock_exclusive()
+
+This takes an object that provides _base_offset(). It will guarantee that
+the storage has taken precautions to be safe for a write.
+
+This returns nothing.
+
+=cut
+
+sub lock_exclusive {
+ my $self = shift;
+ my ($obj) = @_;
+ return $self->storage->lock_exclusive( $obj );
+}
+
+=head2 lock_shared()
+
+This takes an object that provides _base_offset(). It will guarantee that
+the storage has taken precautions to be safe for a read.
+
+This returns nothing.
+
+=cut
+
+sub lock_shared {
+ my $self = shift;
+ my ($obj) = @_;
+ return $self->storage->lock_shared( $obj );
+}
+
+=head2 unlock()
+
+This takes an object that provides _base_offset(). It will guarantee that
+the storage has released the most recently-taken lock.
+
+This returns nothing.
+
+=cut
+
+sub unlock {
+ my $self = shift;
+ my ($obj) = @_;
+
+ my $rv = $self->storage->unlock( $obj );
+
+ $self->flush if $rv;
+
+ return $rv;
+}
+
+=head1 INTERNAL METHODS
+
+The following methods are internal-use-only to DBM::Deep::Engine and its
+child classes.
+
+=cut
+
+=head2 flush()
+
+This takes no arguments. It will do everything necessary to flush all things to
+disk. This is usually called during unlock() and setup().
+
+This returns nothing.
+
+=cut
+
+sub flush {
+ my $self = shift;
+
+ # Why do we need to have the storage flush? Shouldn't autoflush take care of
+ # things? -RobK, 2008-06-26
+ $self->storage->flush;
+
+ return;
+}
+
+=head2 load_sector( $loc )
+
+This takes an id/location/offset and loads the sector based on the engine's
+defined sector type.
+
+=cut
+
+sub load_sector { $_[0]->sector_type->load( @_ ) }
+
+=head2 ACCESSORS
+
+The following are readonly attributes.
+
+=over 4
+
+=item * storage
+
+=back
+
+=cut
+
+sub storage { $_[0]{storage} }
+
+sub sector_type { die "sector_type must be implemented in a child class" }
+
1;
__END__
projects / dbsrgits/DBM-Deep.git / blobdiff