=head1 NAME

ObjStore::Internals - a few notes on the implementation

=head1 SYNOPSIS

You don't have to understand anything about the technical
implementation.  Just know that:

=over 4

=item *

C<ObjectStore> is outrageously powerful; sophisticated; and even
over-engineered.

=item *

The perl interface is optimized to be fun and easy.  Since
C<ObjectStore> is also blindingly fast, you can happily leave
relational databases to collect dust on the bookshelf where they
belong.

=back

So basically, you don't have to understand anything to a greater
depth.  It's not necessary.  You've arrived.  You will be successful.
However, more detail follows.  If you like to turn things inside-out,
read on!

=head1 DESCRIPTION

=head2 Perl & C<C++> APIs: What's The Difference?

Most stuff should be roughly the same.  The few exceptions have
generally arisen because there was a perl way to make the interface
more programmer friendly.

=over 4

=item *

Transactions are perlified.

=item *

Some static methods sit directly under C<ObjStore::> instead of under
their own classes.  (Easier to import.)

=item *

Databases are always blessed according to your pleasure.

=item * 

C<lookup>, C<open>, C<is_open>, and C<lock_timeout> are augmented with
multi-color, pop-tart style interfaces.

=back

=head2 Why not just store perl data with the usual perl structures?

=over 4

=item * CHANGE CONTROL

As perl evolves, new data layouts are introduced.  These changes
must not cause database compatibility problems.

=item * BINARY COMPATIBILITY

Perl doesn't have to worry about binary compatibility between
platforms.  Databases do.  In addition, databases impose a number of
restrictions on persistent data layout that would be onerous and
sub-optimal if adopted by perl.

=item * MEMORY USAGE

Perl often trades memory for speed.  This is the wrong trade for a
database.  Memory usage is much more of a concern when data sets can
be as large or larger than ten million megabytes.  A few percent
difference in compactness can be quite noticable.

=back

=head2 Bless

If you are a suspicious person (like my mom) you might have suspected
that the ObjStore module installs its own version of C<bless>.
Natually it does!  The augmented C<bless> implements extra quality
assurance to insure that blessings are correctly stored persistently.
For example:

    package Scottie;
    use ObjStore;
    use base 'ObjStore::HV';

    sub new {
        my ($class, $near) = @_;
        $class->SUPER::new($near, { fur => 'buffy' });
    }

    my Scottie $dog = Scottie->new($db);

Persistent C<bless> also does some extra work to make evolution
easier.  It stores the current C<@ISA> tree along with the C<$VERSION>
of every class in the C<@ISA> tree.  Furthermore, the C<isa> method is
overridden such that it reports according to the moment of the
C<bless>.  Similarly, the C<versionof> method lets you query the saved
C<$VERSION>s.  This may be helpful when doing evolution, as you can
compare the old C<@ISA> and C<$VERSION>s to figure out what to change
(and how).  C<UNIVERSAL::can> is unmodified.

Technically speaking, C<bless> is re-implemented such that it can be
extended by the I<bless from> and the I<bless to> classes via the
C<BLESS> method.  Both the I<bless from> and I<bless to> operations
are funnelled through a single C<BLESS> method like this:

  sub BLESS {
      my ($r1,$r2);
      if (ref $r1) { warn "$r1 leaving ".ref $r1." for a new life in $r2\n"; }
      else         { warn "$r2 entering $r1\n"; }
      $r1->SUPER::BLESS($r2);
  }

=head2 UNLOADED

Generic tools such as C<posh> or C<ospeek> must C<bless> objects when
reading from an arbitrary database.  To C<bless>, there must be
information about the inheritance tree.  To try to get it, unknown
classes found in a database are C<require>'d.  However, the C<require>
may fail.  If it does fail, a package must be faked-up and
C<${"${package}::UNLOADED"}> is set to true.  This flag is used to
signal that the @ISA tree should not be considered authoritative for a
particular package.

=head2 Representation

All values take a minimum of 8 bytes (OSSV).  These 8 bytes are used
to store 16-bits of type information, a pointer, and a general purpose
16-bit value.

  value stored                   extra allocation (in addition to OSSV)
  ------------------------------ -------------------------------------
  undef                          none
  pointer                        none
  16-bit signed integers         none
  32-bit signed integers         4 byte block (OSPV_iv)
  double                         8 byte block (OSPV_nv)
  string                         length of string (char*)
  object (ref or container)      sizeof object (see subclasses of OSSVPV)
                                 additional references take no extra space
  bless                          .5-1k bytes per class (zero per object)

Since 32-bit integers and doubles are fairly common and should be
stored densely, a pool allocation algorithm is planned.

The ODI FAQ also states: I<In addition, there is an associated entry
in the info segment for the segment in question for each allocation of
the object. This is done in the tag table. The overhead is 16 bits
(i.e., 2 bytes) for each singleton (i.e., non-array) allocation, 32
bits for each character array allocation for character arrays E<lt>=
255 characters, and 48 bits for each character array allocation E<gt>
255 characters, or any array allocation of an object of another type.
Also, depending on the size of an object (i.e., if you allocate a
"huge" object - one that is E<gt>64Kb) there is other overhead caused
by alignment constraints.>

If this seems like a lot of overhead, consider that it is not really
possible to directly compare these numbers to RDBMS statistics.  At
least we can say that relational data can be stored with much less
duplication when moved into C<ObjectStore>.  This is unquestionably
true when you tailor your own C++ extensions to fit data access
patterns.

=head2 Representation Limitations

Most of these limitations are planned to be removed with the release
of C<ObjectStore> 6.0.

=over 4

=item *

Exact width types are preferred.  Specify number of bits per
integer when possible.  It is still mostly unresolved as to how to
deal with 64-bit integer types.

=item *

Try to be as binary compatible as possible between different
platforms.  N-bit width types generally need n-bit alignment.  For
example, 32-bit integers must usually be stored with 32-bit alignment.

=item *

Unions are not supported.  (Don't even think about it! :-)

=item *

Variable length structures are probably not supported.  For example:

  struct varstr {
    int refcnt;
    char string[0];  # sized via malloc
  };

Instead, you must allocate an array separately:

  struct varstr {
    int refcnt;
    char *string;    # string = malloc(sizeof(char) * len)
  };

=item *

Changing the layout of structures after they are stored in a database
is generally a nightmare.  Instead it is recommended that a version
number be appended to the name of the structure (e.g. mystruct1,
mystruct2, mystruct3) and all support code be kept indefinitely.

=back

=head2 Go Extension Crazy

Add your own C<C++> representation.  New families of objects can
inherit from C<ObjStore::UNIVERSAL>.  Suppose you want highly
optimized, persistent bit vectors?  Or matrics?  No problem.

Documentation is slim, but don't let that stop you.  There are many
examples.  See the included representations and also
L<ObjStore::REP::HashRecord> & L<ObjStore::Lib::PDL>.

=head2 Typemap

The typemap is complicated because of the need to insure that
persistent data is not accessed outside of its transaction.

  OODB[SCALAR1 SCALAR2]
         |        |
       BRIDGE  BRIDGE
         |        |
  PERL[SCALAR1 SCALAR2]
  
A bridge has two owners: perl and the current transaction.  The bridge
and the scalar have different lifetimes.  The scalar lives for
MIN(perl,txn), while the bridge must live for MAX(perl,txn) (or at
least until perl is done).

Persistent refcnts are only (can only) be updated during update
transactions.  Fortunately, read-only transactions pose no problem:
refcnts cannot be updated but object cannot be deleted either.

Bridges are also used to store transient cursors associated with
collections.  For example, suppose you need to iterate over a hash
during a read transaction.  The hash is read-only so you create the
cursor transiently and store it in the bridge.

[XXX also mention dynacast stuff]

=head2 Notes On The Source Code

=over 4

=item *

Functions or methods starting with '_' are internal to the C<ObjStore>
extension.  They are subject to change (entirely) without notice.

=item *

Avoid C<const>, privacy & templates.  C<C++> sucks!  Long live C<C++>!

=item *

The relationship between references and cursors is strange.  It's
probably best not to think about it.

=back

=head1 RELATED RESEARCH

 ftp://ftp.cs.utexas.edu/pub/garbage/texas/README

 http://cs-tr.cs.cornell.edu:80/Dienst/UI/1.0/Display/ncstrl.utexas_cs/CS-TR-98-07

 http://www.usenix.org/publications/library/proceedings/usenix98/full_papers/saito/saito_html/saito.html

 http://theory.lcs.mit.edu/~cilk/

=head1 BUGS

=over 4

=item * UNFORGEABLE NOTIFICATION SENDER INFORMATION

ODI feature request #13632: It would be very useful to know which
client sent a given notification.  While the client I<could> fill in
this information as part of the notification, the C<osserver> already
knows the sender's client# and could pass this information
transparently to subscribers.  The additional overhead be just 2 bytes
per received notification.

=item * CROSS DATABASE POINTERS

This feature is highly deprecated and will likely be discontinued,
but at the moment you can allow cross database B<pointers> with:

  $db->_allow_external_pointers;    #never do this!

But you should avoid this if at all possible.  Using real pointers
will affect refcnts, even between two different databases.  Your
refcnts will be wrong if you simply C<osrm> a random database.  This
will cause some of your data to become permenently un-deletable.
Currently, there is no way to safely delete un-deletable data.

Instead, you can use references or cursors to refer to data in other
databases.  References may use the C<os_reference_protected> class
which is designed precisely to address this problem.  Refcnts will not
be updated remotely, but you'll still be protected from accessing
deleted objects or removed databases.  (Imagine the freedom. :-)

=item * C<ObjStore::AVHV> EVOLUTION

Indexed records temporarily cannot be evolved due to const-ness.  For
now, it is recommended that records be removed, changed, and re-added
to the table when changing indexed fields.

=item * WIN32

There might be issues with threads and signal handlers.  I'm not sure
since I don't use Microsoft products regularly.

=item * C<MOP>

This is not a general purpose C<ObjectStore> editor with complete
C<MOP> support.  Actually, I don't think this is a bug!

=item * HIGH VOLITILITY

Everything is subject to change without notice.  (But backward
compatibility will be preserved when possible. :-)

=item * POOR QUALITY DOCUMENTATION

I didn't get a Ph.D in English.  Sorry!

=back

=head1 WHY?

While there have been huge gains in software quality in the form of
GNU, Perl, Apache, Linux, Qt, and Mozilla, the world has been slogging
along in the dark ages (the 1960s in fact) with respect to database
technology.  After avoiding relational databases for years, I sensed a
combination of ObjectStore and Perl could offer the same level of
quality and simplicity that I find invaluable in addressing the
hurdles I face as a software professional.

Combining ObjectStore & Perl might seem obvious in hindsight (doesn't
it always?!) but it is my conviction that it was something more than
luck and hard work that gave me the insight to imagine and implement
this technology years before it would be recognized and adopted.  If
you will suspend disbelief for a moment, I would like to invite you
imagine a hypothetical situation.  What if you found someone who was
absolutely convincing in their understanding of the truth?  What if
you had an opportunity to spend time with them?  What would you do?

Designing software requires a basic subtlety of awareness.  To be
successful you must be able to fluctuate fluidly between the place of
reason and the place of silent knowledge.  The place of concern is the
forerunner to the place of reason.  Similarly, the place of unbending
intent is the forerunner to the place of silent knowledge.  Happily,
this technique is not limited to software design exclusively.

Not that there is anything wrong with software design.  Computers have
an evolutionary function. They (can) help people to understand the
difference between the mechanical and the non-mechanical aspects of
themselves.  Computers are *entirely* mechanical.  Don't believe the
anthropomorphisms.  Computers cannot create anything without our help;
they only do as they are instructed.  Probably this is easy to see.
However, many people seem to have difficulty seeing what should be
equally obvious: the pure, non-mechanical principle in themselves.

Ask yourself these questions: Who is the software designer?  Who
imagines?  Who moves the fingers to type?  IMHO, to try to know
yourself is the most worthy and worthwhile goal.  Therefore, evolution
is served by making software available that is very similar to our
mechanical thinking processes.  By understanding this type of
software, one is aided in understanding how the brain works.  (I am
not referring exclusively to the field of artificial intelligence.
The brain is a data processor in the broadest sense and the algorithms
of artificial intelligence are only a part of that.)

I have clear intention as to why I am developing this software and why
I am (foolishly!) giving it away for free.  By contributing to the
evolution of others' understanding, I in fact contribute to my own
evolution.  The better one understands, the less time is wasted in
states of confusion.  As you learn how this database works, you will
assimilate the perspectives that I have deliberately distilled.  If
you wish to pursue this to a greater depth, please consider visiting
http://www.purposeofcompetition.org .  Best of luck.

=cut
