=head1 NAME 

MICO::internals - MICO module internals

=head1 DESCRIPTION

This document describes the internals of MICO.

=head1 Overview

The Perl MICO module is rather unusual in that it doesn't
generate stubs at all. Instead, it works from a
FullInterfaceDescription loaded from the interface repository.

These descriptions are cached locally so they will be only
ever retrieved once. Retrieval is triggered when:

=over 4

=item *
The user gives an 'ids' argument to 'use MICO'

=item *
The user explicitly calls $orb->preload(ID)

=item *
An unknown method is invoked on an object whose interface
has not yet been loaded.

=back

=head1 Object details

Each CORBA object that is used by a Perl program has a Perl
object associated with it. For objects not implemented in
Perl, this object is an opaque reference. (Currently it
is a hash reference, which could be useful for clients that
want to store temporary data on the stub object, but it
may be changed for efficiency reasons). For objects implemented
in Perl, the type of object is chosen by the implementor.

Attached to this object (via invisible "magic") is a structure
holding information about the Perl object. (Of type
PMicoInstVars) When the perl object is destroyed, the reference
count held on the associated C++ object is released, and for
objects implemented in Perl, the field in the C++ object pointing
to the Perl object is cleared.

A "pin table" is kept to associate C++ CORBA::Object's with Perl
objects. This means that two Perl references for the an same MICO
object will always refer to the same Perl object, and that calls
on a Perl object will be short-circuited locally.

A Perl object holds a reference count on the associated C++
CORBA::Object, but not vice versa. This means, that a Perl server
must hold onto a reference to any live objects that are being
used locally in a different language. (This is identical to the
situation for objects used remotely, so should not present undue
hardship for the implementor)

(See the file C<internals.[fig/ps]> for a graphical view of
the setup)


=head1 Actions upon loading an interface

When an interface is loaded, MICO/Perl:

=over 4

=item *
Initializes the corresponding Perl package by pointing the
@ISA array at the base interfaces, if any, and at 
CORBA::Object otherwise.
Stores the repository ID in a package variable.

=item *
For each exception referenced in the package, creates a
Perl package with @ISA=qw(CORBA::UserException), and
an appropriate $_repoid variable, if one does not exist,
and stores the package name in a global hash table from
repoid to package name

=item *
For each operation in the interface, creates an XS subroutine
pointing to _pmico_callStub, stores the integer index of
the method in:
 
    CvXSUBANY(cv).any_i32

And sets

    CvSTASH (cv)

so it correctly points to the current package.

=item *
For each attribute in the package, does the same thing,
but offsets the stored index so that get methods can
be distinguished from set methods, and from operations.

=back

=head1 Calling a stub routine

When the application calls a stub method on an object,
the call is forwarded to _pmico_callStub, which retrieves
the package and method index. From this information, it
finds the OperationDescription for the method that is
being invoked. (or AttributeDescription, if appropriate)

Then _pmico_callStub builds a DII request using the
OperationDescription and the passed in parameters, invokes
it and translates any return values or exceptions into
Perl terms.


=head1 Handling invocations on a Perl object

When MICO receives a request for an operation on an object
implemented in Perl, it calls the invoke() method of the
PMicoTrueObject. The invoke description finds the 
OperationDescription or AttributeDescription from name which
is passed in in the ServerRequest object. This currently
can be quite slow since it can involve strcmp()'s with every
method name in the interface and its base interfaces. There
probably should be a "method => description" hash table computed
when the interface is loaded, or simply acting as a cache.

The sequence is quite similar to that above - A NamedValue list
is built using the Description, this is used to populate the
arguments for the Perl method from the ServerRequest, the Perl
method is called, then exceptions and results are copied back to
the ServerRequest.


=head1 Translating from and to Perl data structures

Most of the file typefuncs.cc is concerned with translating
Perl data structures from and to MICO's Any's.

It should be realized that a MICO Any is basically a combination
of a buffer and a typecode, so this is quite equivalent
to the marshalling/unmarshalling from a buffer.

The code is quite straightforward - it just uses the information
in a typecode to either walk a Perl structure and create an
Any from it, or to create a Perl structure from an Any.