NAME

omniORB::internals - omniORB module internals

DESCRIPTION

This document describes the internals of the Perl interface to omniORB.

Overview

The Perl omniORB 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:

• The user gives an 'ids' argument to use omniORB

• The user explicitly calls $orb->preload(ID)

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

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 POmniInstVars) 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 to the same omniORB 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 internals.[fig/ps] for a graphical view of the setup)

Actions upon loading an interface

When an interface is loaded, omniORB/Perl:

• 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.

• 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

• For each operation in the interface, creates an XS subroutine pointing to _pomni_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.

• 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.

Calling a stub routine

When the application calls a stub method on an object, the call is forwarded to _pomni_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 _pomni_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.

Handling invocations on a Perl object

When omniORB receives a request for an operation on an object implemented in Perl, it calls the invoke() method of the POmniServant. The invoke description finds the OperationDescription or AttributeDescription from the 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.

Translating from and to Perl data structures

Most of the file types.cc is concerned with translating Perl data structures from and to omniORB's Anys.

It should be realized that a omniORB 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.