MICO::internals - MICO module internals
This document describes the internals of MICO.
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:
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
internals.[fig/ps] for a graphical view of the setup)
When an interface is loaded, MICO/Perl:
so it correctly points to the current package.
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.
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.
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.