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Developers writing applications in statically typed languages such as C++ usually prefer to use the static invocation approach because it provides a more natural programming model The dynamic approach can be useful for applications, such as gateways and bridges, that must receive and forward requests without having compile-time knowledge of the types and interfaces involved 245 Object Adapters In CORBA, object adapters serve as the glue between servants and the ORB As described by the Adapter design pattern [4], which is independent of CORBA, an object adapter is an object that adapts the interface of one object to a different interface expected by a caller In other words, an object adapter is an interposed object that uses delegation to allow a caller to invoke requests on an object without knowing the object's true interface CORBA object adapters fulfill three key requirements They create object references, which allow clients to address objects They ensure that each target object is incarnated by a servant They take requests dispatched by a server-side ORB and further direct them to the servants incarnating each of the target objects Without object adapters, the ORB would have to directly provide these features in addition to all its other responsibilities As a result, it would have a very complex interface that would be difficult for the OMG to manage, and the number of possible servant implementation styles would be limited In C++, servants are instances of C++ objects They are typically defined by deriving from skeleton classes produced by compiling IDL interface definitions To implement operations, you override virtual functions of the skeleton base class You register these C++ servants with the object adapter to allow it to dispatch requests to your servants when clients invoke requests on the objects incarnated by those servants Until version 21, CORBA contained specifications only for the Basic Object Adapter (BOA) The BOA was the original CORBA object adapter, and its designers felt that it would suffice for the majority of applications, with other object adapters filling only niche roles However, CORBA did not evolve as expected because of the following problems with the BOA specification The BOA specification did not account for the fact that, because of their need to support servants, object adapters tend to be language-specific Because CORBA originally provided only a C language mapping, the BOA was written to support only C servants Later attempts to make it support C++ servants proved to be difficult In general, an object adapter that provides solid support for servants in one programming language is not likely to also provide adequate support for servants written in a different language because of differences in implementation style and usage of those servants
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A number of critical features were missing from the BOA specification Certain interfaces were not defined and there were no servant registration operations Even those operations that were specified contained many ambiguities ORB vendors developed their own proprietary solutions to fill the gaps, resulting in poor server application portability between different ORB implementations The Portability Enhancement RFP [27] issued by the OMG in 1995 to address these issues contained a seven-page listing of problems with the BOA specification CORBA version 22 introduced the Portable Object Adapter to replace the BOA Because the POA addresses the full gamut of interactions between CORBA objects and programming language servants while maintaining application portability, the quality of the POA specification is vastly superior to that of the BOA As a result, the BOA specification has been removed from CORBA We provide detailed coverage of the POA in 11 246 Inter-ORB Protocols Before CORBA 20, one of the most common complaints lodged against CORBA was its lack of standard protocol specifications To allow remote ORB applications to communicate, every ORB vendor had to develop its own network protocol or borrow one from another distributed system technology This resulted in "ORB application islands" Each one was built over a particular vendor's ORB, and thus they were unable to communicate with one another CORBA 20 introduced a general ORB interoperability architecture called the General Inter-ORB Protocol (GIOP, pronounced "gee-op") GIOP is an abstract protocol that specifies transfer syntax and a standard set of message formats to allow independently developed ORBs to communicate over any connection-oriented transport The Internet Inter-ORB Protocol (IIOP, pronounced "eye-op") specifies how GIOP is implemented over Transmission Control Protocol/Internet Protocol (TCP/IP) All ORBs claiming CORBA 20 interoperability conformance must implement GIOP and IIOP, and almost all contemporary ORBs do so ORB interoperability also requires standardized object reference formats Object references are opaque to applications, but they contain information that ORBs need in order to establish communications between clients and target objects The standard object reference format, called the Interoperable Object Reference (IOR), is flexible enough to store information for almost any inter-ORB protocol imaginable An IOR identifies one or more supported protocols and, for each protocol, contains information specific to that protocol This arrangement allows new protocols to be added to CORBA without breaking existing applications For IIOP, an IOR contains a host name, a TCP/IP port number, and an object key that identifies the target object at the given host name and port combination
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