Information Technology - Open Distributed Processing - Reference Model: Architectural Semantics

The rapid growth of distributed processing has lead to a need for a coordinating framework for the standardization of Open Distributed Processing (ODP). This Reference Model of ODP provides such a framework. It creates an architecture within which support of distribution, interworking, interoperability and portability can be integrated.

The Basic Reference Model of Open Distributed Processing (RM-ODP), (see ITU-T Recs. X.901 to X.904 | ISO/IEC 10746), is based on precise concepts derived from current distributed processing developments and, as far as possible, on the use of formal description techniques for specification of the architecture.

The RM-ODP consists of:

- ITU-T Rec. X.901 | ISO/IEC 10746-1: Overview: Contains a motivational overview of ODP giving scooping, justification and explanation of key concepts, and an outline of ODP architecture. This part is not normative.

- ITU-T Rec. X.902 | ISO/IEC 10746-2: Foundations: Contains the definition of the concepts and analytical framework and notation for normalized description of (arbitrary) distributed processing systems. This is only to a level of detail sufficient to support ITU-T Rec. X.903 | ISO/IEC 10746-3 and to establish requirements for new specification techniques. This part is normative.

- ITU-T Rec. X.903 | ISO/IEC 10746-3: Architecture: Contains the specification of the required characteristics that qualify distributed processing as open. These are the constraints to which ODP standards must conform. It uses the descriptive techniques from ITU-T Rec. X.902 | ISO/IEC 10746-2. This part is normative.

- ITU-T Rec. X.904 | ISO/IEC 10746-4: Architectural Semantics: Contains a formalisation of the ODP modeling concepts defined in ITU-T Rec. X.902 | ISO/IEC 10746-2, clauses 8 and 9, and a formalisation of the viewpoint languages of ITU-T Rec. X.903 | ISO/IEC 10746-3. The formalisation is achieved by interpreting each concept in terms of the constructs of the different standardized formal description techniques. This part is normative.

The purpose of this Recommendation | International Standard is to provide an architectural semantics for ODP. This essentially takes the form of an interpretation of the basic modeling and specification concepts of ITU-T Rec. X.902 | ISO/IEC 10746-2 and viewpoint languages of ITU-T Rec. X.903 | ISO/IEC 10746-3, using the various features of different formal specification languages. An architectural semantics is developed in four different formal specification languages: LOTOS, ESTELLE, SDL and Z. The result is a formalization of ODP's architecture. Through a process of iterative development and feedback, this has improved the consistency of ITU-T Rec. X.902 | ISO/IEC 10746-2 and ITU-T Rec. X.903 | ISO/IEC 10746-3.

An architectural semantics provides the additional benefits of:

- assisting the sound and uniform development of formal descriptions of ODP systems; and

- of permitting uniform and consistent comparison of formal descriptions of the same standard in different formal specification languages.

Rather than provide a mapping from all the concepts of ITU-T Rec. X.902 | ISO/IEC 10746-2, this Recommendation | International Standard focuses on the most basic. A semantics for the higher level architectural concepts is provided indirectly through their definition in terms of the basic ODP concepts.

Examples of the use of some of the formal specification languages in this report can be found in TR 10167 (Guidelines for the Application of ESTELLE, LOTOS and SDL).

In the following clauses, the concepts are numbered in accordance with the scheme used in ITU-T Rec. X.902 | ISO/IEC 10746-2.

This Recommendation | International Standard specifies an architectural semantics for ODP. This is required to:

- provide formalisation of the ODP modelling concepts;

- assist sound and uniform development of formal descriptions of standards for distributed systems;

- act as a bridge between the ODP modelling concepts and the semantic models of the specification languages: LOTOS, SDL, ESTELLE and Z;

- provide a basis for uniform and consistent comparison between formal descriptions of the same standard in specification languages that are used to develop an architectural semantics.

This part is normative.