An architecture based approach to specifying distributed systems in LOTOS and Z

Specification is difficult. It is often the case that the most difficult aspect of specifying is the structuringof the specification to begin with. Adopting an architectural approach can help to alleviate this structuringproblem. We have investigated how the formal languages LOTOS and Z can be used to develop specificationtemplates suitable for architecting specifications of distributed systems.The immediate question that arises is: where do the architectural concepts come from? We have focusedprimarily on the work of the current standardisation activity of Open Distributed Processing (ODP). Theapproach taken there is to provide an object-oriented set of concepts and to use these as the basis fordeveloping a multi-viewpoint approach. A viewpoint may be regarded as an abstraction of the systemfocusing on some particular aspect, the intention being to help reduce the complexity of the system asa whole. ODP identifies five viewpoints: the enterprise, information, computational, engineering andtechnology viewpoints. In our work, we formalise the foundation set of concepts in LOTOS and Z thenshow how specification architectures based on the computational viewpoint can be developed. We alsohighlight the advantages in a formal approach through the identification of limitations and errors in theODP framework.Central to work on distributed systems (or any system adopting an object-oriented methodology), and tothe computational viewpoint of ODP in particular, is the issue of type management. We have investigatedhow LOTOS and Z can be used to reason about type systems generally and to investigate the issues thathave to be resolved in determining type equivalence. We base our work on the idea of type equivalence asa substitutability relation between types. We address issues such as signature type checking, behaviouraltype checking and non-functional aspects of type checking such as quality of service. Our investigationshave also included a treatment of multimedia type systems where continuous flows of information raiseperformance issues that are particularly important.We have applied our approach to two case studies: the specification of the ODP trader in LOTOS andthe specification of a producer and consumer flow configuration in Z. The advantages and disadvantagesof adopting LOTOS or Z to develop specification architectures for distributed systems are highlighted anddiscussed.