Towards formal specification of a distributed computing system

Onboard spacecraft computing system is a case of a functionally distributed system that requires continuous interaction among the nodes to control the operations at different nodes. A simple and reliable protocol is desired for such an application. This paper discusses a formal approach to specify the computing system with respect to some important issues encountered in the design and development of a protocol for the onboard distributed system. The issues considered in this paper are concurrency, exclusiveness and sequencing relationships among the various processes at different nodes. A 6-tuple model is developed for the precise specification of the system. The model also enables us to check the consistency of specification and deadlock caused due to improper specification. An example is given to illustrate the use of the proposed methodology for a typical spacecraft configuration. Although the theory is motivated by a specific application the same may be applied to other distributed computing system such as those encountered in process control industries, power plant control and other similar environments.     

Self-stabilizing real-time OPS5 production systems

We examine the task of constructing bounded-time self-stabilizing rule-based systems that take their input from an external environment. Bounded response-time and self-stabilization are essential for rule-based programs that must be highly fault-tolerant and perform in a real-time environment. We present an approach for solving this problem using the OPS5 programming language as it is one of the most expressive and widely used rule-based programming languages. Bounded response-time of the program is ensured by constructing the state space graph so that the programmer can visualize the control flow of the program execution. Potential infinite firing sequences, if any, should be detected and the involved rules should be revised to ensure bounded termination. Both the input variables and internal variables are made fault-tolerant from corruption caused by transient faults via the introduction of new self-stabilizing rules in the program. Finally, the timing analysis of the self-stabilizing OPS5 program is shown in terms of the number of rule firings and the comparisons performed in the Rete network.     

Response time analysis of OPS5 production systems

The paper focuses on the problem of determining a priori the maximal response time of rule based programs. The response time analysis problem is an important problem, especially for real time systems. We study this problem in the context of OPS5 production systems. Two aspects of the response time of a program are investigated, the maximal number of rule firings and the maximal number of basic comparisons made by the Rete network during the execution of the program. The response time analysis problem is in general undecidable. However, a program terminates in a finite time if the rule triggering pattern of this program satisfies certain conditions. We present four such termination conditions for OPS5 production systems. An algorithm for computing an upper bound on the number of rule firings is then given. To have a better idea of the time required during execution, we present an algorithm that computes the maximal time required during the match phase in terms of the number of comparisons made by the Rete network. This measurement is sufficient since the match phase consumes about 90 percent of the execution time     

Shortening matching time in OPS5 production systems

A rule-based system must satisfy stringent timing constraints when applied to a real-time environment. As the scale of rule-based expert systems increases, the efficiency of systems becomes a pressing concern. The most critical performance factor in the implementation of a production system is the condition-testing algorithm. We propose a new method based on the widely used RETE match algorithm. We show an approach designed to reduce the response time of rule-based expert systems by reducing the matching time. There are two steps in the method we propose: The first makes an index structure of the tokens to reduce the /spl alpha/-node-level join candidates. The second chooses the highest time tag for certain /spl beta/-nodes to reduce the amount of combinatorial match that is problematical in a real-time production system application. For this purpose, a simple compiler is implemented in C and the response time of test programs is measured.     

The formal specification of a small bookshop information system

A specification, and its development, for a small bookshop information system are discussed. the specification is presented using mathematics and the scheme calculus of C. Morgan and B. Sufrin (see ibid., vol.SE-10, no.2, p.128-142, 1984). An insight is given into how the specification was developed and why a formal specification is appropriate     

Towards an algebraic specification of code generation

We show how the problem of code generation for a simple language can be treated fully algebraically. The algebraic approach enjoys several advantages which should be demonstrated here. First, it allows a uniform specification both of the abstract syntax and of the semantics of the source and the target language on one side and of the code generation on the other side by means of hierarchical abstract data types. Moreover, theorems about the compiler, such as the preservation of the semantics, can be proved by induction on the term structure of the abstract syntax. Furthermore, existing tools for rapid prototyping with abstract data types can be applied to validate the specification against the intention in an early stage. In addition, this paper shows how such a system can also be used for performing induction proofs of conjectures.     

Object modelling and formal specification during real-time system development

Object oriented techniques promote understanding of requirements leading to flexible and extendible designs. The use of formal specification techniques ensures a complete understanding of system requirements and provides sound foundations for subsequent testing and verification. This paper describes the use of the Z and Timed CSP formal specification techniques to support object modelling during real-time system development. Relationships between class attributes are specified within the corresponding Z schemas and inheritance relationships between classes are formally specified using the schema extension mechanism of Z. Z is used to specify the domain types of the attributes of classes identified during object oriented analysis and design. Z is also used to produce model based specifications of the methods within classes that are specified informally during functional analysis. Dynamic analysis identifies events, states and temporal relationships between events. Timed CSP is used to formally specify this information as well as timing information that is necessary during real-time system development.     

A decomposition of a formal specification: an improvedconstraint-oriented method

In this paper, the authors propose a decomposition method for a formal specification that divides the specification into two subspecifications composed by a parallel operator. To make these specification behaviors equivalent before and after decomposition, the method automatically synthesizes an additional control specification, which contains the synchronization information of the decomposed subspecifications. The authors prove that a parallel composition of the decomposed subspecifications synchronized with the control specification is strongly equivalent with the original (monolithic) specification. The authors also write formal specifications of the OSI application layer's association-control service and decompose it using their method as an example of decomposition of a practical specification. Their decomposition method can be applied to top-down system development based on stepwise refinement     

State architecture notation and simulation: A formal technique for the specification and testing of protocol systems

The increasing practical importance of data communications, computer networking and distributed data processing has increased the benefits to be gained from a systematic approach to the design, specification, validation, implementation and testing of these kind of systems.This paper presents an introduction to the State Architecture approach to protocol system specification and also describes an important companion simulation tool.State Architecture concepts and definitions are presented. Complex protocol systems are built up of interconnections of state-system primitives; specifically, combinational functions, finite-state machines, queues, clocks, delays and derivatives. Aspects of component state and input/output behavior are treated explicitly.State Architecture Notation (SAN), a formal machine-readable language for specifying protocol systems, is outlined.The State Architecture Simulator (SAS), a general purpose SAN model simulator is described and example SAS runs on a model of the ADCCP protocol (similar to HDLC and SDLC) are included.A preliminary evaluation of the State Architecture approach is presented along with a brief comparison to other methods of formal specification.     

Design of a knowledge based system for inventory control,an OPS5 implementation

This paper introduces a knowledge based expert system designed to address the fundamental problem of inventory control (how much of an item to order and how often to order) faced by many managers. It provides implementation details about the structure of the system and its programming environment, OPS5.     

Towards the architecture of an instructional multimedia database

Abstract  The applicability of multimedia databases in education may be extended if they can serve multiple target groups, leading to affordable costs per unit for the user. In this contribution, an approach is described to build generic multimedia databases to serve that purpose. This approach is elaborated within the ODB Project ('Instructional Design of an Optical DataBase'); the term optical refers to the use of optical storage media to hold the audiovisual components. The project aims at developing a database in which a hypermedia encyclopedia is combined with instructional multimedia applications for different target groups at different educational levels. The architecture of the Optical Database will allow for switching between application types while working (for instance from tutorial instruction via the encyclopedia to a simulation and back). For instruction, the content of the database is thereby organized around so-called standard instruction routes: one route per target group. In the project, the teacher is regarded as the manager of instruction.
From that perspective, the database is primarily organized as a teaching facility. Central to the research is the condition that the architecture of the Optical Database has to enable teachers to select and tailor instruction routes to their needs in a way that is perceived as logical and easy to use.     

Formal analysis of the Shlaer-Mellor method: Towards a toolkit of formal and informal requirements specification techniques

In this paper, we define a number of tools that we think belong to the core of any toolkit for requirements engineers. The tools are conceptual and hence, they need precise definitions that lay down as exactly as possible what their meaning and possible use is. We argue that this definition can best be achieved by a formal specification of the tool. This means that for each semi-formal requirements engineering tool we should provide a formal specification that precisely specifies its meaning. We argue that this mutually enhances the formal and semi-formal technique: it makes formal techniques more usable and, as we will argue, at the same time simplifies the diagram-based notations.At the same time, we believe that the tools of the requirements engineer should, where possible, resemble the familiar semi-formal specification techniques used in practice today. In order to achieve this, we should search existing requirements specification techniques to look for a common kernel of familiar semi-formal techniques and try to provide a formalisation for these.In this paper we illustrate this approach by a formal analysis of the Shlaer-Mellor method for object-oriented requirements specification. The formal specification language used in this analysis is LCM, a language based on dynamic logic, but similar results would have been achieved by means of another language. We analyse the techniques used in the information model, state model, process model and communication model of the Shlaer-Mellor method, identify ambiguities and redundancies, indicate how these can be eliminated and propose a formalisation of the result. We conclude with a listing of the tools extracted from the Shlaer-Mellor method that we can add to a toolkit that in addition contains LCM as formal specification technique.     

The formal specification of ORN semantics

Object Relationship Notation (ORN) is a declarative scheme that permits a variety of common types of relationships to be conveniently defined to a Database Management System (DBMS), thereby allowing the DBMS to automatically enforce their semantics. Though first proposed for object DBMSs, ORN is applicable to any data model that represents binary entity-relationships or to any DBMS that implements them. In this paper, we first describe ORN semantics informally as has been done in previous papers. We then provide a formal specification of these semantics using the Z-notation. Specifying ORN semantics via formal methods gives ORN a solid mathematical foundation. The semantics are defined in the context of an abstract database of sets and relations in a recursive manner that is precise, unambiguous, and noncircular.     

Execution of OPS5 production systems on a massively parallel machine

In recent years, the development of expert systems implemented by rule-based production systems has emerged as one of the dominant paradigms in the field of artificial intelligence. While production systems offer important advantages in large-scale AI applications, their use in such applications is typically very costly in execution time. In this paper, we describe an algorithm for executing production systems expressed in the OPS5 language on a massively parallel multiple-SIMD machine called NON-VON, portions of which are currently under construction at Columbia University. The algorithm, a parallel adaptation of Forgy's Rete Match, has been implemented and tested on an instruction-level simulator. We present a detailed performance analysis, based on the implemented code, for the averaged characteristics of six production systems having an average of 910 inference rules each. The analysis predicts an execution rate of more than 850 production firings per second using hardware comparable in cost to a VAX 11/780. By way of comparison, a LISP-based OPS5 interpreter running on a VAX 11/780 typically fires 1 to 5 rules per second, while a Bliss-based interpreter executes 5 to 12 rules per second.     

Towards an architecture for virtual enterprises

An approach to the design of an architecture for industrial virtual enterprises (VE), with special emphasis on the identification of main functional requirements, is presented. First, a discussion of the various types of virtual enterprises is provided. Some classification scenarios and discussions of the modelling and reengineering tools and methodologies are described. Due to the importance of the information flows and management in the VE, one section is merely devoted to the analysis of the appropriateness of a federated information management approach. This work is based on and represents the ongoing activities in two European Union funded projects; the Esprit PRODNET II and the INCO-DC SCM+ projects.