Environment-Centric Contracts for Design of Cyber-Physical Systems

Abstract

Abstract. A contract splits the responsibilities between a component and its environment into a guarantee that expresses an intended property under the responsibility of the component, given that the environment fulfills the assumptions. Although current contract theories are limited to express contracts over interfaces of components, specifications that are not limited to interfaces are used in practice and are needed in order to properly express safety requirements. A framework is therefore presented, generalizing current contract theory to environment-centric contracts -contracts that are not limited to the interface of components. The framework includes revised definitions of properties of contracts, as well as theorems that specifies exact conditions for when the properties hold. Furthermore, constraints are introduced, limiting the ports over which an environment-centric contract is expressed where the constraints constitute necessary conditions for the guarantee of the contract to hold in an architecture.

Providing tool support for specifying safety-critical systems by enforcing syntactic contract conditions

“…In [6,68,76], the use of contracts is extended from formal specification of SW to serving as a central systems engineering philosophy to support the design of systems in general. The work in [73] incorporates the work in [6,68,76] and presents a contracts theory that introduces new concepts such as architecture. This contracts theory is extended in [75] to a safety-critical context with the notion of SILs.…”

“…This support is provided by applying the work in [6,68,73,75,76] that present a formal and general contracts [53] theory for modeling and specifying systems.…”

“…These tasks (I)-(IV) are also described in FuSa standards; however, the fact is that in contracts theory [6,68,73,75,76], these tasks are given formal semantics, i.e., interpretations in formal (mathematical) conditions. The present paper capitalizes on this fact by considering the support that can be provided for tasks (I)-(IV) by having a tool that evaluates these formal contract conditions.…”

“…Despite the fact that formal representations have several advantages over non-formal ones, formal languages are difficult to use by non-experts [23] and in industrial practice, 'overcoming the burden of formalization is a major challenge' [10]. Therefore, instead of focusing on enforcing all of the contract conditions in [6,68,73,75,76], the present paper instead identifies necessary conditions of the formal interpretations of tasks (I)-(IV) where these conditions can be evaluated even when requirements and architectures are not represented formally; in the following, such necessary conditions will be called syntactic contract conditions.…”

“…Section 1.1 provides an overview of the contributions and validation approach of the paper. Section 2 introduces relevant parts of the contracts theory in [6,68,73,75,76]. Section 3 identifies syntactic contract conditions of tasks (I)-(IV) in this contracts theory, and as the main contribution, shows how support can be provided for tasks (I)-(IV) by enforcing these syntactic contract conditions.…”