Characterizing the Chain of Evidence for Software Safety Cases: A Conceptual Model Based on the IEC 61508 Standard

Abstract

Critical systems such as those found in the avionics, automotive, maritime, and energy domains are often subject to a formal process known as certification. The goal of certification is to ensure that such systems will operate safely in the presence of known hazards, and without posing undue risks to the users, the public, or the environment. Certification bodies examine such systems based on evidence that the system suppliers provide, to ensure that the relevant safety risks have been sufficiently mitigated. Typically, generic safety standards set forth the general evidence requirements across different industry sectors, and then derived standards specialize the generic standards according to the needs of a specific industry sector. Regardless of whether a generic or sector-specific standard is being used, a key prerequisite for effective collection of evidence is that the supplier be aware of the requirements stipulated in the relevant standard and the evidence they require. This often proves to be a very challenging task because of the sheer size of the standards and the fact that the textual standards are amenable to subjective interpretation. Notably, suppliers find it hard to interpret the evidence requirements imposed by the safety standards within the domain of application; little support exists for recording, querying, and reporting evidence in a structured manner; and there is a general absence of guidelines on how the collected evidence supports the safety objectives. This thesis proposes the application of Model-Driven Engineering as an enabler for performing the various tasks related to safety evidence management. The position taken is that models should serve as the main source of certification information-documents, when needed, should be generated from models. Models are beneficial for the purpose of safety certification in many respects, most notably: (1) Models can be employed to clarify the expectations of safety standards and recommended practices, and develop concrete guidelines for system suppliers; (2) Models expressed in standard notations avoid the ambiguity and redundancy problems associated with text-based documentation; (3) Models provide an ideal vehicle for preserving traceability and the chain of evidence between hazards, requirements, design elements, implementation, and test cases; (4) Models can represent different levels of abstraction and an explicit mapping between the different levels; (5) Models present opportunities for partial or full automation of many laborious safety analysis tasks. The main contribution of this thesis is a model-driven process that enables the automated verification of compliance to standards based on evidence. Specifically, a UML profile is created, based on a conceptual model of a given standard, which provides a succinct and explicit interpretation of the underlying standard. The profile is augmented with constraints that help system suppliers with establishing a relationship between the concepts in the safety standard of interest and the concepts i...

An extended systematic literature review on provision of evidence for safety certification

“…[PS121], address the problem of safety evidence classification through focusing on safety standards such as IEC61508. Further threads, e.g.…”

“…We identified the following approaches in the studies: Sector-specific UML meta-models [PS54][PS122] and UML profiles built specifically for standards such as DO-178B [PS172] and IEC61508 [PS121]. Data modelling using entity-relationship diagrams to structure the data content in large safety cases including the evidence aspects [PS99].…”

Medical Device Software Traceability

“…It is crucial to maintain traceability between the software safety requirements, the decisions taken during design, and their actual implementation in the code. This is a complex task and needs to be performed whilst the system is being developed and not after the development has finished [39].…”

Software Certification: Is There a Case against Safety Cases?

“…For example, a start has been made to develop systematic processes and formalisms that would help increase the level of confidence in the soundness of a safety case [9]. There is also interesting work on how to characterize the chain of evidence, and how to produce it, in making the argument in a safety case [10]. Work is also being done to extend safety claims to more general software properties [11], and a number of researchers have broadened the safety case into an assurance case, see [12] as well.…”