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Using Safety Contracts to Guide the Maintenance of Systems and Safety Cases
Mälardalen University, School of Innovation, Design and Engineering, Embedded Systems.ORCID iD: 0000-0002-9347-1949
Mälardalen University, School of Innovation, Design and Engineering, Embedded Systems.ORCID iD: 0000-0003-2415-8219
2017 (English)In: European Dependable Computing Conference EDCC'17, 2017, p. 95-102Conference paper, Published paper (Refereed)
Abstract [en]

Changes to safety-critical systems are inevitable and can impact the safety confidence about a system as their effects can refute articulated claims about safety or challenge the supporting evidence on which this confidence relies. In order to maintain the safety confidence under changes, system developers need to re-analyse and re-verify the system to generate new valid items of evidence. Identifying the effects of a particular change is a crucial step in any change management process as it enables system developers to estimate the required maintenance effort and reduce the cost by avoiding wider analyses and verification than strictly necessary. This paper presents a sensitivity analysis-based technique which aims at measuring the ability of a system to contain a change (i.e., robustness) without the need to make a major re-design. The proposed technique exploits the safety margins in the budgeted failure probabilities of events in a probabilistic fault-tree analysis to compensate for unaccounted deficits or changes due to maintenance. The technique utilises safety contracts to provide prescriptive data for what is needed to be revisited and verified to maintain system safety when changes happen. We demonstrate the technique on an aircraft wheel braking system.

Place, publisher, year, edition, pages
2017. p. 95-102
National Category
Computer Systems
Identifiers
URN: urn:nbn:se:mdh:diva-37017DOI: 10.1109/EDCC.2017.20ISI: 000419858700017Scopus ID: 2-s2.0-85041210865ISBN: 978-1-5386-0602-5 (print)OAI: oai:DiVA.org:mdh-37017DiVA, id: diva2:1160359
Conference
European Dependable Computing Conference EDCC'17, 04 Sep 2017, Geneva, Switzerland
Projects
SYNOPSIS - Safety Analysis for Predictable Software Intensive SystemsFuture factories in the CloudSafeCOP - Safe Cooperating Cyber-Physical Systems using Wireless Communication
Funder
EU, Horizon 2020, 692529 VinnovaAvailable from: 2017-11-27 Created: 2017-11-27 Last updated: 2019-04-18Bibliographically approved
In thesis
1. Contracts-Based Maintenance of Safety Cases
Open this publication in new window or tab >>Contracts-Based Maintenance of Safety Cases
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Safety critical systems are those systems whose failure could result in loss of life, significant property damage, or damage to the environment. System safety is a major property that shall be adequately assured to avoid any severe outcomes in safety critical systems. Safety assurance should provide justified confidence that all potential risks due to system failures are either eliminated or acceptably mitigated. System developers in many domains (e.g., automotive, avionics, railways) should provide convincing arguments regarding the safe performance of their systems to a national or international regulatory authority and obtain approvals before putting the system into service.  Building 'Safety cases' is a proven technique to argue about and communicate systems' safety and it has become a common practice in many safety critical system domains. System developers use safety cases to articulate claims about how systems meet their safety requirements and objectives, collect and document items of evidence, and construct a safety argument to show how the available items of evidence support the claims.

Safety critical systems are evolutionary and constantly subject to preventive, perfective, corrective or adaptive changes during both the development and operational phases. Changes to any part of those systems can undermine the confidence in safety since changes can refute articulated claims about safety or challenge the supporting evidence on which this confidence relies. Hence, safety cases need to be built as living documents that should always be maintained to justify the safety status of the associated system and evolve as these systems evolve. However, building safety cases are costly since they require a significant amount of time and efforts to define the safety objectives, generate the required evidence and conclude the underlying logic behind the safety case arguments. Safety cases document highly dependent elements such as safety goals, assumptions and evidence. Seemingly minor changes may have a major impact. Changes to a system or its environment can necessitate a costly and painstaking impact analysis for systems and their safety cases. In addition, changes may require system developers to generate completely new items of evidence by repeating the verification activities. Therefore, changes can exacerbate the cost of producing and maintaining safety cases.  

Safety contracts have been proposed as a means for helping to manage changes. There have been works that discuss the usefulness of contracts for reusability and maintainability. However, there has been little attention on how to derive them and how exactly they can be utilised for system or safety case maintenance.

The main goal of this thesis is to support the change impact analysis as a key factor to enhance the maintainability of safety cases. We focus on utilising safety contracts to achieve this goal. To address this, we study how safety contracts can support essential factors for any useful change management process, such as (1) identifying the impacted  elements  and  those  that  are  not  impacted, (2) minimising the number of impacted  safety  case  elements, and (3) reducing the  work  needed  to  make  the  impacted  safety  case  elements valid again. The preliminary finding of our study reveals that using safety contracts can be promising to develop techniques and processes to facilitate safety case maintenance. The absence of safety case maintenance guidelines from safety standards and the lack of systematic and methodical maintenance techniques have motivated the work of this thesis. Our work is presented through a set of developed and assessed techniques, where these techniques utilise safety contracts to achieve the overall goal by various contributions. We begin by a framework for evaluation of the impact of change on safety critical systems and safety cases. Through this, we identify and highlight the most sensitive system components to a particular change. We propose new ways to associate system design elements with safety case arguments to enable traceability. How to identify and reduce the propagation of change impact is addressed subsequently.  Our research also uses safety contracts to enable through-life safety assurance by monitoring and detecting any potential mismatch between the design safety assumptions and the actual behaviour of the system during its operational phase. More specifically, we use safety contracts to capture thresholds of selected safety requirements and compare them with the runtime related data (i.e., operational data) to continuously assess and evolve the safety arguments.

In summary, our proposed techniques pave the way for cost-effective maintenance of safety cases upon preventive, perfective, corrective or adaptive changes in safety critical systems thus helping better decision support for change impact analysis.

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2018
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 280
National Category
Computer Systems
Research subject
Computer Science
Identifiers
urn:nbn:se:mdh:diva-41281 (URN)978-91-7485-417-6 (ISBN)
Public defence
2018-12-03, Kappa, Mälardalens högskola, Västerås, 09:30 (English)
Opponent
Supervisors
Projects
SafeCOP - Safe Cooperating Cyber-Physical Systems using Wireless Communication
Funder
EU, Horizon 2020, 692529 Vinnova
Available from: 2018-11-02 Created: 2018-11-02 Last updated: 2019-04-16Bibliographically approved

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Jaradat, OmarBate, Iain

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