Model constraints define semantic conditions that cannot be defined in the syntactic structure of a metamodel. Since different stakeholders may have different interpretations and the underlying assumptions may be overlooked, ExplicitCase requires to document goal decompositions via strategies. Therefore, a constraint on the assurance case model enforces the existence of a strategy node whenever the user wants to connect two goals. ExplicitCase checks many more constraints to ensure the integrity of assurance cases (e.g., to prevent the creation of invalid relationships). For example, another constraint to ensure the integrity of assurance cases is that only GSN connections permitted by the GSN standard can be modeled (e.g., a context node cannot be connected to a justification node). Avoidance of circular argumentation is another built-in constraint on the semantic level.
ExplicitCase offers on-the-fly checks of arbitrary complexity. We define two types of notifications: warnings and errors. Errors signal missing or erroneous information, whereas warnings indicate assurance case nodes that need to be given further consideration. The type of notifications to be get may be manually selected by the user. For example, an error is signaled when a goal is changed and the supporting solution should be reconsider (see Fig. 6). Warnings are, for instance, raised for option entities that cannot be left in the final version of the assurance case, but must be appropriately resolved (see Fig. 7).
Throughout the operational life of any system, changing regulatory requirements, additional assurance evidence and a changing design can challenge the corresponding assurance case. In order to maintain an accurate account of the assurance of the system, all such challenges must be assessed for their impact on the original assurance argument.
An assurance case consists of many inter-dependent parts: requirements, argument, evidence, design and process information. As a result, a single change to an assurance case may necessitate many other consequential changes - creating a 'ripple effect'. It is significant to recognize the importance of every challenge to an assurance case. Furthermore, the indirect impact is crucial and one of the biggest challenges. Any of these challenges imply re-certification and by extension re-generation of the assurance case of a system. The construction and maintenance of assurance case arguments is expensive and tedious, as it is mainly a manual process that requires a considerable amount of time. Therefore, offering safety engineers tool-supported re-evaluation is a big step forward.
The maintenance algorithm includes the handling of challenges regarding the following different argument elements.
If the challenged item is a Goal, it challenges its relationship to both the parent Goal and to the supporting evidence provided. It also challenges the solutions that support the Goal.
If the challenged item is a Solution, it challenges its role as a solution to all goals relying upon it through the SupportedBy relationship.
If the challenged item is a Context, it challenges the relationship with all goals previously expressed in the context of that item using the InContextOf relationship. More specifically, changing a Context challenges all goals, strategies and solutions that introduce this Context. In addition, it challenges all goals, strategies and solutions which inherit this Context.
The rules described above constitute the potential change effect and not necessarily the actual change. There is a significant difference between actual and potential change. The nodes to which the impact of the challenge in a connected GSN node propagates are called impacted nodes. The potential change includes further analysis of the possible effects on the rest of GSN nodes after one element is challenged. A safety engineer has to review all the potential challenges and decide upon them. ExplicitCase implements as a starting point, the potential change effect.
The assurance case maintenance in ExplicitCase requires the participation of different entities and stakeholders (see Fig. 8). The system modeling is done by the system engineer and the GSN modeling of the assurance cases by the safety engineer. The safety engineer has also responsibilities such as hyperlinking GSN with System Models and annotating GSN assurance cases with maintainability information. ExplicitCase recognizes challenges to validity of GSN assurance cases and identifies the impact of a GSN node challenge. Finally, the safety engineer gives input to the system engineer regarding the reasons why, after a change in one system model element, other system model elements, should be reviewed.
