- Availability : The ability to provide proper services
- Reliability : The ability to sustain said proper services
- Safety : Does not result in, catastrophic events to either the environment or the user
- Integrity : There are no improper changes
- Maintainability : The ability to repair and modify
Jackson have identified that there are 3 requirements for a dependability system software.They are explicit claims, evidence and expertise.The reason for the above requirements is that for a system to be dependable the specific properties of a system should be explicitly claimed and should also present evidence to support the said claim.Dependability cases (D-case) are required to show the significant dependability demands satisfied by a system.
Here on the need for D-case is described in terms of demand of standards, applications and expected results.
1.Demand of StandardsISO 26262 is an international standard for functional safety for electrical / electronic automobiles.In Part10 of ISO 26262, guidelines for functional safety are discussed.In "Understanding the safety case" section 5.3 of the above guidelines, CAE (Claims Argument Evidence)* and GSN (Goal Structuring Notation) 3 is introduced as a notation of safety cases.According to the above guideline it can be pointed out that safety cases are required not only for product assessment but also for the development process of the system.
2.ApplicationsBy using D-Case it can be confirmed that products or systems that interact with the environment possesses desirable properties against uncertainty and risk.Therefore it can be concluded that the product or system will not cause dangerous failures.The consensus building results related to the development process of the product or system`s planning, production, human activities and decision making will be recorded.Stakeholders can discuss whether the uncertainty is tolerable or not, through the creation of a D-case. Therefore also as a result of creating D-case we can incorporate the uncertainty and the impact of the properties of claims.
3.Expected ResultsThe following are the expected effects of D-Case
- Have proof that you can provide the level of service claimed.
- Early detection and correction of system errors.
- Encourage proper management for the development and operations process of a product.
- Early impact assessment of the dependability of a system
- A confirmation process can be provided based on evidence regarding the sufficiency of system requirements
- Encourage process improvement by comprehensively verifying the system development and operations process
Fig.3 illustrates an example of the structure of D-Case regarding a system development and operations process.Fig.4 illustrates the comparison between a dependable development and operations process which uses D-case and a status quo development and operations process.Companies at present are capable of streamlining system development and operations using conventional development and operations documents.But these development and operations documents do not possess claims regarding dependability and explicit proof that the said claim is true.Therefore showing that appropriate steps are being taken towards system`s improvement and system`s failures is difficult.In contrast the development and operations process using D-Case explicitly records the claim, assumption and evidence regarding a systems dependability and thereby demonstrate objectively that the claim is true.For this reason, in case of system failures or system improvements, by using D-case we can easily confirm the validity of system improvements and also the swift correspondence to system failures.