INTERDEPENDENCIES OF REQUIREMENTS. The methods and techniques to apply to ensure that a CIS complies with the above model's requirements are not mutually independent. For a given system, two requirements may be “antagonists”: taking into account one requirement may, if one is not careful, result in an immediate or latent decline in the level of satisfaction with another requirement. The methodological approach is designed to counteract these effects. Conversely, for a given system, two requirements may be “convergent”: taking into account one requirement may inadvertently result in an immediate or latent improvement in the level of satisfaction with another requirement. The methodological approach is designed to reinforce these effects. There is no absolute rule of “systematic” antagonism or convergence between two requirements. Interdependencies between requirements vary for each project or system. However, experience shows that there are “natural” cause-and-effect relations between certain requirements. The figure and the table below illustrate the results of this experience. This is a key consideration. It means that one cannot merely apply a method to suit each requirement, but one must adopt a Example: Impact of uncontrolled actions to improve one requirement over other requirements Focusing exclusively on the security criteria can have: ▪ a positive impact on integrity ▪ a negative impact on performance, availability, capacity and usability ▪ a neutral or undetermined impact on maintainability and resilience “systemic” approach that addresses each requirement as much as its interdependencies with the other requirements. Another interesting example concerns performance. Although focusing on performance might have a negative impact on capacity (high volume of transactions and short response times are antagonistic a priori), capacity may nevertheless benefit from improved performance. Both assertions are arguable. One view is that performance improvements could cause capacity to be exceeded more quickly. On the other hand, they might also improve throughput capacity via faster processing. The table below provides a succinct illustration of the “natural” antagonism and convergence between the specific requirements of CISs. This table does not pretend to define a factual rule since there is no such absolute rules of “systematic” antagonism or convergence between two requirements. Its purpose is to help analyse these interdependencies and to structure the approach of those interdependencies...
INTERDEPENDENCIES OF REQUIREMENTS. The methods and techniques to apply to ensure that a CIS complies with the above model's requirements are not mutually independent. For a given system, two requirements may be “antagonists”: taking into account one requirement may, if one is not careful, result in an immediate or latent decline in the level of satisfaction with another requirement. The methodological approach is designed to counteract these effects. Conversely, for a given system, two requirements may be “convergent”: taking into account one requirement may inadvertently result in an immediate or latent improvement in the level of satisfaction with another requirement. The methodological approach is designed to reinforce these effects. There is no absolute rule of “systematic” antagonism or convergence between two requirements. Interdependencies between requirements vary for each project or system. However, experience shows that there are “natural” cause-and-effect relations between certain requirements. The figure and the table below illustrate the results of this experience. This is a key consideration. It means that one cannot merely apply a method to suit each requirement, but one must adopt a “systemic” approach that addresses each requirement as much as its interdependencies with the other requirements. CW/CIS Best Practices for the Design and Development of Critical Information Systems rev. January 2012
