User-Perceived Instantaneous Service Availability Evaluation

Today's businesses rely ever more on dependable service provision deployed on information and communications technology (ICT) infrastructures. Service dependability is highly influenced by the properties of individual infrastructure components. Combining these properties for consistent dependability analysis is challenging as every service requester might use a different set of components during service usage, constituting the user-perceived view on a service. This paper presents a methodology to evaluate user-perceived instantaneous service availability. It uses three input models: (1) The ICT infrastructure, with failure rates, repair rates and deployment times of all components, (2) an abstract description of complex hierarchical services, (3) a mapping that contains concrete ICT components for the service pair requester and provider, as well as existing replicas, and a duration of usage. The presented methodology sets up the basis for automatic generation of availability models from those parts of the ICT infrastructure needed during provision for the specified pair. To calculate instantaneous availability, the age of the ICT components, the order and time of their usage during service provision are taken into account. The methodology supports generation of different availability models, exemplarily providing reliability block diagrams and fault-trees. We demonstrate the feasibility of the proposed approach by applying it to parts of the network infrastructure of University of Lugano, Switzerland.