An Optimal Value-Based Admission Policy and its Reflective Use in Real-Time Dependable Systems

This paper deals with planning System activities to support applications that have different contrasting requirements including timing constraints on tasks execution and dependability requirements. Our approach is based on a simple yet effective formulation of a value structure associated to the application tasks. Values are associated to each relevant outcome thus accounting for successful as well as erroneous executions. Moreover we assume degradable real time systems equipped with several (possibly fault tolerant) execution techniques characterised by different execution costs and different levels of expected success rate. We propose an admission algorithm to select thè part of thè offered load to be executed, should overload occur. For ali thè admitted tasks the algorithm selects also the most suitable execution technique (among those available) to optimise performability. We show that the algorithm provides the best solution to the optimisation problem resorting to the linear programming theory. Then we discuss the applicability of this result to systems operating in dynamic environments. A planner component is defined, responsible to collect information on the current status of the System and of its environment. The planner decides when a new 'plan' is required, and dynamically executes the admission algorithm to properly tune the usage of System resources .