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 correctness 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 executions as well as for those which violate the application requirements. Moreover we assume degradable real time systems equipped with several execution techniques characterised by different execution costs and different levels of fulfilment of requirements (and associated reward). We propose an admission algorithm to select the part of the offered load to be executed, should overload occur. For all the admitted tasks the algorithm selects also the most suitable execution technique (among those available) to optimise the expected cumulated reward. 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. Aplanner 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.