HIERARCHICAL MODELING OF AVAILABILITY IN DISTRIBUTED SYSTEMS

Distributed computing systems are attractive due to the potential impr ovement in availability, fault-tolerance, performance, and resource sh aring. Modeling and evaluation of such computing systems is an importa nt step in the design process of distributed systems. In this paper, w e present a two-level hierarchical model to analyze the availability o f distributed systems. At the higher level (user level), the availabil ity of the tasks (processes) is analyzed using a graph-based approach. At the lower level (component level), detailed Markov models are deve loped to analyze the component availabilities. These models take into account the hardware/software failures, congestion and collisions in c ommunication links, allocation of resources, and the redundancy level. A systemic approach is developed to apply the two-level hierarchical model to evaluate the availability of the processes and the services p rovided by a distributed computing environment. This approach is then applied to analyze some of the distributed processes of a real distrib uted system, Unified Workstation Environment (UWE), that is currently being implemented at AT&T Bell Laboratories.