A hierarchical modeling of availability in distributed systems

A two-level hierarchical model is proposed to analyze the availability of distributed systems as perceived by their users. At the higher level (user level), the availability of the tasks (processes) is analyzed in terms of the availability of the system components. At the lower level (component level), detailed Markov models are developed to analyze the component availabilities. These models take into account the hardware/software failures, congestion and collisions in communication links, allocation of resources, and the redundancy level. Also presented is the availability analysis of some of the services provided by the unified workstation environment (UWE) currently being implemented at AT&T Bell Laboratories.<<ETX>>