Dynamic dependability models: an overview

The increasing importance of safety-critical and fault-tolerant systems demands an accurate dependability modeling and analysis of these systems. Dependability analysis plays a key role in assessing the effectiveness of redundancy mechanisms and determining optimal system designs. Traditional combinatorial dependability models, such as static fault trees and reliability block diagrams, are very mature and well understood both in industry and academic environments. However, they cannot accurately capture the dynamic dependencies and interactions among components and other system resources. Because these methods are based on simplified assumptions, they deliver inaccurate and often dangerously over-optimistic results. The models that are capable of capturing the dynamic aspects of dependable-fault tolerant systems are called dynamic dependability models. To be practical and useful for modeling large-scale systems, these models must be intuitive, computationally efficient, and able to capture complex scenarios. To achieve these goals, several novel and innovative modeling and analysis frameworks have been proposed including dynamic fault trees, dynamic reliability block diagrams, Boolean logic-driven Markov processes, and stochastic statecharts.

This tutorial presents an overview of the basic concepts and latest research trends of dynamic dependability models including concepts and formalisms, dependability measures, general-purpose and problem-specific methods and algorithms, and available software tools and trends. Special emphasis is given to modeling concepts of commonly occurring scenarios and the scalability issues associated with analyzing large-scale dynamic systems. Specifically presented are the modeling concepts and algorithms related to dependent and disjoint failures, dynamic redundancies, imperfect fault coverage and reconfiguration mechanisms, aging effects and imperfect maintenances, limited repair resources, and priorities in maintenance actions. In addition, the latest algorithms for analyzing multi-state systems, phased missions systems, standby systems, load-sharing systems, and non-coherent systems are discussed. Coverage encompasses the challenges and research trends in estimating dependability model parameters and determining optimal system designs. The tutorial concludes by highlighting the relationships between various dependability models as well as disclosing the gaps between research and commercial software tools.