Architecting Fault-Tolerant Software Systems

The increasing size and complexity of software systems makes it hard to prevent orremove all possible faults. Faults that remain in the system can eventually lead toa system failure. Fault tolerance techniques are introduced for enabling systems torecover and continue operation when they are subject to faults. Many fault tolerancetechniques are available but incorporating them in a system is not always trivial. Weconsider the following problems in designing a fault-tolerant system. First, existingreliability analysis techniques generally do not prioritize potential failures from theend-user perspective and accordingly do not identify sensitivity points of a system.Second, existing architecture styles are not well-suited for specifying, communicatingand analyzing design decisions that are particularly related to the fault-tolerantaspects of a system. Third, there are no adequate analysis techniques that evaluatethe impact of fault tolerance techniques on the functional decomposition of softwarearchitecture. Fourth, realizing a fault-tolerant design usually requires a substantialdevelopment and maintenance effort.To tackle the first problem, we propose a scenario-based software architecture reli-ability analysis method, called SARAH that benefits from mature reliability engi-neering techniques (i.e. FMEA, FTA) to provide an early reliability analysis of thesoftware architecture design. SARAH evaluates potential failures from the end-userperspective to identify sensitive points of a system without requiring an implemen-tation.As a new architectural style, we introduce Recovery Style for specifying fault-tolerantaspects of software architecture. Recovery Style is used for communicating andanalyzing architectural design decisions and for supporting detailed design withrespect to recovery.As a solution for the third problem, we propose a systematic method for optimizingthe decomposition of software architecture for local recovery, which is an effectivefault tolerance technique to attain high system availability. To support the method,we have developed an integrated set of tools that employ optimization techniques,state-based analytical models (i.e. CTMCs) and dynamic analysis on the system.