Method for Reliability Estimation of COTS Components based Software Systems

In this paper a relatively simple and implementable approach is proposed for reliability calculation of software systems which are constructed from COTS components. This approach estimates overall software system reliability based on the individual component reliabilities which are integrated to form the system. Moreover those components having longer execution time contribute more towards overall system reliability. Here we have used operational profile of a component for the calculation of component usage ratio, which uses operational profile for a very different purpose from its traditional use in software reliability engineering. Now-a-days, Commercial Off the Shelf (COTS) components are used frequently in large software systems. Large scale use of COTS components has raised questions on the component's reliability, and the reliability of systems derived out of these components. This paper tries to estimate the overall reliability of the COTS components based Software Systems, following a strict reliability model. Corporate downsizing and decreased government budgets, as well as the spiraling costs of building and maintaining large software systems, have made necessary the reuse of existing software components (1). Moreover, this paradigm shift to COTS components appears inevitable, necessating drastic changes to current software development and business practices (2, 3). Hence we need a model for estimating system reliability, to have a solid foundation and rigid system. In terms of software, the proportion of COTS software in a typical system is beginning to overtake the percentage of custom software. When a system fails, it may well be the COTS software that caused the system to fail, given the well-publicized defect rates for acquired software. Now the question arises why the model should be a strict one, i.e. with more accuracy, because when COTS components are put to use in larger system, they might be the weakest link in the chain so their reliability evaluation should be done at par with the custom components (4). COTS components are "components which are bought from a third-party and integrated into a system" (5). This reliability model should predict reliability of overall system based on individual reliability of COTS components as well as the interaction between them when integrated into a larger system. The approach proposed in this paper takes into the consideration the contribution of a component's reliability to overall system reliability depending upon its usage time when the execution of overall system takes place. In the following text, in section II, we discuss how overall system reliability can be calculated from individual component reliability. The concept of usage time ratio is also discussed. In section III, we discuss the idea of path propagation probability with reasoning needed for understanding the proposed approach. In section IV, the mathematical model is proposed for calculation of reliability of overall system.