Peter Popov

Modeling software design diversity: a review

Design diversity has been used for many years now as a means of achieving a degree of fault tolerance in software-based systems. While there is clear evidence that the approach can be expected to deliver some increase in reliability compared to a single version, there is no agreement about the extent of this. More importantly, it remains difficult to evaluate exactly how reliable a particular diverse fault-tolerant system is. This difficulty arises because assumptions of independence of failures between different versions have been shown to be untenable: assessment of the actual level of dependence present is therefore needed, and this is difficult. In this tutorial, we survey the modeling issues here, with an emphasis upon the impact these have upon the problem of assessing the reliability of fault-tolerant systems. The intended audience is one of designers, assessors, and project managers with only a basic knowledge of probabilities, as well as reliability experts without detailed knowledge of software, who seek an introduction to the probabilistic issues in decisions about design diversity.

Fault Tolerance via Diversity for Off-the-Shelf Products: A Study with SQL Database Servers

If an off-the-shelf software product exhibits poor dependability due to design faults, then software fault tolerance is often the only way available to users and system integrators to alleviate the problem. Thanks to low acquisition costs, even using multiple versions of software in a parallel architecture, which is a scheme formerly reserved for few and highly critical applications, may become viable for many applications. We have studied the potential dependability gains from these solutions for off-the-shelf database servers. We based the study on the bug reports available for four off-the-shelf SQL servers plus later releases of two of them. We found that many of these faults cause systematic noncrash failures, which is a category ignored by most studies and standard implementations of fault tolerance for databases. Our observations suggest that diverse redundancy would be effective for tolerating design faults in this category of products. Only in very few cases would demands that triggered a bug in one server cause failures in another one, and there were no coincident failures in more than two of the servers. Use of different releases of the same product would also tolerate a significant fraction of the faults. We report our results and discuss their implications, the architectural options available for exploiting them, and the difficulties that they may present.

Dynamic Loading of Polycrystalline Shape Memory Alloy Rods

Abstract Shape memory alloys (SMAs) have recently been considered for dynamic loading applications for energy absorbing and vibration damping devices. An SMA body subjected to external dynamic loading will experience large inelastic deformations that will propagate through the body as phase transformation and/or detwinning shock waves. The wave propagation problem in a cylindrical polycrystalline SMA rod induced by an impact loading is considered in this paper. Numerical solutions for various boundary conditions are presented for stress induced martensite and detwinning of martensite. The numerical simulations utilize an adaptive finite element method (FEM) based on the Zienkiewicz–Zhu error estimator. Selected results are compared to known analytical solutions to verify the adaptive FEM approach. The energy dissipation in an SMA rod is evaluated for a square pulse stress input applied at various temperatures involving both stress induced martensite and detwinning of martensite. The dynamic response of a NiTi SMA rod is also studied experimentally in a split Hopkinson bar apparatus under detwinning conditions. Strain history records obtained by strain gauges placed at different locations along the SMA rod are compared with numerical simulations for a square pulse stress input. The quasi-static and dynamic stress–strain hysteretic response of the SMA, both due to detwinning, are found to be nearly identical. The quasi-static tests are used to calibrate the rate independent constitutive model used for the numerical simulations, which are found to match the experimental observations reasonably well.

On designing dependable services with diverse off-the-shelf SQL servers

The most important non-functional requirements for an SQL server are performance and dependability. This paper argues, based on empirical results from our on-going research with diverse SQL servers, in favour of diverse redundancy as a way of improving both. We show evidence that current data replication solutions are insufficient to protect against the range of faults documented for database servers; outline possible fault-tolerant architectures using diverse servers; discuss the design problems involved; and offer evidence of the potential for performance improvement through diverse redundancy.

Assessment of the Reliability of Fault-Tolerant Software: A Bayesian Approach

Fault tolerant systems based on the use of software design diversity may be able to achieve high levels of reliability more cost-effectively than other approaches, such as heroic debugging. Earlier experiments have shown multi-version software systems to be more reliable than the individual versions. However, it is also clear that the reliability benefits are much worse than would be suggested by naive assumptions of failure independence between the versions. It follows that it is necessary to assess the reliability actually achieved in a fault tolerant system. The difficulty here mainly lies in acquiring knowledge of the degree of dependence between the failures processes of the versions. The paper addresses the problem using Byesian inference. In particular, it considers the problem of choosing a prior distribution to represent the beliefs of an expert assessor. It is shown that this is not easy, and some pitfalls for the unwary are identified.

Impact induced propagation of phase transformation in a shape memory alloy rod

Abstract In this work we study the propagation of a phase-transformation front induced by an impact loading. Our main goals in this paper are to demonstrate the wave structure of the solution to the impact loading and to estimate the velocity of phase front propagation using a thermodynamically based constitutive model for SMAs with internal variables. First, we study the impact problem in an isothermal setting; this study provides an important insight into the wave structure of the solution. The non-uniqueness issue inherent to such problems due to nonconvex SMA constitutive models is thoroughly discussed. Then we develop the solution for an adiabatic approximation for a simplified SMA model. Several numerical examples based on a first order Lax-Friedrichs finite difference scheme are presented.

On Upscaling Certain Flows in Deformable Porous Media

We consider certain computational aspects of upscaling fluid flows through deformable porous media. We start with pore level models and discuss upscaled (homogenized) equations and respective cell ...

Stochastic modelling of the effects of interdependencies between critical infrastructure

An approach to Quantitative Interdependency Analysis, in the context of Large Complex Critical Infrastructures, is presented in this paper. A Discrete state-space, Continuous-time, Stochastic Process models the operation of critical infrastructure, taking interdependencies into account. Of primary interest are the implications of both model detail (that is, level of model abstraction) and model parameterisation for the study of dependencies. Both of these factors are observed to affect the distribution of cascade-sizes within and across infrastructure.

Multi-physics and Multi-scale Methods for Modeling Fluid Flow Through Naturally-Fractured Vuggy Carbonate Reservoirs

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract We present a novel approach for flow simulations through naturally-fractured vuggy carbonate reservoirs. This approach generalizes upscaling methods which have been successfully used to perform reservoir simulations on geological (fine) scales. Typically, vugular porous media is described using both Stokes and Darcys equations at the fine-scale. We propose the use of simplified model based on Stokes-Brinkman equations. Stokes and Darcy equations can be obtained from these equations by appropriate choice of parameters. Moreover, in the presence of damaged zones between vugular regions and Darcy regions, Stokes-Brinkman equations allow a seamless transition. The upscaling of fine-scale equations is addressed within homogenization theory. Appropriate local problems are solved to compute the effective permeabilities, which are further used for the simulations on the field scale. We present numerical results for homogeneous and heterogeneous background permeability fields. Our results show that the coarse-scale permeability field is greatly affected when the background permeability is heterogeneous. This is due to the fact that the high flow channels connecting some of the vugs significantly alter the upscaled permeability. We compare the coarse-scale pressure obtained from upscaled equations with the averaged fine-scale pressure. The results are in agreement which indicates that the upscaled models are accurate for practical purposes.

Finite volume discretization of equations describing nonlinear diffusion in Li-Ion batteries

Numerical modeling of electrochemical process in Li-Ion battery is an emerging topic of great practical interest. In this work we present a Finite Volume discretization of electrochemical diffusive processes occurring during the operation of Li-Ion batteries. The system of equations is a nonlinear, time-dependent diffusive system, coupling the Li concentration and the electric potential. The system is formulated at length-scale at which two different types of domains are distinguished, one for the electrolyte and one for the active solid particles in the electrode. The domains can be of highly irregular shape, with electrolyte occupying the pore space of a porous electrode. The material parameters in each domain differ by several orders of magnitude and can be nonlinear functions of Li ions concentration and/or the electrical potential. Moreover, special interface conditions are imposed at the boundary separating the electrolyte from the active solid particles. The field variables are discontinuous across such an interface and the coupling is highly nonlinear, rendering direct iteration methods ineffective for such problems. We formulate a Newton iteration for a purely implicit Finite Volume discretization of the coupled system. A series of numerical examples are presented for different type of electrolyte/electrode configurations and material parameters. The convergence of the Newton method is characterized both as function of nonlinear material parameters and the nonlinearity in the interface conditions.