A probability model of system downtime with implications for optimal warranty design

Traditional approaches toward modeling the availability of a system often do not formally take into account uncertainty over the parameter values of the model. Such models are then frequently criticized because the observed reliability of a system does not match that predicted by the model. This paper extends a recently published segregated failures model so that, rather than providing a single figure for the availability of a system, uncertainty over model parameter values is incorporated and a predictive probability distribution is given. This predictive distribution is generated in a practical way by displaying the uncertainties and dependencies of the parameters of the model through a Bayesian network (BN). Permitting uncertainty in the reliability model then allows the user to determine whether the predicted reliability was incorrect due to inherent variability in the system under study, or due to the use of an inappropriate model. Furthermore, it is demonstrated how the predictive distribution can be used when reliability predictions are employed within a formal decision‐theoretic framework. Use of the model is illustrated with the example of a high‐availability computer system with multiple recovery procedures. An BN is produced to display the relations between parameters of the model in this case and to generate a predictive probability distribution of the system's availability. This predictive distribution is then used to make two decisions under uncertainty concerning the offered warranty policies on the system: a qualitative decision and an optimization over a continuous decision space. Copyright © 2009 John Wiley & Sons, Ltd.     

Availability Evaluation and Sensitivity Analysis of a Mobile Backend‐as‐a‐service Platform

Performance evaluation of mobile applications has received considerable attention as a prominent activity for improving services quality. Because many data stored on mobile device are synchronized with distributed data centers, the system availability is a critical attribute that requires investigation. Mobile backend‐as‐a‐service (MBaaS) allows developers to link the backend of their applications to cloud storage, as well as providing device management and integration with social networking services. The OpenMobster platform offers a complete synchronization service for mobile applications, but its availability is an inherent critical issue, because one failure can result in losses for companies that use this environment. Analytical models can be used to assess availability of this type of environment and perhaps mitigate downtimes. This paper proposes a hierarchical model to assess the availability of the MBaaS OpenMobster platform focusing on two scenarios: the basic architecture and the automatic recovery process. The designed models were validated through testbed measurements by automatically injecting and repairing the infrastructure. Taking into account the three layers: hardware, operating system, and the MBaaS OpenMobster, we observed OpenMobster being the most critical service component. We have applied failover strategy on the Java virtual machine, and we obtained 10% of reduction in annual downtime. This work may guide systems' administrators in planning their maintenance policies. Copyright © 2015 John Wiley & Sons, Ltd.     

Approximate Reliability and Availability Models for High Availability and Fault‐tolerant Systems with Repair

Systems designed for high availability and fault tolerance are often configured as a series combination of redundant subsystems. When a unit of a subsystem fails, the system remains operational while the failed unit is repaired; however, if too many units in a subsystem fail concurrently, the system fails. Under conditions usually met in practical situations, we show that the reliability and availability of such systems can be accurately modeled by representing each redundant subsystem with a constant, ‘effective’ failure rate equal to the inverse of the subsystem mean‐time‐to‐failure (MTTF). The approximation model is surprisingly accurate, with an error on the order of the square of the ratio mean‐time‐to‐repair to mean‐time‐to‐failure (MTTR/MTTF), and it has wide applicability for commercial, high‐availability and fault‐tolerant computer systems. The effective subsystem failure rates can be used to: (1) evaluate the system and subsystem reliability and availability; (2) estimate the system MTTF; and (3) provide a basis for the iterative analysis of large complex systems. Some observations from renewal theory suggest that the approximate models can be used even when the unit failure rates are not constant and when the redundant units are not homogeneous. Copyright © 2004 John Wiley & Sons, Ltd.     

User‐perceived Availability of a Software Rejuvenation Model with Recovery Time Omission

In this paper, the idea of omitting the recovery time when it is too short that it can be considered as negligible is incorporated in a software rejuvenation model. Two new models are introduced, in which the instantaneous availability is defined when firstly the rejuvenation time and secondly both rejuvenation and repair times can be omitted. The main purpose consists in regarding system software as operational when the time spent in a non‐operational state, like the rejuvenation and the failure states, does not exceed a predefined time threshold. By modeling the evolution of such a software rejuvenation model using continuous‐time Markov chains, we define its availability depending on whether the rejuvenation and the repair critical times are constant or random variables. Copyright © 2015 John Wiley & Sons, Ltd.     

Diagnosis Modelling for Dependability Assessment of Fault‐Tolerant Systems Based on Stochastic Activity Networks

The growing demand for safety, reliability, availability and maintainability in modern technological systems has led these systems to become more and more complex. To improve their dependability, many features and subsystems are employed like the diagnosis system, control system, backup systems, and so on. These subsystems have all their own dynamic, reliability and performances and interact with each other in order to provide a dependable and fault‐tolerant system. This makes the dependability analysis and assessment very difficult. This paper proposes a method to completely model the diagnosis procedure in fault‐tolerant systems using stochastic activity networks. Combined with Monte Carlo simulation, this will allow the dependability assessment by including the diagnosis parameters and performances explicitly. Copyright © 2014 John Wiley & Sons, Ltd.     

Multi-state systems with multi-fault coverage

The paper introduces a new model of fault level coverage for multi-state systems in which the effectiveness of recovery mechanisms depends on the coexistence of multiple faults in related elements. Examples of this effect can be found in computing systems, electrical power distribution networks, pipelines carrying dangerous materials, etc. For evaluating reliability and performance indices of multi-state systems with imperfect multi-fault coverage, a modification of the generalized reliability block diagram (RBD) method is suggested. This method, based on a universal generating function technique, allows performance distribution of complex multi-state series–parallel system with multi-fault coverage to be obtained using a straightforward recursive procedure. Illustrative examples are presented.     

Availability of Maintained Systems: A State-of-the-Art Survey

Availability appears to be a more appropriate measure than reliability for measuring the effectiveness of maintained systems because it includes reliability as well as maintainability. This paper is a survey and a systematic classification of the literature relevant to availability. Emphasis in this paper is centered on the current state of the art on a variety of topics related to availability. Among the topics discussed are: the definition and concepts of the availability; the probability density functions (pdf) of failure times, the pdf of repair times, system configurations, and the various approaches employed to obtain the availability models; confidence intervals of availability; effect of preventive maintenance policies on availability; availability parameters in the model; and systems optimization.     

Flexible disassembly planning considering product conditions

To increase the economic benefit of product recovery at the end of life of a consumer product, the profit margins should be augmented. This can be realised by utilising the existing flexibility of today’s mostly manually-conducted disassembly processes. Moreover, with increased economic benefits, the recovery becomes even more attractive, which is also beneficial to the environment. A key component of product recovery is disassembly. Allowing different disassembly states (or levels) per core (i.e. recovered product) increases flexibility in planning. This should result in higher profits, as long as the flexibility is still manageable for the companies. This study focuses on flexible disassembly planning, i.e. the integration of sequencing aspects into disassembly (process) planning. In addition, we further incorporate the condition of items in the core, item damaging, purity requirements, special treatment of hazardous items and several limitations like core availability, item, module and material distribution, disposal and labour time limit. We base our developed mixed integer linear programme on graphs, such as the disassembly state graph for sequencing, and a hypergraph to model the core condition. Lastly, our considerations are illustrated by a numerical example.     

基于P2P的空间信息服务组合执行引擎

提出了一种基于P2P的空间信息服务组合执行引擎设计.引擎使用空间信息服务团体模型和服务质量参数,在P2P网络中动态地选择最佳的执行引擎,通过P2P的消息和通信机制进行协作,完成复杂的空间信息服务组合执行任务.P2P的执行机制避免了集中式执行引擎带来的网络拥塞和单点失效问题,提高了空间信息服务组合的可靠性和可用性.     

A full history proportional hazards model for preventive maintenance scheduling

This paper is concerned with the development of a realistic preventive maintenance (PM) scheduling model. A heuristic approach for implementing the semi-parametric proportional-hazards model (PHM) to schedule the next preventive maintenance interval on the basis of the equipment's full condition history is introduced. This heuristic can be used with repairable systems and does not require the unrealistic assumption of renewal during repair, or even during PM. Two PHMs are fitted, for the life of equipment following corrective work and the life of equipment following PM, using appropriate explanatory variables. These models are then used within a simulation framework to schedule the next preventive maintenance interval. Optimal PM schedules are estimated using two different criteria, namely maximizing availability over a single PM interval and over a fixed horizon. History data from a set of four pumps operating in a continuous process industry is also used to demonstrate the proposed approach. The results indicate a higher availability for the recommended schedule than the availability resulting from applying the optimal PM intervals as suggested by using the conventional stationary models. © 1997 John Wiley & Sons, Ltd.     

Reliability and fault-tolerant issues of multiprocessor and multicomputer systems

This paper deals with the reliability and fault-tolerance evaluation of multiprocessor and multicomputer architectures considering the degradation of both computation and communication capabilities. Reliability and performance availability (pa) are used to characterize and evaluate the dependability of these architectures. Bandwidth availability (ba) and computation-communication availability (cca) are used to quantify thepa of multiprocessors and multicomputers, respectively. These measures are based on the system requirements for the parallel execution of a task (job) that consists of a few subtasks. We present two different dependability models for multiprocessors, namely: a bus-oriented model (bom) and a switch-oriented model (som). Thebom is an analytical model and is used to evaluate multiprocessors with crossbar and multiple-bus interconnections. Thesom uses simulation to analyze all types of multiprocessors. A simulation technique is also presented to compute the reliability andcca of various types of multicomputer networks suggested in the literature. This research was conducted when Das was with the University of Southwestern Louisiana. It was supported in part by thensf Grant No.dmc-8513041.     

Proactive service migration for long-running Byzantine fault-tolerant systems

A proactive recovery scheme based on service migration for long-running Byzantine fault-tolerant systems is described. Proactive recovery is an essential method for ensuring the long-term reliability of fault-tolerant systems that are under continuous threats from malicious adversaries. The primary benefit of our proactive recovery scheme is a reduced vulnerability window under normal operation. This is achieved in two ways. First, the time-consuming reboot step is removed from the critical path of proactive recovery. Second, the response time and the service migration latency are continuously profiled and an optimal service migration interval is dynamically determined during runtime based on the observed system load and the user-specified availability requirement.     

Reliability of a k-out-of-n: G system with common-mode outages

This paper presents the model of a k-out-of-n: G system with common mode outages. The objective is to analytically derive the mean operating mode between failures for a non-repairable component system. The average system failure time and the system availability are also considered. Then, the model is extended to a system with repairable components and unrestricted repair, in which service times are exponentially distributed.     

Web service availability—impact of error recovery and traffic model

Internet is often used for transaction based applications such as online banking, stock trading and shopping, where the service interruption or outages are unacceptable. Therefore, it is important for designers of such applications to analyze how hardware, software and performance related failures affect the quality of service delivered to the users. This paper presents analytical models for evaluating the service availability of web cluster architectures. A composite performance and availability modeling approach is defined considering various causes of service unavailability. In particular, web cluster systems are modeled taking into account: two error recovery strategies (client transparent and non-client-transparent) as well as two traffic models (Poisson and modulated Poisson). Sensitivity analysis results are presented to show their impact on the web service availability. The obtained results provide useful guidelines to web designers.     

Availability consideration in the optimal selection of multiple-aspect RMS configurations

Machine availability has a profound influence on the performance of manufacturing systems. This paper extends a model for optimizing reconfigurable manufacturing systems (RMS) configurations with multiple-aspects to incorporate the effect of machine availability using the universal generating function (UGF). Two powerful meta-heuristic optimization techniques, namely genetic algorithms (GAs) and tabu search (TS), are used for optimizing the capital cost and system availability of the RMS configurations. The optimized configurations can handle multiple-parts and their structure is that of flow lines allowing paralleling of identical machines in each production stage. The various aspects considered in the RMS configurations include arrangement of machines, equipment selection and assignment of operations. A case study is presented and implementation of the optimization model is carried out using MATLAB software. The results of using both GAs and TS to solve the problem are then reported and compared for validation. Analysis of different cases of availability consideration including infinite and no buffer capacity is performed and results are compared to those obtained when machine availability is not considered. It has been shown that considering availability affects the optimal configuration selection and increases the required equipment. This increases the costs of the near-optimal configurations obtained especially in the case without buffers. The presented model can support the manufacturing systems configuration selection decisions at both the initial design and reconfiguration stages.     

System degraded availability

The concept of degraded availability is introduced, and the required definitions and assumptions are presented. Appropriate metrics are formulated for the quantification of degraded availability at function, mission and system level. This degraded availability model is an extension to the model for availability of multifunctional systems (Sols, A., Availability of continuously operated, coherent, multifunctional systems. Master's thesis, Virginia Polytechnic Institute and State University, 1992).     

基于知识的卫星故障诊断与预测方法

卫星结构的复杂性、运行环境的独特性和诱发故障的多源性，使得卫星故障的诊断与预测较一般设备困难。通常，一种形式的推理只能诊断和预测卫星的一类故障。文章提出了同时应用多种形式推理进行卫星故障诊断和预测的新方法，此方法已成功地应用于基于知识的卫星故障诊断与恢复系统的开发，并取得了显著的效果。     

Determination of maintenance floats using Buzen's algorithm

This paper applies Buzen's algorithm to the problem of determining the maintenance float needed to support an operating system with N circulating units. A maintenance network with load-independent servers is assumed. The operating system availability and maintenance floats are obtained using this iterative algorithm. It is shown that the normalizing constant used in the algorithm can easily be computed for the maintenance float problem by using geometric series. This result is further used to obtain a simple model for the expected number of operating units, thus making it easy to compute the operating system availability for any given N. Buzen's algorithm therefore provides a simple solution to the maintenance float problem.     

Linear consecutive-k-out-of-n: F system reliability with common-mode forced outages

This paper presents a model of consecutive-k-out-of-n: F system subject to common-mode forced outages, whose interarrival times are independent and exponentially distributed. The objective is to analytically derive the mean operating time between failures for a non-repairable component system. The average system failure time and the system availability are also considered. Then, the model is extended to a system with repairable components and unrestricted repair, in which service times are exponentially distributed.     

A combined approach to solve the redundancy optimization problem for multi-state systems under repair policies

This paper combines universal moment generating function technique with stochastic Petri nets to solve the redundancy optimization problem for multi-state systems under repair policies. Redundant elements are included in order to achieve a desirable level of production availability. The elements of the system are characterized by their cost, performance and availability. These elements are chosen from a list of products available on the market. The number of repair teams is less than the number of reparable elements, and a repair policy specifies the maintenance priorities between the system elements. A heuristic is proposed to determine the minimal cost system configuration under availability constraints. This heuristic, first applies universal moment generating function technique to evaluate the system availability, assuming unlimited maintenance resources. Once a preliminary solution is found by the optimization algorithm, stochastic Petri nets are used to model different repair policies, and to find the best system configuration (architecture and number of repairmen) in terms of global performance (availability and cost). This combined procedure is applied to a reference example.