Extension of PRISM by Synthesis of Optimal Timeouts in Fixed-Delay CTMC

We present a practically appealing extension of the probabilistic model checker PRISM rendering it to handle fixed-delay continuous-time Markov chains (fdCTMCs) with rewards, the equivalent formalism to the deterministic and stochastic Petri nets (DSPNs). fdCTMCs allow transitions with fixed-delays (or timeouts) on top of the traditional transitions with exponential rates. Our extension supports an evaluation of expected reward until reaching a given set of target states. The main contribution is that, considering the fixed-delays as parameters, we implemented a synthesis algorithm that computes the epsilon-optimal values of the fixed-delays minimizing the expected reward. We provide a performance evaluation of the synthesis on practical examples

Colored Petri net modelling and evaluation of drone inspection methods for distribution networks

The UAV industry is developing rapidly and drones are increasingly used for monitoring industrial facilities. When designing such systems, operating companies have to find a system configuration of multiple drones that is near-optimal in terms of cost while achieving the required monitoring quality. Stochastic influences such as failures and maintenance have to be taken into account. Model-based systems engineering supplies tools and methods to solve such problems. This paper presents a method to model and evaluate such UAV systems with coloured Petri nets. It supports a modular view on typical setup elements and different types of UAVs and is based on UAV application standards. The model can be easily adapted to the most popular flight tasks and allows for estimating the monitoring frequency and determining the most appropriate grouping and configuration of UAVs, monitoring schemes, air time and maintenance periods. An important advantage is the ability to consider drone maintenance processes. Thus, the methodology will be useful in the conceptual design phase of UAVs, in monitoring planning, and in the selection of UAVs for specific monitoring tasks

Evaluating the impact of maintenance policies associated to SLA contracts on the dependability of data centers electrical infrastructures

Due to the growth of cloud computing, data center environment has grown in importance and in use. Data centers are responsible for maintaining and processing several critical-value applications. Therefore, data center infrastructures must be evaluated in order to improve the high availability and reliability demanded for such environments. This work adopts Stochastic Petri Nets (SPN) to evaluate the impact of maintenance policies on the data center dependability. The main goal is to analyze maintenance policies, associated to SLA contracts, and to propose improvements. In order to accomplish this, an optimization strategy that uses Euclidean distance is adopted to indicate the most appropriate solution assuming conflicting requirements (e.g., cost and availability). To illustrate the applicability of the proposed models and approach, this work presents case studies comparing different SLA contracts and maintenance policies (preventive and corrective) applied on data center electrical infrastructures

Performability evaluation of the ERTMS/ETCS - Level 3

Abstract Level 3 of the ERTMS/ETCS improves the capacity of railways by replacing fixed-block signalling, which prevents a train to enter a block occupied by another train, with moving block signalling, which allows a train to proceed as long as it receives radio messages ensuring that the track ahead is clear of other trains. If messages are lost, a train must stop for safety reasons within a given deadline, even though the track ahead is clear, making the availability of the communication link crucial for successful operation. We combine analytic evaluation of failures due to burst noise and connection losses with numerical solution of a non-Markovian model representing also failures due to handovers between radio stations. In so doing, we show that handovers experienced by a pair of chasing trains periodically affect the availability of the radio link, making behavior of the overall communication system recurrent over the hyper-period of periodic message releases and periodic arrivals at cell borders. As a notable aspect, non-Markovian transient analysis within two hyper-periods is sufficient to derive an upper bound on the first-passage time distribution to an emergency brake, permitting to achieve a trade-off between railway throughput and stop probability. A sensitivity analysis is performed with respect to train speed and headway distance, permitting to gain insight into the consequences of system-level design choices

Toward a decision support system for the clinical pathways assessment

This paper presents a decision support system to be used in hospital management taskswhich is based on the clinical pathways. We propose a very simple graphical modeling lan-guage based on a small number of primitive elements through which the medical doctorscould introduce a clinical pathway for a specific disease. Three essential aspects relatedto a clinical pathway can be specified in this language: (1) patient flow; (2) resource uti-lization; and (3) information interchange. This high-level language is a domain specificmodeling language calledHealthcare System Specification (HSS), and it is defined as anUnified Modeling Language (UML) profile. A model to model transformation is also pro-posed in order to obtain, from the pathways HSS specification, a Stochastic Well-formedNet (SWN) model that enables a formal analysis of the modeled system and, if needed, toapply synthesis methods enforcing specified requirements. The transformation is based onthe application of local rules. The clinical pathway of hip fracture from the “Lozano Blesa”University hospital in Zaragoza is taken as an example

MACHS: Mitigating the Achilles Heel of the Cloud through High Availability and Performance-aware Solutions

Cloud computing is continuously growing as a business model for hosting information and communication technology applications. However, many concerns arise regarding the quality of service (QoS) offered by the cloud. One major challenge is the high availability (HA) of cloud-based applications. The key to achieving availability requirements is to develop an approach that is immune to cloud failures while minimizing the service level agreement (SLA) violations. To this end, this thesis addresses the HA of cloud-based applications from different perspectives. First, the thesis proposes a component’s HA-ware scheduler (CHASE) to manage the deployments of carrier-grade cloud applications while maximizing their HA and satisfying the QoS requirements. Second, a Stochastic Petri Net (SPN) model is proposed to capture the stochastic characteristics of cloud services and quantify the expected availability offered by an application deployment. The SPN model is then associated with an extensible policy-driven cloud scoring system that integrates other cloud challenges (i.e. green and cost concerns) with HA objectives. The proposed HA-aware solutions are extended to include a live virtual machine migration model that provides a trade-off between the migration time and the downtime while maintaining HA objective. Furthermore, the thesis proposes a generic input template for cloud simulators, GITS, to facilitate the creation of cloud scenarios while ensuring reusability, simplicity, and portability. Finally, an availability-aware CloudSim extension, ACE, is proposed. ACE extends CloudSim simulator with failure injection, computational paths, repair, failover, load balancing, and other availability-based modules

Modeling and Evaluation of Stochastic Petri Nets With TimeNET 4.1

Abstract—The paper presents a major update of the software tool TimeNET, a package for the modeling and performance evaluation of standard and colored stochastic Petri nets. Among its main characteristics are simulation and analysis modules for stationary and transient evaluation of Petri nets including nonexponentially distributed delays, as well as an efficient simulation module for complex colored models. An overview of the tool is given as well as a description of the new features, which are demonstrated using a manufacturing system application example. The tool is available free of charge for non-commercial use

Pattern-Based Generation of AMF Configurations

Information Technology service providers aim at attracting customers by providing services that meet a high level of quality. They should not only satisfy functional requirements, but also non-functional requirements. An important non-functional requirement is the level of service availability. Service Availability Forum (SAForum), a consortium of communications and computing companies, has developed a set of services and standard Application Programming Interfaces (APIs) to address the issue of high availability for the Commercial-off-the-shelf (COTS)-based systems and enable portability. Among the services standardized by the SAForum, the Availability Management Framework (AMF) has the important role of ensuring the high availability of an application and its services, by managing the redundant application components deployed on the cluster. To achieve this task, AMF requires a configuration that represents the logical organization of the application components and their services. The design of AMF configurations is a complex and error prone task. Automation of the process is the first step towards improving the quality of such configurations. It also enables exploring different potential solutions for a given set of requirements. An automated approach to generate configurations for applications to deploy on top of the SAForum middleware has been proposed in the context of the MAGIC project. This approach, however, may generate several configurations among which some may not meet the required level of service availability. Therefore, the system designer needs to evaluate the generated configurations using an availability analysis tool to select an appropriate one for deployment. One may want to improve this process by targeting directly in the generation process, the configurations that can guarantee the requested level of service availability. The objective of this thesis is to propose solutions to enhance this configuration generation process and generate configurations that can guarantee the required level of service availability without using advanced analysis tools. For this purpose, we propose configuration design patterns to improve the expected level of service availability and quantitative methods that eliminate some configurations that do not meet the availability requirement. The configuration design patterns improve the expected level of service availability by selecting the best configuration options. The methods estimate service availability for the different possible combinations of software components, which can provide the requested services, taking into account the properties of these components and the behavior of the SAForum middleware. As a proof of concept, we have embedded our proposed solutions into a prototype tool as an eclipse plug-in and validated our work with case studies