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1

Dependability aspects of language technology infrastructure

Walkowiak T., Pol M.

Journal of Polish Safety and Reliability Association

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2018

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Vol. 9, No. 3

101--108

EN

The paper presents dependability analysis of CLARIN-PL Centre of Language Technology (CLT). It describes infrastructure, high availability aspects and micro-service architecture used in CLARIN-PL applications. Microservices architecture improves dependability in respect to availability and reliability and to some extent safety. It is comprised of the mechanisms of reliable communication of applications, replication, recovery, and transaction processing. CLT has also a set of components for failure detection, monitoring and autonomic management, and distributed security policy enforcement.

2

Open Stylometric System WebSty : Integrated Language Processing, Analysis and Visualisation

Piasecki M., Walkowiak T., Eder M.

Computational Methods in Science and Technology

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2018

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Vol. 24, No. 1

43--58

EN

The paper presents an open, web-based system for stylometric analysis named WebSty, which is a part of the CLARIN-PL research infrastructure. WebSty does not require local installation by users, can be used via any web browser, offers rich set-up, and runs on a computing cluster.We discuss the underlying ideas of the system, its architecture, a pipeline of language tools for processing Polish, and its integration with systems for clustering, visualizing the results of clustering, and identifying the features of the strongest discrimination power. The techniques used for feature weighting and text similarity measuring are also concisely overviewed. In conclusions, we present preliminary evaluation of WebSty on the corpus of 1000 literary works, and we report on the results of the first research applications of WebSty. Even if the system was initially focused on processing Polish texts, we also briefly discuss its development towards a multilingual system, which already supports English, German and Hungarian.

3

The impact of administrator working hours on the reliability of the Centre of Language Technology

Walkowiak T., Pol M.

Journal of Polish Safety and Reliability Association

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2017

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Vol. 8, No. 1

167--174

EN

The paper presents reliability analysis of CLARIN-PL Centre of Language Technology (CLT). The CLT is a Polish part of the language technology infrastructure developed by CLARIN project. The main goal of which is to support researchers in humanities and social sciences. The infrastructure is a complex computer system that enables combining language tools with language resources into processing chains. Authors present the system structure, analyse types of faults and define the CLT reliability model. The model takes into account the fact the time gap, between the failure and the repair of the system is not exponential since repair actions are taken only when administrators are at work (assuming that administrators are not working 24/7). The model is used to estimate reliability metrics (mean time and 90th percentile of relative down time and relative partial operational time) by a use of Monte-Carlo simulation. Moreover, analysis of possible improvement in the CLT organisation and its influence on the estimated metrics is given.

4

A repair time model of a web based system including administrator working hours

Walkowiak T.

Journal of Polish Safety and Reliability Association

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2016

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Vol. 7, No. 1

233--240

EN

The paper presents a web based system reliability analysis. We propose to model different types of faults (hardware one, software and security incidents) taking consideration only the effect of the failure, not the source of the fault. It is assumed that failure events are independent and the time to failure is exponential. Whereas the time to repair is not exponential since repair actions are taken only when administrators are at work. We assume that administrators are not working 24/7. The paper presents an algorithmic model of the repair time including administrator working hours. The model is used to estimate reliability parameters (mean time, standard deviation and 90th percentile of yearly down time) by a use of Monte-Carlo simulation. The numerical results are compared with results from the analytical model (Markov one) that assumes exponential distribution of all repair times. Results for two web exemplar web systems (with reliability model consisting of two and five states) show the range of error caused by the exponential distribution assumption.

5

Fuzzy quality analysis of web services implemented in virtualised environment

Walkowiak T.

Journal of Polish Safety and Reliability Association

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2015

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Vol. 6, No. 2

147--152

EN

The paper describes the quality analysis of web services implemented in virtualised environment. It takes into account the reliability and performance aspects of software and hardware elements of the web service. The presented state-transitional model considers the reconfiguration process to maintain the continuity of business services. The reconfiguration is based in on the redeployment of virtual machines so for some time the system operates in a degraded state. Author propose two quality measures based on the web service availability and maximum handled requests calculated at each of degradation levels. Moreover, authors proposed to model the web service reliability parameters by fuzzy numbers. A method for calculating fuzzy membership functions of quality measures are also presented.

6

Simulation based availability assessment of services provided by web applications with realistic repair time

Walkowiak T.

Eksploatacja i Niezawodność

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2014

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Vol. 16, no. 2

341--346

PL

W artykule przedstawiono metodę numerycznego wyznaczania zmian gotowości usług internetowych w czasie. Bierze ona pod uwagę aspekty niezawodnościowe i funkcjonalne systemu komputerowego świadczącego usługi. Analiza niezawodnościowa pozwala na wyznaczenie prawdopodobieństwa, że system będzie zdatny w danym momencie. Uwzględnia ona strukturę systemu komputerowego, losowe czasy do uszkodzeń (sprzętowych jak i oprogramowania związanych z naruszeniami zabezpieczeń) oraz szczegółowy model odnowy biorący pod uwagę godziny pracy administratorów oraz czas związany z dostarczeniem elementów do wymiany. Analiza funkcjonalna pozwala na wyznaczanie prawdopodobieństwa, że użytkownik zostanie obsłużony w czasie mniejszym niż zadany. Oparta jest ona na modelowaniu procesu realizacji usługi jako sekwencji zadań wykonywanych na jednym lub kilku komputerach. Bierze ona pod uwagę zmienność intensywności napływu użytkowników w ciągu tygodnia oraz parametry takie jak: sekwencję zadań, alokację zadań na komputerach oraz parametry techniczne komputerów i zadań. Opisana metoda była podstawą do stworzenia aplikacji komputerowej wyznaczającej techniką symulacji Monte-Carlo zmienność gotowości systemu w ciągu tygodnia. Artykuł zawiera numeryczne rezultaty przykładowej analizy.

EN

Paper presents a numerical method for determining changes of availability of services provided by web applications in time. It takes into account the reliability and functional aspects. The reliability analysis allows determining the probability that the system is operational at a given time. The analysis includes the structure of a computer system, random times to failures (hardware or software – related to security breaches) and the detailed repair time model. The repair time model takes into account working hours of administrators and a time associated with delivering components to exchange. Functional analysis allows the determination of the probability that the user will be served in less than a specified time limit. It is based on modelling the interaction between a user and a server as a sequence of tasks on one or more hosts. It takes into account the variability of workload over a week and web application parameters such as the choreography of service, allocation of tasks on hosts and technical parameters of hosts and tasks. The described method was the basis for development of a Monte-Carlo simulator that allows variability of service availability over a week to be calculated. The paper contains the numerical results of the sample analysis.

7

Critical states reliability analysis of discrete transportation systems

Caban D., Walkowiak T.

Prace Naukowe Politechniki Warszawskiej. Transport

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2013

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z. 96

99--108

EN

The paper presents a resource constrained model of a discrete transportation system that can be used to simulate its operation in presence of faults. The simulation results are used to assess the conditional probability of system failure after a short time after it reaches a specific set of reliability states, i.e. the set of critical states. These are determined as the states when the system is still operational but the probability of failure in near future is unacceptably high. The critical states are shown to be a practical indicator that the system is dangerously degraded.

PL

W artykule przedstawiono model dyskretnych systemów transportowych z ograniczonymi zasobami, wykorzystywany do analizy symulacyjnej ich działania w warunkach występowania uszkodzeń. Wyniki symulacji wykorzystywane są do oceny prawdopodobieństw warunkowych upadku systemu w krótkim horyzoncie czasowym po osiągnięciu określonego zbioru stanów sprawności. Stany krytyczne określane są w oparciu o to prawdopodobieństwo, jako stany, gdy system jest jeszcze sprawny, ale ryzyko upadku w bliskiej przyszłości jest zbyt wysokie. Wykazano, że stany krytyczne mogą być praktycznym wskaźnikiem nadmiernej degradacji systemu.

8

Analiza efektywności systemu transportu dyskretnego z wykorzystaniem detekcji stanów krytycznych

Mazurkiewicz J., Walkowiak T.

Prace Naukowe Politechniki Warszawskiej. Transport

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2013

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z. 96

309--319

PL

Artykuł prezentuje analizę efektywności funkcjonowania systemu transportu dyskretnego. Zaproponowano formalny model tego typu systemów uwzględniając aspekty funkcjonalne i niezawodnościowe. Estymacja miar efektywności odbywa się z użyciem symulacji Monte Carlo. Miara ta staje się także punktem wyjścia do określenia trzech poziomów zdatności systemu – jako zdatnego, niezdatnego oraz pozostającego w stanie krytycznym wobec realizowanych zadań przewozowych. Zaproponowane rozwiązanie pozwala prognozować efektywność systemu w krótkim horyzoncie czasowym. Opisane podejście zostało zweryfikowane analizą zachowania rzeczywistego systemu transportu dyskretnego.

EN

The paper presents the analysis of discrete transportation systems efficiency. The formal model of the transportation system is presented. It takes into consideration functional and reliability aspects. Monte Carlo simulation is used for estimating the system quality metric. The quality of the system is assessed in three levels, as: operational, critical and failed. The proposed solution allows to predict the system quality within the short time horizon. The paper includes numerical results for real mail distribution system.

9

Risk in systems with virtualization : test case analysis

Stanclik M., Walkowiak T.

Journal of Polish Safety and Reliability Association

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2013

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Vol. 4, No. 2

249--258

EN

The paper presents an approach to risk analysis of exemplar test case information systems. Authors point out the common practice to implement virtualization and put away security considerations for future[5]. The overview of virtualization techniques, focusing on server virtualization is given. Next, authors present risk analysis of exemplar GIS system. First of all identification of threats is taken out focusing on virtualization aspects, but it also includes common threats for both that could have a significant impact on safety when using a virtualization. The risk assessment for the test case system was performed using qualitative method. Assessment of the likelihood and magnitude of impact of identified risks was performed on the basis of the expert’s knowledge and experience. The obtained results were used to develop risk rankings, which indicate the risks that need special attention when designing and managing a virtual system.

10

Web server performance and availability model for simulation

Walkowiak T.

Journal of Polish Safety and Reliability Association

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2012

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Vol. 3, No. 2

189--196

EN

The paper presents an approach to modelling and simulation of web systems. The systems are being modelled from the point of view of services realized by them. The formal model of a system, accompanied with a model of a request realization is presented. The main aspect of the paper is a simulation model of client-server interactions that adequately describes the relationship between the server response time, its availability and resource utilization. The model was constructed based on the the results of multiple experiments presented in a paper. Moreover, it was implemented in the simulation tool and its accuracy verified against testbed web servers. Apache and IIS servers were analysed.

11

Fuzzy approach to complex system analysis

Mazurkiewicz J., Nowak K., Walkowiak T.

Journal of Polish Safety and Reliability Association

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2012

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Vol. 3, No. 2

169--178

EN

The paper presents a new approach to reliability and functional analysis of sophisticated complex systems using fuzzy logic. We propose to use such methodology since dependability parameters of the system are mostly approximated by experts instead of classical sources of data. Presented approach show different idea – modeling system using the unified structure – in functional sense. We assess the reliability of the system by accumulated down time. Fuzzy logic based reliability analysis, as well as Computer Information System and Discrete Transport System modeling and simulation are presented. Moreover, results of numerical experiment performed on a test case scenario related to the economic and functional aspects using proposed methodology are given. Fuzzy approach allows reducing the problem of assumptions of reliability distributions and – this way – seems to be very interesting for real systems management and tuning.

12

Simulation approach to Web system dependability analysis

Walkowiak T.

Journal of Polish Safety and Reliability Association

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2011

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Vol. 2, No. 1

197--204

EN

The paper is concerned with the analysis of under-keel clearance of ships based on real data from Szczecin Maritime Office. Using probabilistic method for UKC assessment give it possibility to create the distribution of under-keel clearance in several ship’s passages and in further step to determine the probability of ship’s grounding accident during the port approach. The results of using this type of model could be used in risk assessment of ships entering to the ports, therefore it is possible to used probabilistic model in Maritime Office employees’ everyday decision making practice. Research results present verification of probabilistic model, based on data from Vessel Traffic Service database and probability of touching the bottom during ships’ passages on Swinoujscie – Szczecin waterway. Moreover there are presented examples of these situations when entering to the port for analysed ships is not allowed according to the model.

13

Critical situations and human resources in discrete transportation systems analysis

Walkowiak T., Mazurkiewicz J.

Journal of Polish Safety and Reliability Association

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2011

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Vol. 2, No. 1

205--216

EN

The paper presents an approach to critical situations analysis related to discrete transportation systems (DTS). The critical situation is understood as sudden shortage of some system recourses resulting in the transportation system performance degradation. The analysis is realized based on availability calculation of DTS. The system is described by the formal model, which includes reliability and functional parameters of DTS as well as human resources – drivers and management system. The availability and average availability of the system, defined in a functional way, is discussed as a function of different essential parameters of DTS. The proposed analysis is based on the modelling and simulating of the system behaviour using Monte-Carlo approach. The paper presents some exemplar systems – based on real Polish Post DTS – modelling and results.

14

Simulation of dispatcher driven discrete transport system for reliability and functional analysis

Walkowiak T., Mazurkiewicz J.

Archives of Transport

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2005

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Vol. 17, iss.3-4

221-235

EN

A new model of transport discrete systems with dispatcher allowing to perform the functional and reliability analysis of real transport systems is presented. The position of dispatcher is to mimic a function of real transport logistics. The Monte Carlo simulation is used for proper reliability and functional parameters calculation. The paper presents some exemplary system modelIing and the economic decision problems which are a base for system management. The example concerns the selection of optimum number and time to failures of vehicles. AIso a method of analysis of the risk understood as probability of not fulfilling any economic or functional requirements is presented. lntroduced exemplary analysis shows, that the described method of transport system modelling can serve for practical solving of essential decision problems related to an organisation and parameters of transport systems.

PL

Artykuł prezentuje model systemu transportu dyskretnego z dyspozytorem opracowany na potrzeby realizacji funkcjonalnej i niezawodnościowej analizy rzeczywistych systemów transportowych. Przedstawiony jest przykładowy system transportu wraz z określeniem ekonomicznych problemów decyzyjnych będących podstawą zarządzania jego funkcjonowaniem. Analiza przykładowego systemu koncentruje się wokól określenia optymalnej liczby pojazdów eksploatowanych w systemie oraz doboru średniego czasu do uszkodzenia tych pojazdów. Dyskusji podlega również ryzyko eksploatacji systemu rozumiane jako prawdopodobieństwo spełnienia wymagań ekonomicznych i funkcjonalnych stawianych przed systemem. Wykonane badania pokazują, że zaproponowana metoda modelowania systemów transportu dyskretnego może być z powodzeniem stosowana do rozwiązywania istotnych problemów decyzyjnych związanych z organizacją przewozów, jak i doborem różnego typu parametrów cechujących rzeczywiste systemy transportowe.