Dependability of IT Systems in Emergency Situations – Theory and Practice

As our dependence on IT systems increases, evaluating the dependability of critical IT systems becomes more important. One of the main challenges in software reliability engineering is the sensitivity of software systems to a changing usage. This is especially important for systems that are critical in the aftermath of a crisis and for which reliability is the most important aspect of dependability. The crisis might change the usage of the system, and this could have a negative effect on the reliability. Because crisis situations are typically rare events, both the reliability and the criticality of IT systems after a crisis situation are hard to predict. The first part of this thesis focuses on the analysis of the sensitivity of the reliability of IT systems to changes in their usage. With the help of statistical methods the effects of changing usage profiles, modelled through the use of Markov models, can be examined. After a theoretical derivation of the properties of different models for the usage of software systems, the results were validated by applying the models to the data collected from the logfiles of a webserver. Swedish municipalities also depend more and more on IT systems for their daily work. Because of their important role in the relief coordination after a crisis, the dependability of their IT systems during these emergency situations is especially critical. The evaluation of this dependability requires the combination of two kinds of information: how critically needed the IT systems are in the aftermath of a crisis and how trustworthy the critical systems are. To avoid that a failing IT system disturbs the relief work, risk and vulnerability analyses need to take into account the dependability of critical IT systems. This way, municipalities can make sure that the relief work is not critically dependent on systems that are not sufficiently reliable. The second part of this thesis describes a case study on how two Swedish municipalities deal with these issues. The study focuses especially on the division of responsibilities in the municipalities and on their current methods. The study shows that today there is much room for improvement, especially in the communication between IT personnel and emergency managers. The main goal of these case studies is to form a basis for the development of practical methods that can assist Swedish municipalities in evaluating the dependability of their IT systems and integration of this information in their emergency planning in the near future

Sensitivity of Website Reliability to Usage Profile Changes

To measure the reliability of a website from a user’s point of view, the uncertainty on the usage of the website has to be taken into account. In this paper we investigate the influence of this uncertainty on the reliability estimate for a web server. For this purpose a session based Markov model is used to model the usage extracted from the server’s logfiles. From these logfiles a complete user profile can be extracted together with an estimate of the uncertainty on this user profile. This paper investigates the applicability of this kind of Markov model on web server reliability and discusses the difficulties with data extraction from the logfiles. Advantages and disadvantages of this approach are discussed and the approach is applied to data from a university department’s web server to demonstrate its applicability