Several recent studies have reported & examined the phenomenon that long-running software systems show an increasing failure rate and/or a progressive degradation of their performance. Causes of this phenomenon, which has been referred to as “software aging”, are the accumulation of internal error conditions, and the depletion of operating system resources. A proactive technique called “software rejuvenation” has been proposed as a way to counteract software aging. It involves occasionally terminating the software application, cleaning its internal state and/or its environment, and then restarting it. Due to the costs incurred by software rejuvenation, an important question is when to schedule this action. While periodic rejuvenation at constant time intervals is straightforward to implement, it may not yield the best results. The rate at which software ages is usually not constant, but it depends on the time-varying system workload. Software rejuvenation should therefore be planned & initiated in the face of the actual system behavior. This requires the measurement, analysis, and prediction of system resource usage. In this paper, we study the development of resource usage in a web server while subjecting it to an artificial workload. We first collect data on several system resource usage & activity parameters. Non-parametric statistical methods are then applied toward detecting & estimating trends in the data sets. Finally, we fit time series models to the data collected. Unlike the models used previously in the research on software aging, these time series models allow for seasonal patterns, and we show how the exploitation of the seasonal variation can help in adequately predicting the future resource usage. Based on the models employed here, proactive management techniques like software rejuvenation triggered by actual measurements can be built.

Recently, the phenomenon of software aging, one in which the state of the software system degrades with time, has been reported. This phenomenon, which may eventually lead to system performance degradation and/or crash/hang failure, is the result of exhaustion of operating system resources, data corruption, and numerical error accumulation. To counteract software aging, a technique called software rejuvenation has been proposed, which essentially involves occasionally terminating an application or a system, cleaning its internal state and/or its environment, and restarting it. Since rejuvenation incurs an overhead, an important research issue is to determine optimal times to initiate this action. In this paper, we first describe how to include faults attributed to software aging in the framework of Gray's software fault classification (deterministic and transient), and study the treatment and recovery strategies for each of the fault classes. We then construct a semi-Markov reward model based on workload and resource usage data collected from the UNIX operating system. We identify different workload states using statistical cluster analysis, estimate transition probabilities, and sojourn time distributions from the data. Corresponding to each resource, a reward function is then defined for the model based on the rate of resource depletion in each state. The model is then solved to obtain estimated times to exhaustion for each resource. The result from the semi-Markov reward model are then fed into a higher-level availability model that accounts for failure followed by reactive recovery, as well as proactive recovery. This comprehensive model is then used to derive optimal rejuvenation schedules that maximize availability or minimize downtime cost.

Quite often failures in network based services and server systems may not be accidental, but rather caused by deliberate security intrusions. We would like such systems to either completely preclude the possibility of a security intrusion or design them to be robust enough to continue functioning despite security attacks. Not only is it important to prevent or tolerate security intrusions, it is equally important to treat security as a QoS attribute at par with, if not more important than other QoS attributes such as availability and performability. This paper deals with various issues related to quantifying the security attribute of an intrusion tolerant system, such as the SITAR system. A security intrusion and the response of an intrusion tolerant system to the attack is modeled as a random process. This facilitates the use of stochastic modeling techniques to capture the attacker behavior as well as the system's response to a security intrusion. This model is used to analyze and quantify the security attributes of the system. The security quantification analysis is first carried out for steady-state behavior leading to measures like steady-state availability. By transforming this model to a model with absorbing states, we compute a security measure called the "mean time (or effort) to security failure" and also compute probabilities of security failure due to violations of different security attributes.

Recently, the phenomenon of "software aging", one in which the state of a software system gradually degrades with time and eventually leads to performance degradation or crash/hang failure, has been reported. Preventive maintenance of operational software systems is used specifically to counteract this phenomenon. However preventive maintenance incurs an overhead in terms of downtime and cost and this must be traded off with the cost of failures to obtain maximum benefits. We present an analytical model of a software system employing inspection-based preventive maintenance, through a Markov Regenerative Process (MRGP) with a subordinated semi-Markov reward process. Furthermore, we consider preemptive-resume type transitions. The model is solved for steady state as well as transient conditions and expressions for expected downtime and expected cost are derived. Numerical examples are presented to illustrate the applicability of the models. With the help of these models, optimal strategies for preventive maintenance techniques such as "software rejuvenation" could be formulated.

Software systems are known to suffer from outages due to transient errors. Recently, the phenomenon of "software aging", in which the state of the software system degrades with time, has been reported (S. Garg et al., 1998). The primary causes of this degradation are the exhaustion of operating system resources, data corruption and numerical error accumulation. This may eventually lead to performance degradation of the software or crash/hang failure, or both. Earlier work in this area to detect aging and to estimate its effect on system resources did not take into account the system workload. In this paper, we propose a measurement-based model to estimate the rate of exhaustion of operating system resources both as a function of time and the system workload state. A semi-Markov reward model is constructed based on workload and resource usage data collected from the UNIX operating system. We first identify different workload states using statistical cluster analysis and build a state-space model. Corresponding to each resource, a reward function is then defined for the model based on the rate of resource exhaustion in the different states. The model is then solved to obtain trends and the estimated exhaustion rates and the time-to-exhaustion for the resources. With the help of this measure, proactive fault management techniques such as "software rejuvenation" (Y. Huang et al., 1995) may be employed to prevent unexpected outages.

The phenomenon of software aging refers to the accumulation of errors during the execution of the software which eventually results in it's crash/hang failure. A gradual performance degradation may also accompany software aging. Pro-active fault management techniques such as "software rejuvenation" (Y. Huang et al., 1995) may be used to counteract aging if it exists. We propose a methodology for detection and estimation of aging in the UNIX operating system. First, we present the design and implementation of an SNMP based, distributed monitoring tool used to collect operating system resource usage and system activity data at regular intervals, from networked UNIX workstations. Statistical trend detection techniques are applied to this data to detect/validate the existence of aging. For quantifying the effect of aging in operating system resources, we propose a metric: "estimated time to exhaustion", which is calculated using well known slope estimation techniques. Although the distributed data collection tool is specific to UNIX, the statistical techniques can be used for detection and estimation of aging in other software as well.