Mohammad S. Raunak

Maintaining mutual consistency for cached Web objects

Existing Web proxy caches employ cache consistency mechanisms to ensure that locally cached data is consistent with that at the server. We argue that techniques for maintaining consistency of individual objects are not sufficient; a proxy should employ additional mechanisms to ensure that related Web objects are mutually consistent with one another. We formally define the notion of mutual consistency and the semantics provided by a mutual consistency mechanism to end users. We then present techniques for maintaining mutual consistency in the temporal and value domains. A novel aspect of our techniques is that they can adapt to the variations in the rate of change of the source data, resulting in judicious use of proxy and network resources. We evaluate our approaches using real-world Web traces and show that: (i) careful tuning can result in substantial savings in the network overhead incurred without any substantial loss in fidelity, of the consistency guarantees, and (ii) the incremental cost of providing mutual consistency guarantees over mechanisms to provide individual consistency guarantees is small.

Simulating patient flow through an Emergency Department using process-driven discrete event simulation

This paper suggests an architecture for supporting discrete event simulations that is based upon using executable process definitions and separate components for specifying resources. The paper describes the architecture and indicates how it might be used to suggest efficiency improvements for hospital Emergency Departments (EDs). Preliminary results suggest that the proposed architecture provides considerable ease of use and flexibility for specifying a wider range of simulation problems, thus creating the possibility of carrying out a wide range of comparisons of different approaches to ED improvement. Some early comparisons suggest that the simulations are likely to be of value to the medical community and that the simulation architecture offers useful flexibility.

Resource Management for Complex, Dynamic Environments

This paper describes an approach to the specification and management of the agents and resources that are required to support the execution of complex systems and processes. The paper suggests that a resource should be viewed as a provider of a set of capabilities that are needed by a system or process, where that set may vary dynamically over time and with circumstances. This view of resources is defined and then made the basis for the framework of an approach to specifying, managing, and allocating resources in the presence of real-world complexity and dynamism. The ROMEO prototype resource management system is presented as an example of how this framework can be instantiated. Some case studies of the use of ROMEO to support system execution are presented and used to evaluate the framework, the ROMEO prototype, and our view of the nature of resources.

Definition and analysis of election processes

This paper shows that process definition and analysis technologies can be used to reason about the vulnerability of election processes with respect to incorrect or fraudulent behaviors by election officials. The Little-JIL language is used to model example election processes, and various election worker fraudulent behaviors. The FLAVERS finite-state verification system is then used to determine whether different combinations of election worker behaviors cause the process to produce incorrect election results or whether protective actions can be used to thwart these threats.

On effective testing of health care simulation software

Health care professionals rely on software to simulate anatomical and physiological elements of the human body for purposes of training, prototyping, and decision making. Software can also be used to simulate medical processes and protocols to measure cost effectiveness and resource utilization. Whereas much of the software engineering research into simulation software focuses on validation (determining that the simulation accurately models real-world activity), to date there has been little investigation into the testing of simulation software itself, that is, the ability to effectively search for errors in the implementation. This is particularly challenging because often there is no test oracle to indicate whether the results of the simulation are correct. In this paper, we present an approach to systematically testing simulation software in the absence of test oracles, and evaluate the effectiveness of the technique.

Implications of proxy caching for provisioning networks and servers

In this paper, we examine the potential benefits of web proxy caches in improving the effective capacity of servers and networks. Since networks and servers are typically provisioned based on a high percentile of the load, we focus on the effects of proxy caching on the tail of the load distribution. We find that, unlike their substantial impact on the average load, proxies have a diminished impact on the tail of the load distribution. The exact reduction in the tail and the corresponding capacity savings depend on the percentile of the load distribution chosen for provisioning networks and servers—the higher the percentile, the smaller the savings. In particular, compared to over a 50% reduction in the average load, the savings in network and server capacity is only 20-35% for the 99th percentile of the load distribution. We also find that while proxies can be somewhat useful in smoothing out some of the burstiness in web workloads; the resulting workload continues, however, to exhibit substantial burstiness and a heavy-tailed nature. We identify large objects with poor locality to be the limiting factor that diminishes the impact of proxies on the tail of load distribution. We conclude that, while proxies are immensely useful to users due to the reduction in the average response time, they are less effective in improving the capacities of networks and servers.

A framework for simulation validation coverage

Although verification and validation have been studied for modeling and simulation for many decades, we do not yet have a quantitative measure of the level of validation performed on a simulation model. Validation is especially important as it determines whether or not the results from the simulation model can be trusted and used to make statements about the studied system. We propose a validation coverage metric to quantify the validation performed on a simulation model based on the possible validation that could be performed on it. This metric takes into account the aspects of the simulation model that should be validated. To show how such a metric could be utilized, we propose a version of the metric specific to agent-based models, and analyze three example models. We find that the coverage metric can be used to quantify validation on a variety of simulation models.

An Analysis of Vulnerability Trends, 2008-2016

Computer security has been a subject of serious study for at least 40 years, and a steady stream of innovations has improved our ability to protect networks and applications. But attackers have adapted and changed methods over the years as well. Where do we stand today in the battle between attackers and defenders? Are attackers gaining ground, as it often seems when reading press accounts of the latest data exposure? This analysis seeks to answer these questions using data from the US National Vulnerability Database (NVD), and to identify classes of vulnerabilities where improvements will be most cost effective.

Quantifying validation of discrete event simulation models

Simulation model validation and its rigorous assessment is known to be a difficult task. Quantification of validation is necessary to answer the question “how much validation is adequate?” One can answer this question by developing adequacy criteria to measure the validation performed on a simulation model. Developing test adequacy criteria for verifying computer programs has been useful for improving the quality and increasing confidence of regular software systems. We argue that from the validation and verification (V&V) perspective, simulation models are no different than software that are generally termed as “non-testable” due to the absence of a test oracle. There has been little research to develop V&V related adequacy criteria for this type of software. We present a validation coverage criterion, show in detail how it applies to discrete-event simulation models (DES), and discuss how it can be extended to develop V&V coverage criteria for other “non-testable” software.

Process definition language support for rapid simulation prototyping

This paper suggests how an appropriately designed and architected process definition language can be an effective aid to the rapid generation of simulations, which are, in turn, capable of providing important insights. The paper describes how the features of the Little-JIL process definition language helped in the rapid generation of simulations that shed important new light on the effectiveness of various collusion strategies in influencing the outcomes of various auction approaches. The paper describes how Little-JIL’s approach to modular reuse and its separation of process concerns both turn out to be of particular value in supporting rapid prototyping. The simulation results obtained are themselves interesting, as the paper also suggests that the auction idiom is highly relevant to resource allocation in software development. Thus, the insights gained into the efficacy of various collusion approaches have particular relevance to software process research.