Marios D. Dikaiakos

Scheduling workflows with budget constraints

Grids are emerging as a promising solution for resource and computation demanding applications. However, the heterogeneity of resources in Grid computing, complicates resource management and scheduling of applications. In addition, the commercialization of the Grid requires policies that can take into account user requirements, and budget considerations in particular. This paper considers a basic model for workflow applications modelled as Directed Acyclic Graphs (DAGs) and investigates heuristics that allow to schedule the nodes of the DAG (or tasks of a workflow) onto resources in a way that satisfies a budget constraint and is still optimized for overall time. Two different approaches are implemented, evaluated and presented using four different types of basic DAGs.

Location-Aware Services over Vehicular Ad-Hoc Networks using Car-to-Car Communication

Recent advances in wireless inter-vehicle communication systems enable the establishment of vehicular ad-hoc networks (VANET) and create significant opportunities for the deployment of a wide variety of applications and services to vehicles. In this work, we investigate the problem of developing services that can provide car drivers with time-sensitive information about traffic conditions and roadside facilities. We introduce the vehicular information transfer protocol (VITP), a location- aware, application-layer, communication protocol designed to support a distributed service infrastructure over vehicular ad- hoc networks. We describe the key design concepts of the VITP protocol and infrastructure. We provide an extensive simulation study of VITP performance on large-scale vehicular networks under realistic highway and city traffic conditions. Our results demonstrate the viability and effectiveness of VITP in providing location-aware services over VANETs.

VITP: an information transfer protocol for vehicular computing

Recent advances in wireless inter-vehicle communication systems enable the development of Vehicular Ad-Hoc Networks (VANET) and create significant opportunities for the deployment of a wide variety of vehicular applications and services. In this paper, we introduce the Vehicular Information Transfer Protocol (VITP), an application-layer communication protocol, which is designed to support the establishment of a distributed, ad-hoc service infrastructure over VANET. The VITP infrastructure can be used to provide location-based, traffic-oriented services to drivers, using information retrieved from vehicular sensors and taking advantage of on-board GPS navigation systems. In this paper, we present the key design concepts of the protocol and the infrastructure, the protocol specification, simple examples of protocol interactions that support driver inquiries, and a simulation study of VITP performance properties.

A distributed middleware infrastructure for personalized services

In this paper, we present an overview of extensible Retrieval, Annotation and Caching Engine (eRACE), a modular and distributed intermediary infrastructure that collects information from heterogeneous Internet sources according to registered profiles or end-user requests. Collected information is stored for filtering, transformation, aggregation, and subsequent personalized or wide-area dissemination on the wireline or wireless-Internet. We study the architecture and implementation of the main module of eRACE, an HTTP proxy named WebRACE. WebRACE consists of a high-performance, distributed and multithreaded Web crawler, a multithreaded filtering processor and an Object Cache. We discuss the implementation of WebRACE in Java, describe a number of performance optimizations, and present its performance assessment.

An investigation of web crawler behavior: characterization and metrics

In this paper, we present a characterization study of search-engine crawlers. For the purposes of our work, we use Web-server access logs from five academic sites in three different countries. Based on these logs, we analyze the activity of different crawlers that belong to five search engines: Google, AltaVista, Inktomi, FastSearch and CiteSeer. We compare crawler behavior to the characteristics of the general World-Wide Web traffic and to general characterization studies. We analyze crawler requests to derive insights into the behavior and strategy of crawlers. We propose a set of simple metrics that describe qualitative characteristics of crawler behavior, vis-a-vis a crawlers preference on resources of a particular format, its frequency of visits on a Web site, and the pervasiveness of its visits to a particular site. To the best of our knowledge, this is the first extensive and in depth characterization of search-engine crawlers. Our results and observations provide useful insights into crawler behavior and serve as basis of our ongoing work on the automatic detection of Web crawlers.

Web robot detection: A probabilistic reasoning approach

In this paper, we introduce a probabilistic modeling approach for addressing the problem of Web robot detection from Web-server access logs. More specifically, we construct a Bayesian network that classifies automatically access log sessions as being crawler- or human-induced, by combining various pieces of evidence proven to characterize crawler and human behavior. Our approach uses an adaptive-threshold technique to extract Web sessions from access logs. Then, we apply machine learning techniques to determine the parameters of the probabilistic model. The resulting classification is based on the maximum posterior probability of all classes given the available evidence. We apply our method to real Web-server logs and obtain results that demonstrate the robustness and effectiveness of probabilistic reasoning for crawler detection.

Online Social Networks: Status and Trends

The rapid proliferation of Online Social Network (OSN) sites has made a profound impact on the WWW, which tends to reshape its structure, design, and utility. Industry experts believe that OSNs create a potentially transformational change in consumer behavior and will bring a far-reaching impact on traditional industries of content, media, and communications. This chapter starts out by presenting the current status of OSNs through a taxonomy which delineates the spectrum of attributes that relate to these systems. It also presents an overall reference system architecture that aims at capturing the building blocks of prominent OSNs. Additionally, it provides a state-of-the-art survey of popular OSN systems, examining their architectural designs and business models. Finally, the chapter explores the future trends of OSN systems, presents significant research challenges and discusses their societal and business impact.

On the structure and evolution of vehicular networks

Vehicular ad hoc networks have emerged recently as a platform to support intelligent inter-vehicle communication and improve traffic safety and performance. The road-constrained and high mobility of the vehicles, their unbounded power source, and the emergence of roadside wireless infrastructures make VANETs a challenging research topic. A key to the development of protocols for intervehicle communication and services lies in the knowledge of the topological characteristics of the VANET communication graph. This article provides answers to the general question: how does a VANET communication graph look like over time and space? This study is the first one that examines a very large-scale VANET graph and conducts a thorough investigation of its topological characteristics using several metrics, not examined in previous studies. Our work characterizes a VANET graph at the connectivity (link) level, quantifies the notion of “qualitative” nodes as required by routing and dissemination protocols, and examines the existence and evolution of communities (dense clusters of vehicles) in the VANET. Several latent facts about the VANET graph are revealed and incentives for their exploitation in protocol design are examined.

GridBench: a tool for benchmarking grids

The aim of the GridBench suite of benchmarks is to bring together a core set of benchmarks for characterizing grid nodes or collections of grid resources. In order to do this in an organized and flexible way we provide a framework for running benchmarks on grid environments as well as collecting, archiving, and publishing the results. This framework allows for convenient integration of new and existing benchmarks into the suite.

Mobile agent platforms for Web databases: a qualitative and quantitative assessment

We present practical experiences gathered from the employment of two popular Java-based mobile-agent platforms, IBMs Aglets and Mitsubishis Concordia. We present some basic distributed computing models and describe their adaptation to the mobile-agent paradigm. Upon these models we develop a set of frameworks for distributed database access over the World Wide Web, using IBMs Aglets and Mitsubishis Concordia platforms. We compare the two platforms both quantitatively and qualitatively. For the quantitative comparison, we propose, employ, and validate an approach to evaluate and analyse mobile-agent framework performance. For the qualitative assessment, we present our observations about the programmability and robustness of, and mobility provided by, the two platforms.