Nowadays, an increasing number of blockchain architectures provide well-promising protocols for pseudonymous online payments via proposed cryptocurrencies. Most of them suffer from a number of extensibility and scalability issues, as their capacity regarding the number of transactions they are capable of processing per second is limited. Security is also a challenge for this kind of architectures. This paper presents the design and implementation of the Adrestus system, a blockchain-based transaction system with a novel consensus mechanism that is able to tolerate Byzantine faults and is designed to scale without compromising system security. One of the main components of the Adrestus design is a consistent hashing mechanism for the efficient assignment of transactions on parallel regions, called zones, and for solving load balancing problems. We claim that the Adrestus blockchain system scales linearly without compromising system security and achieves its goals without introducing the unnecessary overhead and by eliminating energy and computational waste. Preliminary theoretical simulations and results reflect that Adrestus exceeds the average throughput of the most well-known cryptocurrencies like Bitcoin, and thus, it achieves a higher performance. In this paper, we present this proposed approach along with simulation results and examine the conditions for the proposed fault-tolerant system to meet safety and liveness.

Despite the steady growth of e-commerce communities in the past two decades, little has changed in the way these communities manage reputation for building trust and for protecting their member's financial interests against fraud. As these communities mature and the defects of their reputation systems are revealed, further potential for deception against their members is created, that pushes the need for novel reputation mechanisms. Although a high volume of research works has explored the concepts of reputation and trust in e-communities, most of the proposed reputation systems target decentralized e-communities, focusing on issues related with the decentralized reputation management; they have not thus been integrated in e-commerce platforms. This work's objective is to provide an attackresilient feedback-based reputation system for modern e-commerce platforms, while minimizing the incurred financial burden of potent security schemes. Initially, we discuss a series of attacks and issues in reputation systems and study the different approaches of these problems from related works, while also considering the structural properties, defense mechanisms and policies of existing platforms. Then we present our proposition for a robust reputation system which consists of a novel reputation metric and attack prevention mechanisms. Finally, we describe the simulation framework and tool that we have implemented for thoroughly testing and evaluating the metric's resilience against attacks and present the evaluation experiments and their results. We consider the presented simulation framework as the second contribution of our article, aiming at facilitating the simulation and elaborate evaluation of reputation systems which specifically target e-commerce platforms by thoroughly presenting it, exhibiting its usage and making it available to the research community.

This article presents BPELcube, a framework comprising a scalable architecture and a set of distributed algorithms, which support the decentralized enactment of BPEL processes. In many application domains, BPEL processes are long-running, involve the exchange of voluminous data with external Web services, and are concurrently accessed by large numbers of users. In such context, centralized BPEL process execution engines pose considerable limitations in terms of scalability and performance. To overcome such problems, a scalable hypercube peer-to-peer topology is employed by BPELcube in order to organize an arbitrary number of nodes, which can then collaborate in the decentralized execution and monitoring of BPEL processes. Contrary to traditional clustering approaches, each node does not fully take charge of executing the whole process; rather, it contributes to the overall process execution by running a subset of the process activities and maintaining a subset of the process variables. Hence, the hypercube-based infrastructure acts as a single execution engine, where workload is evenly distributed among the participating nodes in a fine-grained manner. An experimental evaluation of BPELcube and a comparison with centralized and clustered BPEL engine architectures demonstrate that the decentralized approach yields improved process execution times and throughput.

In this paper, we propose Proteus, a generic query model for the discovery of operations offered by heterogeneous services. We demonstrate the need for such a model, and show how it unifies the task of service discovery through abstractions, which allow for the technology-independent formulation of service advertisements, queries, and query responses. On top of these generic elements, we build an intuitive, fuzzy-based query evaluation mechanism that supports the service matchmaking process by employing and appropriately combining existing similarity metrics. Thanks to the generality of Proteus, it is possible to seamlessly accommodate the discovery of operations provided by various types of services without the need of changing the existing service infrastructure. Thus, our approach is applicable to a variety of settings ranging from traditional web services to service-oriented grids, peer-to-peer networks, geospatial information systems, and so on. Overall, compared to the existing query models supported by standard service discovery technologies, our approach is marked by openness, flexibility, and improved performance in terms of precision and recall. The feasibility and efficiency of Proteus are verified by a series of experiments.

P2P communities are online communities of entities which offer services to each other without a central administration. Community members that need a specific service have to choose the transaction partner which they believe that will provide them with a required service of the expected quality, thus, they need mechanisms to support trust decisions regarding who they will transact with. P2P reputation systems provide trust mechanisms for P2P communities. Due to their decentralized and social nature, they are vulnerable to various types of attacks which distort their credibility and effectiveness. In our paper we first identify the credibility factors of a P2P reputation system. Then we describe a dynamic reputation mechanism for P2P communities, which integrates some of these credibility factors and which can be used as part of a comprehensive reputation system enhancing its robustness against some types of attacks. Simulation results show the effectiveness of our mechanism in relation to other reputation mechanisms which do not integrate such factors.

The challenge of publishing and discovering Web services has recently received lots of attention. Various solutions to this problem have been proposed which, apart from their offered advantages, suffer the following disadvantages: (i) most of them are syntactic-based, leading to poor precision and recall, (ii) they are not scalable to large numbers of services, and (iii) they are incompatible, thus yielding in cumbersome service publication and discovery. This article presents the principles, the functionality, and the design of PYRAMID-S which addresses these disadvantages by providing a scalable framework for unified publication and discovery of semantically enhanced services over heterogeneous registries. PYRAMID-S uses a hybrid peer-to-peer topology to organize Web service registries based on domains. In such a topology, each Registry retains its autonomy, meaning that it can use the publication and discovery mechanisms as well as the ontology of its choice. The viability of this approach is demonstrated through the implementation and experimental analysis of a prototype.

As the Web service technology matures, other computing paradigms such as peer-to-peer gradually adopt the service-oriented approach and are beginning to expose functionality as services. Hence there is a need for integration of these heterogeneous services, for the development of service-oriented applications. The first step to accomplish this is to establish a unified approach for service discovery. In this paper, we briefly present a query language along with its enacting service search engine which is used for effectively discovering Web and P2P services in a unified manner

This paper presents a comparison of several technologies for developing distributed applications. The specific technologies into consideration are CORBA, Enterprise JavaBeans and Web Services. The comparison involves both qualitative factors, like data representation, transfer protocols and discovery services, as well as quantitative factors, which measure the performance of the models into consideration. In order to facilitate the comparison, an application has been developed that implements exactly the same functionality in all three technologies.