Luigi Sauro

Robust and Scalable Linked Data Reasoning Incorporating Provenance and Trust Annotations

In this paper, we leverage annotated logic programs for tracking indicators of provenance and trust during reasoning, specifically focussing on the use-case of applying a scalable subset of OWL 2 RL/RDF rules over static corpora of arbitrary Linked Data (Web data). Our annotations encode three facets of information: (i) blacklist: a (possibly manually generated) boolean annotation which indicates that the referent data are known to be harmful and should be ignored during reasoning; (ii) ranking: a numeric value derived by a PageRank-inspired technique—adapted for Linked Data—which determines the centrality of certain data artefacts (such as RDF documents and statements); (iii) authority: a boolean value which uses Linked Data principles to conservatively determine whether or not some terminological information can be trusted. We formalise a logical framework which annotates inferences with the strength of derivation along these dimensions of trust and provenance; we formally demonstrate some desirable properties of the deployment of annotated logic programming in our setting, which guarantees (i) a unique minimal model (least fixpoint); (ii) monotonicity; (iii) finitariness; and (iv) finally decidability. In so doing, we also give some formal results which reveal strategies for scalable and efficient implementation of various reasoning tasks one might consider. Thereafter, we discuss scalable and distributed implementation strategies for applying our ranking and reasoning methods over a cluster of commodity hardware; throughout, we provide evaluation of our methods over 1 billion Linked Data quadruples crawled from approximately 4 million individual Web documents, empirically demonstrating the scalability of our approach, and how our annotation values help ensure a more robust form of reasoning. We finally sketch, discuss and evaluate a use-case for a simple repair of inconsistencies detectable within OWL 2 RL/RDF constraint rules using ranking annotations to detect and defeat the “marginal view”, and in so doing, infer an empirical “consistency threshold” for the Web of Data in our setting.

Reasoning about action and cooperation

We present a logic for reasoning both about the ability of agents to cooperate to execute complex actions, and how this relates to their ability to reach certain states of affairs. We show how the logic can be obtained in a modularised way, by combining a model for reasoning about actions and their effects with a model that describes what actions an agent can perform. More precisely, we show how one can combine an action logic which resembles Propositional Dynamic Logic with a cooperation logic which resembles Coalition Logic. We give a sound and complete axiomatisation for the logic, illustrate its use by means of an example, and discuss possible future extensions to it.

Defeasible inclusions in low-complexity DLs

Some of the applications of OWL and RDF (e.g. biomedical knowledge representation and semantic policy formulation) call for extensions of these languages with nonmonotonic constructs such as inheritance with overriding. Nonmonotonic description logics have been studied for many years, however no practical such knowledge representation languages exist, due to a combination of semantic difficulties and high computational complexity. Independently, low-complexity description logics such as DL-lite and EL have been introduced and incorporated in the OWL standard. Therefore, it is interesting to see whether the syntactic restrictions characterizing DL-lite and EL bring computational benefits to their nonmonotonic versions, too. In this paper we extensively investigate the computational complexity of Circumscription when knowledge bases are formulated in DL-liteR, EL, and fragments thereof. We identify fragments whose complexity ranges from P to the second level of the polynomial hierarchy, as well as fragments whose complexity raises to PSPACE and beyond.

A new semantics for overriding in description logics

Abstract Many modern applications of description logics (DLs, for short), such as biomedical ontologies and semantic web policies, provide fresh motivations for extending DLs with nonmonotonic inferences—a topic that has attracted a significant amount of attention along the years. Despite this, nonmonotonic inferences are not yet supported by DL technology due to a number of issues related to expressiveness, computational complexity, and optimizations. This paper contributes to the practical support of nonmonotonic inferences in description logics by introducing a new semantics expressly designed to address knowledge engineering needs. This formalism has appealing expressiveness, enjoys nice computational properties, and constitutes an interesting solution to an ample class of application needs. The formalism is validated through extensive comparison with the other nonmonotonic DLs, and systematic scalability tests. The test case generator and its novel validation methodology constitute a further contribution of this paper.

On the Boundary of Behavioral Strategies

In the setting of multi-agent games, considerable effort has been devoted to the definition of modal logics for strategic reasoning. In this area, a recent contribution is given by the introduction of Strategy Logic (SL, for short) by Mogavero, Murano, and Vardi. This logic allows to reason explicitly about strategies as first order objects and express in a very natural and elegant way several solution concepts like Nash, resilient, and secure equilibria, dominant strategies, etc. The price that one has to pay for the high expressiveness of SL semantics is that agents strategies it admits may be not behavioral, i.e., a choice of an agent, at a given moment of a play, may depend on the choices another agent can make in another counterfactual play. As the latter moves are unpredictable, this kind of strategies cannot be synthesized in practice. In this paper, we investigate two syntactical fragments of SL, namely the conjunctive-goal and disjunctive-goal, called SL[CG] and SL[DG] for short, and prove that their semantics admit behavioral strategies only. These logics are obtained by forcing SL formulas to be only of the form of conjunctions or disjunctions of goals, which are temporal assertions associated with a binding of agents with strategies. As SL formulas with any Boolean combination of goals turn out to be non behavioral, we have that SL[CG] and SL[DG] represent the maximal fragments of SL describing agent behaviors that are synthesizable. As a consequence of the above results, the model-checking problem for both SL[CG] and SL[DG] is shown to be solvable in 2EXPTIME, as it is for the subsumed logic ATL*.

A Rule-Based Trust Negotiation System

Open distributed environments, such as the World Wide Web, facilitate information sharing but provide limited support to the protection of sensitive information and resources. Trust negotiation (TN) frameworks have been proposed as a better solution for open environments, in which parties may get in touch and interact without being previously known to each other. In this paper, we illustrate Protune, a rule-based TN system. By describing Protune, we will illustrate the advantages that arise from an advanced rule-based approach in terms of deployment efforts, user friendliness, communication efficiency, and interoperability. The generality and technological feasibility of Protunes approach are assessed through an extensive analysis and experimental evaluations.

Rule-based policy representations and reasoning

Trust and policies are going to play a crucial role in enabling the potential of many web applications. Policies are a well-known approach to protecting security and privacy of users in the context of the Semantic Web: in the last years a number of policy languages were proposed to address different application scenarios. The first part of this chapter provides a broad overview of the research field by accounting for twelve relevant policy languages and comparing them on the strength of ten criteria which should be taken into account in designing every policy language. By comparing the choices designers made in addressing such criteria, useful conclusions can be drawn about strong points and weaknesses of each policy language. The second part of this chapter is devoted to the description of the Protune framework, a system for specifying and cooperatively enforcing security and privacy policies on the Semantic Web developed within the network of excellence REWERSE. We describe the frameworks functionalities, provide details about their implementation, and report the results of performance evaluation experiments.

Power and dependence relations in groups of agents

We present a formal model of multiagent systems to analyze the relations of power and dependence underlying group behaviors such as cooperation. Inspired by the work of Castelfranchi we define these relations by means of a description of goals and skills of single agents. We show how our framework can be used to describe social and organizational structures as emergent properties of a collection of individuals.

Dependency in Cooperative Boolean Games

Cooperative boolean games (CBG) are a family of coalitional games where agents may depend on each other for the satisfaction of their personal goals. In Dunne et al. (2008, Cooperative Boolean games. In Proceedings of the 7th International Joint Conference on Autonomous Agents and Multiagent Systems (AAMAS 2008), 1015–1022), the authors define as solution concept the notion of core showing that several decision problems, such as core-emptiness, are Π2p-complete. In this work, we investigate how to improve the computation of the core. In particular, we introduce two different types of dependence networks, abstract dependence networks and refined dependence networks, that are used to define the notion of stable coalitions and Δ-reduction, respectively. Stable coalitions enable to focus on a subset of the agents and use results to determinate the core of the whole game. Δ-reduction prunes the search space by returning a set of actions that are not admissible to be executed. We present an algorithm based on stable coalitions and a Δ-reduction implemented in Prolog and experimental results that show how they effectively improve the computation of the core.

E-Auctions for Multi-Cloud Service Provisioning

The cloud computing paradigm requires solutions supporting customers in the selection of services that satisfy their functional and non-functional requirements. These solutions must i) support the dynamic, multi-cloud nature of service provisioning, ii) manage scenarios where no total preference relation over service properties is available, and iii) prevent providers from misrepresenting or overstating their properties. In this paper we put forward the idea of modeling multi-cloud provisioning scenarios as procurement e-auctions (where the auctioneer is the customer and the bidders are service providers). We introduce a service selection process based on matching and ranking algorithms, and an e-auction mechanism that addresses the above requirements, encouraging trustworthy bids and therefore improving the truthfulness on the e-auction outcome. Finally we describe the implementation of a prototype used to evaluate the performance of our approach with respect to traditional query-based engines.