Soft Concurrent Constraint Programming

Soft constraints extend classical constraints to represent multiple consistency levels, and thus provide a way to express preferences, fuzziness, and uncertainty. While there are many soft constraint solving formalisms, even distributed ones, by now there seems to be no concurrent programming framework where soft constraints can be handled. In this paper we show how the classical concurrent constraint (cc) programming framework can work with soft constraints, and we also propose an extension of cc languages which can use soft constraints to prune and direct the search for a solution. We believe that this new programming paradigm, called soft cc (scc), can be also very useful in many web-related scenarios. In fact, the language level allows web agents to express their interaction and negotiation protocols, and also to post their requests in terms of preferences, and the underlying soft constraint solver can find an agreement among the agents even if their requests are incompatible.Comment: 25 pages, 4 figures, submitted to the ACM Transactions on Computational Logic (TOCL), zipped file

Analysis of Integrity Policies using Soft Constraints

An integrity policy defines the situations when modification of information is authorized and is enforced by the security mechanisms of the system. However, in a complex application system it is possible that an integrity policy may have been incorrectly specified and, as a result, a user may be authorized to modify information that can lead to an unexpected system compromise. In this paper we outline a scalable and quantitative technique that uses constraint solving to model and analyze the effectiveness of application system integrity policie

A Soft Constraint-Based Approach to QoS-Aware Service Selection

Service-based systems should be able to dynamically seek replacements for faulty or underperforming services, thus performing self-healing. It may however be the case that available services do not match all requirements, leading the system to grind to a halt. In similar situations it would be better to choose alternative candidates which, while not fulfilling all the constraints, allow the system to proceed. Soft constraints, instead of the traditional crisp constraints, can help naturally model and solve replacement problems of this sort. In this work we apply soft constraints to model SLAs and to decide how to rebuild compositions which may not satisfy all the requirements, in order not to completely stop running systems

Many-Valued Institutions for Constraint Specification

We advance a general technique for enriching logical systems with soft constraints, making them suitable for specifying complex software systems where parts are put together not just based on how they meet certain functional requirements but also on how they optimise certain constraints. This added expressive power is required, for example, for capturing quality attributes that need to be optimised or, more generally, for formalising what are usually called service-level agreements. More specifically, we show how institutions endowed with a graded semantic consequence can accommodate soft-constraint satisfaction problems. We illustrate our approach by showing how, in the context of service discovery, one can quantify the compatibility of two specifications and thus formalise the selection of the most promising provider of a required resource.Peer Reviewe

Stochastic modeling and analysis of the bitcoin protocol in the presence of block communication delays

International audienceWe analyze the protocol of the Bitcoin blockchain by using the PRISM probabilistic model checker. In particular, we (i) extend PRISM with the ledger data type, (ii) model the behaviour of the key participants in the protocol-the miners-and (iii) describe the whole protocol as a parallel composition of processes. The probabilistic analysis of the model highlights how forks happen and how they depend on specific parameters of the protocol, such as the difficulty of the cryptopuzzle and the network communication delays. Our results confirm that considering transactions in blocks at depth larger than 5 as permanent is reasonable because the majority of miners have consistent blockchains up-to that depth with probability of almost 1. We also study the behaviour of networks with churn miners, which may leave the network and rejoin afterwards, and with different topologies

A Component-oriented Framework for Autonomous Agents

The design of a complex system warrants a compositional methodology, i.e., composing simple components to obtain a larger system that exhibits their collective behavior in a meaningful way. We propose an automaton-based paradigm for compositional design of such systems where an action is accompanied by one or more preferences. At run-time, these preferences provide a natural fallback mechanism for the component, while at design-time they can be used to reason about the behavior of the component in an uncertain physical world. Using structures that tell us how to compose preferences and actions, we can compose formal representations of individual components or agents to obtain a representation of the composed system. We extend Linear Temporal Logic with two unary connectives that reflect the compositional structure of the actions, and show how it can be used to diagnose undesired behavior by tracing the falsification of a specification back to one or more culpable components

ABAplus: Attack Reversal in Abstract and Structured Argumentation with Preferences

We present ABAplus, a system that implements reasoning with the argumentation formalism ABA+. ABA+ is a structured argumentation formalism that extends Assumption-Based Argumentation (ABA) with preferences and accounts for preferences via attack reversal. ABA+ also admits as instance Preference-based Argumentation which accounts for preferences by reversing attacks in abstract argumentation (AA). ABAplus readily implements attack reversal in both AA and ABAstyle structured argumentation. ABAplus affords computation, visualisation and comparison of extensions under five argumentation semantics. It is available both as a stand-alone system and as a web application

Abstract Constraint Data Types

Martin Wirsing is one of the earliest contributors to the area of Algebraic Specification (e.g., [2]), which he explored in a variety of domains over many years. Throughout his career, he has also inspired countless researchers in related areas. This paper is inspired by one of the domains that he explored thirty years or so after his first contributions when leading the FET Integrated Project SENSORIA [14]: the use of constraint systems to deal with non-functional requirements and preferences [13,8]. Following in his footsteps, we provide an extension of the traditional notion of algebraic data type specification to encompass soft-constraints as formalised in [1]. Finally, we relate this extension with institutions [6] and recent work on graded consequence in institutions [3].Peer ReviewedPostprint (author’s final draft