This paper presents an extension of Disjunctive Datalog (DATALOG/sup V,/spl sim//) by integrity constraints. These are of two types: strong, that is, classical integrity constraints and weak, that is, constraints that are satisfied if possible. While strong constraints must be satisfied, weak constraints express desiderata, that is, they may be violated-actually, their semantics tends to minimize the number of violated instances of weak constraints. Weak constraints may be ordered according to their importance to express different priority levels. As a result, the proposed language (call it, DATALOG/sup V,/spl sim/,c/) is well-suited to represent common sense reasoning and knowledge-based problems arising in different areas of computer science such as planning, graph theory optimizations, and abductive reasoning. The formal definition of the language is first given. The declarative semantics of DATALOG/sup V,/spl sim/,c/ is defined in a general way that allows us to put constraints on top of any existing (model-theoretic) semantics for DATALOG/sup V,/spl sim// programs. Knowledge representation issues are then addressed and the complexity of reasoning on DATALOG/sup V,/spl sim/,c/ programs is carefully determined. An in-depth discussion on complexity and expressiveness of DATALOG/sup V,/spl sim/,c/ is finally reported. The discussion contrasts DATALOG/sup V,/spl sim/,c/ to DATALOG/sup V,/spl sim// and highlights the significant increase in knowledge modeling ability carried out by constraints.

The Internet of Things is constantly capturing interest from modern applications, changing our everyday life and empowering industrial applications. Interaction and the collaboration among smart devices offer new challenges to security since they conflict with economic and energy consumption requirement constraints. On the other hand, the lack of security measures could negatively impact the concrete adoption of this paradigm. This paper focuses on the Message Queuing Telemetry Transport (MQTT) protocol, widely adopted in the Internet of Things. This protocol does not implement natively secure authentication mechanisms, which are demanded to developers. Hence, this paper proposes a novel OTP (one-time password)-authentication schema for MQTT, which uses the Ethereum blockchain to implement a second-factor out-of-band channel. The proposal enables the authentication of both local and remote devices preserving user privacy and guaranteeing trust and accountability via Ethereum smart contracts.