Lending Petri nets and contracts

Choreography-based approaches to service composition typically assume that, after a set of services has been found which correctly play the roles prescribed by the choreography, each service respects his role. Honest services are not protected against adversaries. We propose a model for contracts based on a extension of Petri nets, which allows services to protect themselves while still realizing the choreography. We relate this model with Propositional Contract Logic, by showing a translation of formulae into our Petri nets which preserves the logical notion of agreement, and allows for compositional verification

Contract agreements via logic

We relate two contract models: one based on event structures and game theory, and the other one based on logic. In particular, we show that the notions of agreement and winning strategies in the game-theoretic model are related to that of provability in the logical model.Comment: In Proceedings ICE 2013, arXiv:1310.401

Debits and Credits in Petri Nets and Linear Logic

Exchanging resources often involves situations where a participant gives a resource without obtaining immediately the expected reward. For instance, one can buy an item without paying it in advance, but contracting a debt which must be eventually honoured. Resources, credits and debits can be represented, either implicitly or explicitly, in several formal models, among which Petri nets and linear logic. In this paper we study the relations between two of these models, namely intuitionistic linear logic with mix and Debit Petri nets. In particular, we establish a natural correspondence between provability in the logic, and marking reachability in nets

Vicious circles in contracts and in logic

Contracts are formal promises on the future interactions of participants, which describe the causal dependencies among their actions. An inherent feature of contracts is that such dependencies may be circular: for instance, a buyer promises to pay for an item if the seller promises to ship it, and vice versa. We establish a bridge between two formal models for contracts, one based on games over event structures, and the other one on Propositional Contract Logic. In particular, we show that winning strategies in the game-theoretic model correspond to proofs in the logi

Automata for specifying and orchestrating service contracts

An approach to the formal description of service contracts is presented in terms of automata. We focus on the basic property of guaranteeing that in the multi-party composition of principals each of them gets his requests satisfied, so that the overall composition reaches its goal. Depending on whether requests are satisfied synchronously or asynchronously, we construct an orchestrator that at static time either yields composed services enjoying the required properties or detects the principals responsible for possible violations. To do that in the asynchronous case we resort to Linear Programming techniques. We also relate our automata with two logically based methods for specifying contracts

A theory of agreements and protection

In this thesis we propose a theory of contracts. Contracts are modelled as interacting processes with an explicit association of obligations and objectives. Obligations are specified using event structures. In this model we formalise two fundamental notions of contracts, namely agreement and protection. These notions arise naturally by interpreting contracts as multi-player concurrent games. A participant agrees on a contract if she has a strategy to reach her objectives (or to make another participant sanctionable for a violation), whatever the moves of her counterparts. A participant is protected by a contract when she has a strategy to defend herself in all possible contexts, even in those where she has not reached an agreement. When obligations are represented using classical event structures, we show that agreement and protection mutually exclude each other for a wide class of contracts. To reconcile agreement with protection we propose a novel formalism for modelling contractual obligations: event structures with circular causality. We study this model from a foundational perspective, and we relate it with classical event structures. Using this model, we show how to construct contracts which guarantee both agreement and protection. We relate our contract model with Propositional Contract Logic, by establishing a correspondence between provability in the logic and the notions of agreement and strategies. This is a first step towards reducing the gap between two main paradigms for modelling contracts, that is the one which interprets them as interactive systems, and the one based on logic

A theory of agreements and protection

In this thesis we propose a theory of contracts. Contracts are modelled as interacting processes with an explicit association of obligations and objectives. Obligations are specified using event structures. In this model we formalise two fundamental notions of contracts, namely agreement and protection. These notions arise naturally by interpreting contracts as multi-player concurrent games. A participant agrees on a contract if she has a strategy to reach her objectives (or to make another participant sanctionable for a violation), whatever the moves of her counterparts. A participant is protected by a contract when she has a strategy to defend herself in all possible contexts, even in those where she has not reached an agreement. When obligations are represented using classical event structures, we show that agreement and protection mutually exclude each other for a wide class of contracts. To reconcile agreement with protection we propose a novel formalism for modelling contractual obligations: event structures with circular causality. We study this model from a foundational perspective, and we relate it with classical event structures. Using this model, we show how to construct contracts which guarantee both agreement and protection. We relate our contract model with Propositional Contract Logic, by establishing a correspondence between provability in the logic and the notions of agreement and strategies. This is a first step towards reducing the gap between two main paradigms for modelling contracts, that is the one which interprets them as interactive systems, and the one based on logic

Specification and Verification of Contract-Based Applications

Nowadays emerging paradigms are being adopted by several companies, where applications are built by assembling loosely-coupled distributed components, called services. Services may belong to possibly mutual distrusted organizations and may have conflicting goals. New methodologies for designing and verifying these applications are necessary for coping with new scenarios in which a service does not adhere with its prescribed behaviour, namely its contract. The thesis tackles this problem by proposing techniques for specifying and verifying distributed applications. The first contribution is an automata-based model checking technique for ensuring both service compliance and security requirements in a composition of services. We further develop the automata-based approach by proposing a novel formal model of contracts based on tailored finite state automata, called contract automata. The proposed model features several notions of contract agreement described from a language-theoretic perspective, for characterising the modalities in which the duties and requirements of services are fulfilled. Contract automata are equipped with different composition operators, to uniformly model both single and composite services, and techniques for synthesising an orchestrator to enforce the properties of agreement. Algorithms for verifying these properties are introduced, based on control theory and linear programming techniques. The formalism assumes the existence of possible malicious components trying to break the overall agreement, and techniques for detecting and banning eventually liable services are described. We study the conditions for dismissing the central orchestrator in order to generate a distributed choreography of services, analysing both closed and open choreographed systems, with synchronous or asynchronous interactions. We relate contract automata with different intutionistic logics for contracts, introduced for solving mutual circular dependencies between the requirements and the obligations of the parties, with either linear or non-linear availability of resources. Finally, a prototypical tool implementing the theory developed in the thesis is presented

Lending Petri Nets and Contracts

International audienceChoreography-based approaches to service composition typically assume that, after a set of services has been found which correctly play the roles prescribed by the choreography, each service respects his role. Honest services are not protected against adversaries. We propose a model for contracts based on an extension of Petri nets, which allows services to protect themselves while still realizing the choreography. We relate this model with Propositional Contract Logic, by showing a translation of formulae into our Petri nets which preserves the logical notion of agreement, and allows for compositional verification