Selected Papers of the 22nd International Conference on Coordination Models and Languages (COORDINATION 2020)

Editors: Simon Bliudze, Laura Bocchi

1. A theory of transaction parallelism in blockchains

Massimo Bartoletti ; Letterio Galletta ; Maurizio Murgia.
Decentralized blockchain platforms have enabled the secure exchange of crypto-assets without the intermediation of trusted authorities. To this purpose, these platforms rely on a peer-to-peer network of byzantine nodes, which collaboratively maintain an append-only ledger of transactions, called blockchain. Transactions represent the actions required by users, e.g. the transfer of some units of crypto-currency to another user, or the execution of a smart contract which distributes crypto-assets according to its internal logic. Part of the nodes of the peer-to-peer network compete to append transactions to the blockchain. To do so, they group the transactions sent by users into blocks, and update their view of the blockchain state by executing these transactions in the chosen order. Once a block of transactions is appended to the blockchain, the other nodes validate it, re-executing the transactions in the same order. The serial execution of transactions does not take advantage of the multi-core architecture of modern processors, so contributing to limit the throughput. In this paper we develop a theory of transaction parallelism for blockchains, which is based on static analysis of transactions and smart contracts. We illustrate how blockchain nodes can use our theory to parallelize the execution of transactions. Initial experiments on Ethereum show that our technique can improve the performance of nodes.

2. Time-Fluid Field-Based Coordination through Programmable Distributed Schedulers

Danilo Pianini ; Roberto Casadei ; Mirko Viroli ; Stefano Mariani ; Franco Zambonelli.
Emerging application scenarios, such as cyber-physical systems (CPSs), the Internet of Things (IoT), and edge computing, call for coordination approaches addressing openness, self-adaptation, heterogeneity, and deployment agnosticism. Field-based coordination is one such approach, promoting the idea of programming system coordination declaratively from a global perspective, in terms of functional manipulation and evolution in "space and time" of distributed data structures called fields. More specifically regarding time, in field-based coordination (as in many other distributed approaches to coordination) it is assumed that local activities in each device are regulated by a fair and unsynchronised fixed clock working at the platform level. In this work, we challenge this assumption, and propose an alternative approach where scheduling is programmed in a natural way (along with usual field-based coordination) in terms of causality fields, each enacting a programmable distributed notion of a computation "cause" (why and when a field computation has to be locally computed) and how it should change across time and space. Starting from low-level platform triggers, such causality fields can be organised into multiple layers, up to high-level, collectively-computed time abstractions, to be used at the application level. This reinterpretation of time in terms of articulated causality relations allows us to express what we call "time-fluid" coordination, […]

3. A new operational representation of dependencies in Event Structures

G. Michele Pinna.
The execution of an event in a complex and distributed system where the dependencies vary during the evolution of the system can be represented in many ways, and one of them is to use Context-Dependent Event structures. Event structures are related to Petri nets. The aim of this paper is to propose what can be the appropriate kind of Petri net corresponding to Context-Dependent Event structures, giving an operational flavour to the dependencies represented in a Context/Dependent Event structure. Dependencies are often operationally represented, in Petri nets, by tokens produced by activities and consumed by others. Here we shift the perspective using contextual arcs to characterize what has happened so far and in this way to describe the dependencies among the various activities.

4. Architectures in parametric component-based systems: Qualitative and quantitative modelling

Maria Pittou ; George Rahonis.
One of the key aspects in component-based design is specifying the software architecture that characterizes the topology and the permissible interactions of the components of a system. To achieve well-founded design there is need to address both the qualitative and non-functional aspects of architectures. In this paper we study the qualitative and quantitative formal modelling of architectures applied on parametric component-based systems, that consist of an unknown number of instances of each component. Specifically, we introduce an extended propositional interaction logic and investigate its first-order level which serves as a formal language for the interactions of parametric systems. Our logics achieve to encode the execution order of interactions, which is a main feature in several important architectures, as well as to model recursive interactions. Moreover, we prove the decidability of equivalence, satisfiability, and validity of first-order extended interaction logic formulas, and provide several examples of formulas describing well-known architectures. We show the robustness of our theory by effectively extending our results for parametric weighted architectures. For this, we study the weighted counterparts of our logics over a commutative semiring, and we apply them for modelling the quantitative aspects of concrete architectures. Finally, we prove that the equivalence problem of weighted first-order extended interaction logic formulas is decidable in a large class […]

5. Verifying liquidity of recursive Bitcoin contracts

Massimo Bartoletti ; Stefano Lande ; Maurizio Murgia ; Roberto Zunino.
Smart contracts - computer protocols that regulate the exchange of crypto-assets in trustless environments - have become popular with the spread of blockchain technologies. A landmark security property of smart contracts is liquidity: in a non-liquid contract, it may happen that some assets remain frozen, i.e. not redeemable by anyone. The relevance of this issue is witnessed by recent liquidity attacks to Ethereum, which have frozen hundreds of USD millions. We address the problem of verifying liquidity on BitML, a DSL for smart contracts with a secure compiler to Bitcoin, featuring primitives for currency transfers, contract renegotiation and consensual recursion. Our main result is a verification technique for liquidity. We first transform the infinite-state semantics of BitML into a finite-state one, which focusses on the behaviour of a chosen set of contracts, abstracting from the moves of the context. With respect to the chosen contracts, this abstraction is sound, i.e. if the abstracted contract is liquid, then also the concrete one is such. We then verify liquidity by model-checking the finite-state abstraction. We implement a toolchain that automatically verifies liquidity of BitML contracts and compiles them to Bitcoin, and we assess it through a benchmark of representative contracts.

6. Specification and Verification of Timing Properties in Interoperable Medical Systems

Mahsa Zarneshan ; Fatemeh Ghassemi ; Ehsan Khamespanah ; Marjan Sirjani ; John Hatcliff.
To support the dynamic composition of various devices/apps into a medical system at point-of-care, a set of communication patterns to describe the communication needs of devices has been proposed. To address timing requirements, each pattern breaks common timing properties into finer ones that can be enforced locally by the components. Common timing requirements for the underlying communication substrate are derived from these local properties. The local properties of devices are assured by the vendors at the development time. Although organizations procure devices that are compatible in terms of their local properties and middleware, they may not operate as desired. The latency of the organization network interacts with the local properties of devices. To validate the interaction among the timing properties of components and the network, we formally specify such systems in Timed Rebeca. We use model checking to verify the derived timing requirements of the communication substrate in terms of the network and device models. We provide a set of templates as a guideline to specify medical systems in terms of the formal model of patterns. A composite medical system using several devices is subject to state-space explosion. We extend the reduction technique of Timed Rebeca based on the static properties of patterns. We prove that our reduction is sound and show the applicability of our approach in reducing the state space by modeling two clinical scenarios made of several […]