arXiv subjects

Fabrizio Montesi

Publications and source records attributed to Fabrizio Montesi.

At least 37 records · Page 2Linked to original sources

arXiv2022

A Formal Theory of Choreographic Programming

Choreographic programming is a paradigm for writing coordination plans for distributed systems from a global point of view, from which correct-by-construction decentralised implementations can be generated automatically. Theory of choreographies typically includes a number of complex results that are proved by structural induction. The high number of cases and the subtle details in some of these proofs has led to important errors being found in published works. In this work, we formalise the theory of a choreographic programming language in Coq. Our development includes the basic properties of this language, a proof of its Turing completeness, a compilation procedure to a process language, and an operational characterisation of the correctness of this procedure. Our formalisation experience illustrates the benefits of using a theorem prover: we get both an additional degree of confidence from the mechanised proof, and a significant simplification of the underlying theory. Our results offer a foundation for the future formal development of choreographic languages.

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arXiv2022

Functional Choreographic Programming

Choreographic programming is an emerging programming paradigm for concurrent and distributed systems, whereby developers write the communications that should be enacted and then a distributed implementation is automatically obtained by means of a compiler. Theories of choreographic programming typically come with strong theoretical guarantees about the compilation process, most notably: the generated implementations operationally correspond to their source choreographies and are deadlock-free. Currently, the most advanced incarnation of the paradigm is Choral, an object-oriented choreographic programming language that targets Java. Choral deviated significantly from known theories of choreographies, and introduced the possibility of expressing higher-order choreographies (choreographies parameterised over choreographies) that are fully distributed. As a consequence, it is unclear if the usual guarantees of choreographies can still hold in the more general setting of higher-order ones. We introduce Chorλ, the first functional choreographic programming language: it introduces a new formulation of the standard communication primitive found in choreographies as a function, and it is based upon the λ-calculus. Chorλ is the first theory that explains the core ideas of higher-order choreographic programming (as in Choral). Bridging the gap between practice and theory requires developing a new evaluation strategy and typing discipline for λ terms that accounts for the distributed nature of computation in choreographies. We illustrate the expressivity of Chorλ with a series of examples, which include reconstructions of the key examples from the original presentation of Choral. Our theory supports the expected properties of choreographic programming and bridges the gap between the communities of functional and choreographic programming.

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arXiv2022

From Infinity to Choreographies: Extraction for Unbounded Systems

Choreographies are formal descriptions of distributed systems, which focus on the way in which participants communicate. While they are useful for analysing protocols, in practice systems are written directly by specifying each participant's behaviour. This created the need for choreography extraction: the process of obtaining a choreography that faithfully describes the collective behaviour of all participants in a distributed protocol. Previous works have addressed this problem for systems with a predefined, finite number of participants. In this work, we show how to extract choreographies from system descriptions where the total number of participants is unknown and unbounded, due to the ability of spawning new processes at runtime. This extension is challenging, since previous algorithms relied heavily on the set of possible states of the network during execution being finite.

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arXiv2022

Implementing Choreography Extraction

Choreographies are global descriptions of interactions among concurrent components, most notably used in the settings of verification and synthesis of correct-by-construction software. They require a top-down approach: programmers first write choreographies, and then use them to verify or synthesize their programs. However, most software does not come with choreographies yet, which prevents their application. To attack this problem, previous work investigated choreography extraction, which automatically constructs a choreography that describes the behaviour of a given set of programs or protocol specifications. We propose a new extraction methodology that improves on the state of the art: we can deal with programs that are equipped with state and internal computation and time complexity is dramatically better. We also implement this theory and show that, in spite of its theoretical exponential complexity, it is usable in practice. We discuss the data structures needed for an efficient implementation, introduce some optimisations, and perform a systematic practical evaluation.

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arXiv2022

Model-Driven Generation of Microservice Interfaces: From LEMMA Domain Models to Jolie APIs

We formally define and implement a translation from domain models in the LEMMA modelling framework to microservice APIs in the Jolie programming language. Our tool enables a software development process whereby microservice architectures can first be designed with the leading method of Domain-Driven Design, and then corresponding data types and service interfaces (APIs) in Jolie are automatically generated. Developers can extend and use these APIs as guides in order to produce compliant implementations. Our tool thus contributes to enhancing productivity and improving the design adherence of microservices.

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arXiv2021

Certifying Choreography Compilation

Choreographic programming is a paradigm for developing concurrent and distributed systems, where programs are choreographies that define, from a global viewpoint, the computations and interactions that communicating processes should enact. Choreography compilation translates choreographies into the local definitions of process behaviours, given as terms in a process calculus. Proving choreography compilation correct is challenging and error-prone, because it requires relating languages in different paradigms (global interactions vs local actions) and dealing with a combinatorial explosion of proof cases. We present the first certified program for choreography compilation for a nontrivial choreographic language supporting recursion.

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arXiv2021

Sliceable Monolith: Monolith First, Microservices Later

We propose Sliceable Monolith, a new methodology for developing microservice architectures and perform their integration testing by leveraging most of the simplicity of a monolith: a single codebase and a local execution environment that simulates distribution. Then, a tool compiles a codebase for each microservice and a cloud deployment configuration. The key enabler of our approach is the technology-agnostic service definition language offered by Jolie.

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arXiv2021

Linear Logic, the $π$-calculus, and their Metatheory: A Recipe for Proofs as Processes

Initiated by Abramsky [1994], the Proofs as Processes agenda is to establish a solid foundation for the study of concurrent languages, by researching the connection between linear logic and the $π$-calculus. To date, Proofs as Processes is still a partial success. Caires and Pfenning [2010] showed that linear propositions correspond to session types, which prescribe the observable behaviour of processes. Further, Carbone et al. [2018] demonstrated that adopting devices from hypersequents [Avron 1991] shapes proofs such that they correspond to the expected syntactic structure of processes in the $π$-calculus. What remains is reconstructing the expected metatheory of session types and the $π$-calculus. In particular, the hallmark of session types, session fidelity, still has to be reconstructed: a correspondence between the observable behaviours of processes and their session types, in terms of labelled transitions. In this article, we present $π$LL, a new process calculus rooted in linear logic. The key novelty of $π$LL is that it comes with a carefully formulated design recipe, based on a dialgebraic view of labelled transition systems. Thanks to our recipe, $π$LL offers the expected transition systems of session types, which we use to establish session fidelity. We use $π$LL to carry out the first thorough investigation of the metatheoretical properties enforced by linear logic on the observable behaviour of processes, uncovering connections with similarity and bisimilarity. We also prove that $π$LL and our recipe form a robust basis for the further exploration of Proofs as Processes, by considering different features: polymorphism, process mobility, and recursion.

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arXiv2021

Jolie & LEMMA: Model-Driven Engineering and Programming Languages Meet on Microservices

In the field of microservices, Model-Driven Engineering has emerged as a powerful methodology for architectural design, and new programming languages have introduced language abstractions to deal with microservice development more effectively. In this article, we present the first preliminary investigation of how the two approaches can be married, taking the LEMMA framework and the Jolie programming language as respective representatives. By developing a conceptual metamodel for Jolie, we elicit a strong link between the two approaches, which shows that there is much to gain. We discuss a few low-hanging fruits that come from our finding and present some interesting future directions that arise from our new viewpoint.

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arXiv2021

Formalising a Turing-Complete Choreographic Language in Coq

Theory of choreographic languages typically includes a number of complex results that are proved by structural induction. The high number of cases and the subtle details in some of them lead to long reviewing processes, and occasionally to errors being found in published proofs. In this work, we take a published proof of Turing completeness of a choreographic language and formalise it in Coq. Our development includes formalising the choreographic language and its basic properties, Kleene's theory of partial recursive functions, the encoding of these functions as choreographies, and proving this encoding correct. With this effort, we show that theorem proving can be a very useful tool in the field of choreographic languages: besides the added degree of confidence that we get from a mechanised proof, the formalisation process led us to a significant simplification of the underlying theory. Our results offer a foundation for the future formal development of choreographic languages.

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arXiv2020

Applied Choreographies

Choreographic Programming is a correct-by-construction paradigm where a compilation procedure synthesises deadlock-free, concurrent, and distributed communicating processes from global, declarative descriptions of communications, called choreographies. Previous work used choreographies for the synthesis of programs. Alas, there is no formalisation that provides a chain of correctness from choreographies to their implementations. This problem originates from the gap between existing theoretical models, which abstract communications using channel names (à la CCS/π-calculus), and their implementations, which use low-level mechanisms for message routing. As a solution, we propose the theoretical framework of Applied Choreographies. In the framework, developers write choreographies in a language that follows the standard syntax and name-based communication semantics of previous works. Then, they use a compilation procedure to transform a choreography into a low-level, implementation-adherent calculus of Service-Oriented Computing (SOC). To manage the complexity of the compilation, we divide its formalisation and proof in three stages, respectively dealing with: a) the translation of name-based communications into their SOC equivalents (namely, using correlation mechanisms based on message data); b) the projection of a choreography into a composition of partial, single-participant choreographies (towards their translation into SOC processes); c) the translation of partial choreographies and the distribution of choreography-level state into SOC processes. We provide results of behavioural correspondence for each stage. Thus, given a choreography specification, we guarantee to synthesise its faithful and deadlock-free service-oriented implementation.

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arXiv2019

Implementing choreography extraction

Choreography extraction deals with the generation of a choreography (a global description of communication behaviour) from a set of local process behaviours. In this work, we implement a previously proposed theory for extraction and show that, in spite of its theoretical exponential complexity, it is usable in practice. We discuss the data structures needed for an efficient implementation, introduce some optimizations, and perform a systematic practical evaluation.

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arXiv2019

No more, no less - A formal model for serverless computing

Serverless computing, also known as Functions-as-a-Service, is a recent paradigm aimed at simplifying the programming of cloud applications. The idea is that developers design applications in terms of functions, which are then deployed on a cloud infrastructure. The infrastructure takes care of executing the functions whenever requested by remote clients, dealing automatically with distribution and scaling with respect to inbound traffic. While vendors already support a variety of programming languages for serverless computing (e.g. Go, Java, Javascript, Python), as far as we know there is no reference model yet to formally reason on this paradigm. In this paper, we propose the first formal programming model for serverless computing, which combines ideas from both the $λ$-calculus (for functions) and the $π$-calculus (for communication). To illustrate our proposal, we model a real-world serverless system. Thanks to our model, we are also able to capture and pinpoint the limitations of current vendor technologies, proposing possible amendments.

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arXiv2019

Ephemeral Data Handling in Microservices - Technical Report

In modern application areas for software systems --- like eHealth, the Internet-of-Things, and Edge Computing --- data is encoded in heterogeneous, tree-shaped data-formats, it must be processed in real-time, and it must be ephemeral, i.e., not persist in the system. While it is preferable to use a query language to express complex data-handling logic, their typical execution engine, a database external from the main application, is unfit in scenarios of ephemeral data-handling. A better option is represented by integrated query frameworks, which benefit from existing development support tools (e.g., syntax and type checkers) and execute within the application memory. In this paper, we propose one such framework that, for the first time, targets tree-shaped, document-oriented queries. We formalise an instantiation of MQuery, a sound variant of the widely-used MongoDB query language, which we implemented in the Jolie language. Jolie programs are microservices, the building blocks of modern software systems. Moreover, since Jolie supports native tree data-structures and automatic management of heterogeneous data-encodings, we can provide a uniform way to use MQuery on any data-format supported by the language. We present a non-trivial use case from eHealth, use it to concretely evaluate our model, and to illustrate our formalism.

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arXiv2019

Taking Linear Logic Apart

Process calculi based on logic, such as $π$DILL and CP, provide a foundation for deadlock-free concurrent programming. However, in previous work, there is a mismatch between the rules for constructing proofs and the term constructors of the $π$-calculus: the fundamental operator for parallel composition does not correspond to any rule of linear logic. Kokke et al. (2019) introduced Hypersequent Classical Processes (HCP), which addresses this mismatch using hypersequents (collections of sequents) to register parallelism in the typing judgements. However, the step from CP to HCP is a big one. As of yet, HCP does not have reduction semantics, and the addition of delayed actions means that CP processes interpreted as HCP processes do not behave as they would in CP. We introduce HCP-, a variant of HCP with reduction semantics and without delayed actions. We prove progress, preservation, and termination, and show that HCP- supports the same communication protocols as CP.

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arXiv2018

Multiparty Classical Choreographies

We present Multiparty Classical Choreographies (MCC), a language model where global descriptions of communicating systems (choreographies) implement typed multiparty sessions. Typing is achieved by generalising classical linear logic to judgements that explicitly record parallelism by means of hypersequents. Our approach unifies different lines of work on choreographies and processes with multiparty sessions, as well as their connection to linear logic. Thus, results developed in one context are carried over to the others. Key novelties of MCC include support for server invocation in choreographies, as well as logic-driven compilation of choreographies with replicated processes.

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arXiv2018

Better Late Than Never: A Fully Abstract Semantics for Classical Processes

We present Hypersequent Classical Processes (HCP), a revised interpretation of the "Proofs as Processes" correspondence between linear logic and the π-calculus initially proposed by Abramsky [1994], and later developed by Bellin and Scott [1994], Caires and Pfenning [2010], and Wadler [2014], among others. HCP mends the discrepancies between linear logic and the syntax and observable semantics of parallel composition in the π-calculus, by conservatively extending linear logic to hyperenvironments (collections of environments, inspired by the hypersequents by Avron [1991]). Separation of environments in hyperenvironments is internalised by $\otimes$ and corresponds to parallel process behaviour. Thanks to this property, for the first time we are able to extract a labelled transition system (lts) semantics from proof rewritings. Leveraging the information on parallelism at the level of types, we obtain a logical reconstruction of the delayed actions that Merro and Sangiorgi [2004] formulated to model non-blocking I/O in the π-calculus. We define a denotational semantics for processes based on Brzozowski derivatives, and uncover that non-interference in HCP corresponds to Fubini's theorem of double antiderivation. Having an lts allows us to validate HCP using the standard toolbox of behavioural theory. We instantiate bisimilarity and barbed congruence for HCP, and obtain a full abstraction result: bisimilarity, denotational equivalence, and barbed congruence coincide.

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arXiv2018

A Core Model for Choreographic Programming

Choreographic Programming is a programming paradigm for building concurrent programs that are deadlock-free by construction, as a result of programming communications declaratively and then synthesising process implementations automatically. Despite strong interest on choreographies, a foundational model that explains which computations can be performed with the hallmark constructs of choreographies is still missing. In this work, we introduce Core Choreographies (CC), a model that includes only the core primitives of choreographic programming. Every computable function can be implemented as a choreography in CC, from which we can synthesise a process implementation where independent computations run in parallel. We discuss the design of CC and argue that it constitutes a canonical model for choreographic programming.

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