Hugo Torres Vieira

Conversation types

We present a type theory for analyzing concurrent multiparty interactions as found in service-oriented computing. Our theory introduces a novel and flexible type structure, able to uniformly describe both the internal and the interface behavior of systems, referred respectively as choreographies and contracts in web-services terminology. The notion of conversation builds on the fundamental concept of session, but generalizes it along directions up to now unexplored; in particular, conversation types discipline interactions in conversations while accounting for dynamical join and leave of an unanticipated number of participants. We prove that well-typed systems never violate the prescribed conversation constraints. We also present techniques to ensure progress of systems involving several interleaved conversations, a previously open problem.

The conversation calculus: a model of service-oriented computation

We present a process-calculus model for expressing and analyzing service-based systems. Our approach addresses central features of the service-oriented computational model such as distribution, process delegation, communication and context sensitiveness, and loose coupling. Distinguishing aspects of our model are the notion of conversation context, the adoption of a context sensitive, message-passing-based communication, and of a simple yet expressive mechanism for handling exceptional behavior. We instantiate our model by extending a fragment of the π-calculus, illustrate its expressiveness by means of many examples, and study its basic behavioral theory; in particular, we establish that bisimilarity is a congruence.

Foundations of Session Types and Behavioural Contracts

Behavioural type systems, usually associated to concurrent or distributed computations, encompass concepts such as interfaces, communication protocols, and contracts, in addition to the traditional input/output operations. The behavioural type of a software component specifies its expected patterns of interaction using expressive type languages, so types can be used to determine automatically whether the component interacts correctly with other components. Two related important notions of behavioural types are those of session types and behavioural contracts. This article surveys the main accomplishments of the last 20 years within these two approaches.

Conversation Types

We present a type theory for analyzing concurrent multiparty interactions as found in service-oriented computing. Our theory introduces a novel and flexible type structure, able to uniformly describe both the internal and the interface behavior of systems, referred respectively as choreographies and contracts in web-services terminology. The notion of conversation builds on the fundamental concept of session, but generalizes it along directions up to now unexplored; in particular, conversation types discipline interactions in conversations while accounting for dynamical join and leave of an unanticipated number of participants. We prove that well-typed systems never violate the prescribed conversation constraints. We also present techniques to ensure progress of systems involving several interleaved conversations, a previously open problem.

A Process Calculus Analysis of Compensations

Conversations in service-oriented computation are frequently long running. In such a setting, traditional ACID properties of transactions cannot be reasonably implemented, and compensation mechanisms seem to provide convenient techniques to, at least, approximate them. In this paper, we investigate the representation and analysis of structured compensating transactions within a process calculus model, by embedding in the Conversation Calculus certain structured compensation programming abstractions inspired by the ones proposed by Butler, Ferreira, and Hoare. We prove the correctness of the embedding after developing a general notion of stateful model for structured compensations and related results, and showing that the embedding induces such a model.

A Type System for Flexible Role Assignment in Multiparty Communicating Systems

Communication protocols in distributed systems often specify the roles of the parties involved in the communications, namely for enforcing security policies or task assignment purposes. Ensuring that implementations follow role-based protocol specifications is challenging, especially in scenarios found, e.g., in business processes and web applications, where multiple peers are involved, single peers impersonate several roles, or single roles are carried out by several peers. We present a type-based analysis for statically verifying role-based multi-party interactions, based on a simple π-calculus model and prior work on conversation types. Our main result ensures that well-typed systems follow the role-based protocols prescribed by the types, including systems where roles are flexibly assigned to processes.

Advanced mechanisms for service combination and transactions

Languages and models for service-oriented applications usually include primitives and constructs for exception and compensation handling. Exception handling is used to react to unexpected events while compensation handling is used to undo previously completed activities. In this chapter we investigate the impact of exception and compensation handling in message-based process calculi and the related theories developed within SENSORIA.

Typing Progress in Communication-Centred Systems

We present a type system for the analysis of progress in session-based communication centred systems. Our development is carried out in a minimal setting considering classic (binary) sessions, but building on and generalising previous work on progress analysis in the context of conversation types. Our contributions aim at underpinning forthcoming works on progress for session-typed systems, so as to support richer verification procedures based on a more foundational approach. Although this work does not target expressiveness, our approach already addresses challenging scenarios which are unaccounted for elsewhere in the literature, in particular systems that interleave communications on received session channels.

Type-based access control in data-centric systems

Data-centric multi-user systems, such as web applications, require flexible yet fine-grained data security mechanisms. Such mechanisms are usually enforced by a specially crafted security layer, which adds extra complexity and often leads to error prone coding, easily causing severe security breaches. In this paper, we introduce a programming language approach for enforcing access control policies to data in data-centric programs by static typing. Our development is based on the general concept of refinement type, but extended so as to address realistic and challenging scenarios of permission-based data security, in which policies dynamically depend on the database state, and flexible combinations of column- and row-level protection of data are necessary. We state and prove soundness and safety of our type system, stating that well-typed programs never break the declared data access control policies.

Extensionality of Spatial Observations in Distributed Systems

We discuss the tensions between intensionality and extensionality of spatial observations in distributed systems, showing that there are natural models where extensional observational equivalences may be characterized by spatial logics, including the composition and void operators. Our results support the claim that spatial observations do not need to be always considered intensional, even if expressive enough to talk about the structure of systems. For simplicity, our technical development is based on a minimalist process calculus, that already captures the main features of distributed systems, namely local synchronous communication, local computation, asynchronous remote communication, and partial failures.