Dynamic Web data : a process algebraic approach

Peer to peer systems, exchanging dynamic documents through Web services, are a simple and effective platform for data integration on the internet. Dynamic documents can contain both data and references to external sources in the form of links, calls to web services, or coordination scripts. XML standards, and industrial platforms for web services, provide the technological basis for building such systems. We argue that process algebras are a promising tool for studying and understanding their formal properties. In this thesis, we define the Xdπ-calculus with the aim of reasoning about dynamic Web data. Xdπ terms represent networks of peers, each consisting of an XML data repository and a working space where processes are allowed to run. Processes, inspired by the π-calculus, can communicate with each other, query and update the local repository, or migrate to other peers to continue execution. Data can contain scripted processes, which can be executed by other processes. For example, Xdπ processes can be used to embed service calls in documents and to model Web services. We investigate behavioural equivalences for Xdπ, comparing several observable properties, such as the shape of data trees and the communication actions attempted by processes. To simplify reasoning on equivalences, we introduce Core Xdπ, a calculus which is semantically equivalent to Xdπ, but where processes are located explicitly and are separated from the data repository. To help proving equivalences, which require a costly property of closure under contexts, we define a coinductive relation (called domain bisimilarity) which does not quantify over contexts and which entails process equivalence. Its definition is non-standard, because scripts are part of the values, and process equivalences are sensitive to the set of locations constituting the network. We apply bisimilarity to study some communication patterns used by servers in distributed query systems, and we propose a new pattern involving mobile code. Declaration about conjoint work The composition of the thesis is entirely my own. The Xdπ model and the behavioural equivalences as presented in this thesis, are a revised and extended version of the ones presented in earlier work done in collaboration with my supervisor Philippa Gardner. The technical development of the behavioural equivalences is mostly my own. The design of Core Xdπ is inspired by previous work on polyadic synchronization done in collaboration with Marco Carbone.