An Analysis Tool for Models of Virtualized Systems

This paper gives an example-driven introduction to modelling and analyzing virtualized systems in, e.g., cloud computing, using virtually timed ambients, a process algebra developed to study timing aspects of resource management for (nested) virtual machines. The calculus supports nested virtualization and virtual machines compete with other processes for the resources of their host environment. Resource provisioning in virtually timed ambients extends the capabilities of mobile ambients to model the dynamic creation, migration, and destruction of virtual machines. Quality of service properties for virtually timed ambients can be formally expressed using modal contracts describing aspects of resource provisioning and verified using a model checker for virtually timed ambients, implemented in the rewriting system Maude

An Algebra of Hierarchical Graphs and its Application to Structural Encoding

We define an algebraic theory of hierarchical graphs, whose axioms characterise graph isomorphism: two terms are equated exactly when they represent the same graph. Our algebra can be understood as a high-level language for describing graphs with a node-sharing, embedding structure, and it is then well suited for defining graphical representations of software models where nesting and linking are key aspects. In particular, we propose the use of our graph formalism as a convenient way to describe configurations in process calculi equipped with inherently hierarchical features such as sessions, locations, transactions, membranes or ambients. The graph syntax can be seen as an intermediate representation language, that facilitates the encodings of algebraic specifications, since it provides primitives for nesting, name restriction and parallel composition. In addition, proving soundness and correctness of an encoding (i.e. proving that structurally equivalent processes are mapped to isomorphic graphs) becomes easier as it can be done by induction over the graph syntax

Ten virtues of structured graphs

This paper extends the invited talk by the first author about the virtues of structured graphs. The motivation behind the talk and this paper relies on our experience on the development of ADR, a formal approach for the design of styleconformant, reconfigurable software systems. ADR is based on hierarchical graphs with interfaces and it has been conceived in the attempt of reconciling software architectures and process calculi by means of graphical methods. We have tried to write an ADR agnostic paper where we raise some drawbacks of flat, unstructured graphs for the design and analysis of software systems and we argue that hierarchical, structured graphs can alleviate such drawbacks

Basic completion strategies as another application of the Maude strategy language

The two levels of data and actions on those data provided by the separation between equations and rules in rewriting logic are completed by a third level of strategies to control the application of those actions. This level is implemented on top of Maude as a strategy language, which has been successfully used in a wide range of applications. First we summarize the Maude strategy language design and review some of its applications; then, we describe a new case study, namely the description of completion procedures as transition rules + control, as proposed by Lescanne.Comment: In Proceedings WRS 2011, arXiv:1204.531

UML for Global Computing

Abstract. Global systems – systems which may operate over transient networks including mobile elements and in which computation itself may be mobile – are gaining in importance. Nevertheless, the means for their modelling are still underdeveloped. The Unified Modelling Language (UML) is well developed for convenient modelling of behavior, but is not yet so useful for modelling aspects of design relevant to global systems, such as mobility. Non-functional requirements such as performance and security also assume an increased importance in the context of global systems, and here too, UML requires enhancement. In this paper we present an extension to UML class, sequence and activity diagrams to model mobile systems. We also describe extensions to model performance and security characteristics. We will describe how, wherever possible, we reuse existing work in these areas.

On symbolic semantics for name-decorated contexts

Under several regards, various of the recently proposed computational paradigms are open-ended, i.e. they may comprise components whose behaviour is not or cannot be fully specified. For instance, applications can be distributed across different administration domains that do not fully disclose their internal business processes to each other, or the dynamics of the system may allow reconfigurations and dynamic bindings whose specification is not available at design time. While a large set of mature design and analysis techniques for closed systems have been developed, their lifting to the open case is not always straightforward. Some existing approaches in the process calculi community are based on the need of proving properties for components that may hold in any, or significantly many, execution environments. Dually, frameworks describing the dynamics of systems with unspecified components have also been presented. In this paper we lay some preliminary ideas on how to extend a symbolic semantics model for open systems in order to deal with name-based calculi. Moreover, we also discuss how the use of a simple type system based on name-decoration for unknown components can improve the expressiveness of the framework. The approach is illustrated on a simple, paradigmatic calculus of web crawlers, which can be understood as a term representation of a simple class of graphs

Bigraphs and Their Algebra

AbstractBigraphs are a framework in which both existing process calculi and new models of behaviour can be formulated, yielding theory that is shared among these models. A short survey of the main features of bigraphs is presented, showing how they can be developed from standard graph theory using elementary category theory. The algebraic manipulation of bigraphs is outlined with the help of illustrations. The treatment of dynamics is then summarised. Finally, origins and some related work are discussed. The paper provides a motivating introduction to bigraphs

On Hierarchical Graphs: Reconciling Bigraphs, Gs-monoidal Theories and Gs-graphs

Abstract. Compositional graph models for global computing systems must account for two relevant dimensions, namely nesting and linking. In Milner’s bigraphs the two dimensions are made explicit and represented as loosely coupled structures: the place graph and the link graph. Here, bigraphs are compared with an earlier model, gs-graphs, based on gs-monoidal theories and originally conceived for modelling the syntactical structure of agents with α-convertible declarations. We show that gs-graphs are quite convenient also for the new purpose, since the two dimensions can be recovered by introducing two types of nodes. With respect to bigraphs, gs-graphs can be proved essentially equivalent, with minor differences at the interface level. We argue that gs-graphs have a simpler and more standard algebraic structure for representing both states and transitions, and can be equipped with a simple type system (in the style of relational separation logic) to check the well-formedness of bounded gs-graphs. Another advantage concerns a textual form in terms of sets of assignments, which can make implementation easier in rewriting frameworks like Maude. Vice versa, the reactive system approach developed for bigraphs needs yet to be addressed in gs-graphs.