Towards a Theory of Glue

We propose and study the notions of behaviour type and composition operator making a first step towards the definition of a formal framework for studying behaviour composition in a setting sufficiently general to provide insight into how the component-based systems should be modelled and compared. We illustrate the proposed notions on classical examples (Traces, Labelled Transition Systems and Coalgebras). Finally, the definition of memoryless glue operators, takes us one step closer to a formal understanding of the separation of concerns principle stipulating that computational aspects of a system should be localised within its atomic components, whereas coordination layer responsible for managing concurrency should be realised by memoryless glue operators.Comment: In Proceedings ICE 2012, arXiv:1212.345

Extended Connectors: Structuring Glue Operators in BIP

Based on a variation of the BIP operational semantics using the offer predicate introduced in our previous work, we extend the algebras used to model glue operators in BIP to encompass priorities. This extension uses the Algebra of Causal Interaction Trees, T(P), as a pivot: existing transformations automatically provide the extensions for the Algebra of Connectors. We then extend the axiomatisation of T(P), since the equivalence induced by the new operational semantics is weaker than that induced by the interaction semantics. This extension leads to canonical normal forms for all structures and to a simplification of the algorithm for the synthesis of connectors from Boolean coordination constraints.Comment: In Proceedings ICE 2013, arXiv:1310.401

Architectures: Design patterns for component-based systems

Architectures depict design principles, paradigms that can be understood by all, allow thinking on a higher plane and avoiding low-level mistakes. They provide means for ensuring correctness by construction by enforcing global properties characterizing the coordination between components. An architecture can be considered as an operator A that, applied to a set of components B, builds a composite component A(B) meeting a characteristic property P. A theory of architectures must address several fundamental questions: 1. How does one model the architectures? In many existing approaches, architecture description is limited to drawing the structure of the system by connecting boxes with lines. A rigorous theory requires a formal definition of their operational semantics. 2. How does one specify the architectures? Characteristic properties of architectures must be clearly stated, understandable and verifiable by engineers. Specifications must be sufficiently versatile for the architectures to be applicable to a variety of components. 3. How does one combine architectures? Composition of architectures must preserve their respective characteristic properties. 4. How does one efficiently implement architectures? The cost of raising the abstraction level is the coordination overhead entailed by any centralised framework imposing the operational semantics. This can be reduced by applying transformation techniques to internalise and distribute the coordination constraints. We propose a formal and general framework for studying architectures. In particular, composability is based on an associative, commutative and idempotent architecture composition operator. The main result is that if two architectures A1 and A2 enforce respectively state invariants P1 and P2 , the composed architecture enforces the state invariant P1 & P2, that is both invariants are preserved by architecture composition. We also discuss preservation of liveness properties and internalisation of architectures

Approach for Highly Dependable Software-Intensive Systems

Author-contributed print itemhe objectives of the proposal are creating new methods and tools for effective constructing software-intensive systems based on multiple perspectives that are used to reflect differing stakeholder's concerns. The models and methods will incorporate rapid prototyping, explicit architecting and consistent engineering techniques into a synthesis approach for highly dependable software-intensive systems (HDSIS)

For Coordination, State Component Transitions

Coordinating component behaviour and, in particular, concurrent access to resources is among the key difficulties of building large concurrent systems. To address this, developers must be able to manipulate high-level concepts, such as Finite State Machines and separate functional and coordination aspects of the system behaviour. OSGi associates to each bundle a simple state machine representing the bundle’s lifecycle. However, once the bundle has been started, it remains in the state Active — the functional states are not represented. Therefore, this mechanism is not sufficient for coordination of active components. In this talk, we presented a methodology for functional component coordination in OSGi by using BIP coordination mechanisms. In BIP, systems are constructed by superposing three layers of modelling: Behaviour, Interaction and Priority. This approach allows us to clearly separate the system-wide coordination policies from the component behaviour and the interface that components expose for interaction. By using BIP, we have shown how the allowed global states and state transitions of the modular system can be taken into account in a non-invasive manner and without any impact on the technology stack within an OSGi container. We illustrated our approach on real-life application use-case

Data abstraction in coordination constraints

Communications in Computer and Information Science 393, 2013This paper studies complex coordination mechanisms based on constraint satisfaction. In particular, it focuses on data-sensitive connectors from the Reo coordination language. These connectors restrict how and where data can flow between loosely-coupled components taking into account the data being exchanged. Existing engines for Reo provide a very limited support for data-sensitive connectors, even though data constraints are captured by the original semantic models for Reo. When executing data-sensitive connectors, coordination constraints are not exhaustively solved at compile time but at runtime on a per-need basis, powered by an existing SMT (satisfiability modulo theories) solver.To deal with a wider range of data types and operations, we abstract data and reduce the original constraint satisfaction problem to a SAT problem, based on a variation of predicate abstraction. We show soundness and completeness of the abstraction mechanism for well-defined constraints, and validate our approach by evaluating the performance of a prototype implementation with different test cases, with and without abstraction.(undefined

Offer Semantics: Achieving Compositionality, Flattening and Full Expressiveness for the Glue Operators in BIP

Based on a concise but comprehensive overview of some fundamental properties required from component-based frameworks, namely compositionality, incrementality, flattening, modularity and expressiveness, we review three modifications of the semantics of glue operators in the Behaviour-Interaction-Priority (BIP) framework. We provide theoretical results and examples illustrating the degree, to which the three semantics meet these requirements. In particular, we show that the latest semantics, based on the offer predicate is the only one that satisfies all of them. The classical and offer semantics are not comparable: there are systems that can be assembled in the classical semantics, but not in the offer one. We present a strict characterisation of the behaviour hierarchy determining the conditions, under which systems in the classical semantics can be transposed into the offer semantics directly, with minor modifications, by introducing a new type of synchronisation or not at all. The offer semantics allows us to extend the algebras, which are used to model glue operators in BIP, to encompass priorities. This extension uses the Algebra of Causal Interaction Trees, T(P), as a pivot: existing transformations automatically provide the extensions for the Algebra of Connectors. We then extend the axiomatisation of T(P), since the equivalence induced by the new operational semantics is weaker than that induced by the interaction semantics. This extension leads to canonical normal forms for all structures and to a simplification of the algorithm for the synthesis of connectors from Boolean coordination constraints

Rigorous System Design

The monograph advocates rigorous system design as a coherent and accountable model-based process leading from requirements to correct implementations. It presents the current state of the art in system design, discusses its limitations, and identifies possible avenues for overcoming them. A rigorous system design flow is defined as a formal accountable and iterative process composed of steps, and based on four principles: (1) separation of concerns; (2) component-based construction; (3) semantic coherency; and (4) correctness-by-construction. The combined application of these principles allows the definition of a methodology clearly identifying where human intervention and ingenuity are needed to resolve design choices, as well as activities that can be supported by tools to automate tedious and error-prone tasks. An implementable system model is progressively derived by source-to-source automated transformations in a single host component-based language rooted in well-defined semantics. Using a single modeling language throughout the design flow enforces semantic coherency. Correct-by-construction techniques allow well-known limitations of a posteriori verification to be overcome and ensure accountability. It is possible to explain, at each design step, which among the requirements are satisfied and which may not be satisfied. The presented view for rigorous system design has been amply implemented in the BIP (Behavior, Interaction, Priority) component framework and substantiated by numerous experimental results showing both its relevance and feasibility. The monograph concludes with a discussion advocating a system-centric vision for computing, identifying possible links with other disciplines, and emphasizing centrality of system design