Section: Overall Objectives

Overall Objectives

The objective of the project is the realization by algorithmic methods of reactive and distributed systems from partial and heterogeneous specifications. Methods, algorithms and tools are developed to synthesize reactive software from one or several incomplete descriptions of the system's expected behavior, regarding functionality (synchronization, conflicts, communication), control (safety, reachability, liveness), deployment architecture (mapping, partitioning, segregation), or even quantitative performances (response time, communication cost, throughput).

These techniques are better understood on fundamental models, such as automata, Petri nets, event structures and their timed extensions. The results obtained on these basic models are then adapted to those realistic but complex models commonly used to design embedded and telecommunication systems.

The behavioral views of the Unified Modeling Language (UML) (sequence diagrams and statecharts), the High-Level Message Sequence Charts (HMSC) and the synchronous reactive language Signal are the heart of the software prototypes being developed and the core of the technology transfer strategy of the project.

The scientific objectives of the project can be characterized by the following elements:

A focus on a precise type of applications:

The design of real-time embedded software to be deployed over dedicated distributed architectures. Engineers in this field face two important challenges. The first one is related to system specification. Behavioral descriptions should be adaptable and composable. Specifications are expressed as requirements on the system to be designed. These requirements fall into four categories: (i) functional (synchronization, conflict, communication), (ii) control (safety, reachability, liveness), (iii) architectural (mapping, segregation) and (iv) quantitative (response time, communication cost, throughput, etc). The second challenge is the deployment of the design on a distributed architecture. Domain-specific software environments, known as middleware or real-time operating systems or communication layers, are now part of the usual software design process in industry. They provide a specialized and platform-independent distributed environment to higher-level software components. Deployment of software components and services should be done in a safe and efficient manner.

A specific methodology:

The development of methods and tools which assist engineers since the very first design steps of reactive distributive software. The main difficulty is the adequacy of the proposed methods with standard design methods based on components and model engineering, which most often rely on heterogeneous formalisms and require correct-by-construction component assembly.

A set of scientific and technological foundations:

Those models and methods which encompass (i) the distributed nature of the systems being considered, (ii) true concurrency, and (iii) real-time.

The contribution of the S4 Project-Team consists of algorithms and tools producing distributed reactive software from partial heterogeneous specifications of the system to be synthesized (functionality, control, architecture, quantitative performances). This means that several heterogeneous specifications (for instance, sequence diagrams and state machines) can be combined, analyzed (are the specifications consistent?) and mapped to lower-level specifications (for instance, communicating automata, or Petri nets).

The scientific approach of Team S4 begins with a rigorous modeling of problems and the development of sound theoretical foundations. This not only allows to prove the correctness (functionality and control) of the proposed transformations or analysis; but this can also guarantee the optimality of the quantitative performances of the systems produced with our methods (communication cost, response time).

Synthesis and verification methods are best studied within fundamental models, such as automata, Petri nets, event structures, synchronous transition systems. Then, results can be adapted to more realistic but complex formalisms, such as the UML. The research work of Team S4 is divided in four main tracks:

Petri net synthesis:

Petri nets and apparented concurrency models have found applications in several fields, from telecommunications, to embedded systems, to process mining. Team S4 has been on the forefront of Petri net theory since its inception. In particular, the team has been very active on the theory of regions and Petri net synthesis algorithms, which has found applications in asynchronous hardware circuit design, communicating supervisory control synthesis and process mining applications.

Heterogeneous systems:

This track contributes to the extension of the well-established synchronous paradigm to distributed systems. The aim is to provide a unified framework in which both synchronous systems, and particular asynchronous systems (so-called weakly-synchronous systems) can be expressed, combined, analyzed and transformed.

Reactive components:

The design of reusable components calls for rich specification formalisms, with which the interactions of a component with its environment combines expectations with guarantees on its environment.We are investigating questions related to reactive component refinement and composition. We are also investigating the issues of coherence of views and modularity in complex specifications.

Discrete event system synthesis and supervisory control:

Many synthesis and supervisory control problems can be expressed with full generality in the quantified mu-calculus, including the existence of optimal solutions to such problems. Algorithms computing winning strategies in parity games (associated with formulas in this logic) provide effective methods for solving such control problems. This framework offers means of classifying control problems, according to their decidability or undecidability, but also according to their algorithmic complexity.