2.1 Scientific Objectives

2.2 Concurrency

The increasing size and the networked nature of communication systems, controls, distributed services, etc. confront us with an ever higher degree of parallelism between local processes. This field of application for our work includes telecommunication systems and composite web services. The challenge is to provide sound theoretical foundations and efficient algorithms for management of such systems, ranging from controller synthesis and fault diagnosis to integration and adaptation. While these tasks have received considerable attention in the sequential setting, managing non-sequential behavior requires profound modifications for existing approaches, and often the development of new approaches altogether. We see concurrency in distributed systems as an opportunity rather than a nuisance. Our goal is to exploit asynchronicity and distribution as an advantage. Clever use of adequate models, in particular partial order semantics (ranging from Mazurkiewicz traces to event structures to MSCs) actually helps in practice. In fact, the partial order vision allows us to make causal precedence relations explicit, and to perform diagnosis and test for the dependency between events. This is a conceptual advantage that interleaving-based approaches cannot match. The two key features of our work will be (i) the exploitation of concurrency by using asynchronous models with partial order semantics, and (ii) distribution of the agents performing management tasks.

2.3 Interaction

Systems and services exhibit non-trivial interaction between specialized and heterogeneous components. A coordinated interplay of several components is required; this is challenging since each of them has only a limited, partial view of the system's configuration. We refer to this problem as distributed synthesis or distributed control. An aggravating factor is that the structure of a component might be semi-transparent, which requires a form of grey box management.

2.4 Quantitative Features

Besides the logical functionalities of programs, the quantitative aspects of component behavior and interaction play an increasingly important role.

• Real-time properties cannot be neglected even if time is not an explicit functional issue, since transmission delays, parallelism, etc, can lead to time-outs striking, and thus change even the logical course of processes. Again, this phenomenon arises in telecommunications and web services, but also in transport systems.
• In the same contexts, probabilities need to be taken into account, for many diverse reasons such as unpredictable functionalities, or because the outcome of a computation may be governed by race conditions.
• Last but not least, constraints on cost cannot be ignored, be it in terms of money or any other limited resource, such as memory space or available CPU time.

2.5 Evolution and Perspectives

Since the creation of MExICo, the weight of quantitative aspects in all parts of our activities has grown, be it in terms of the models considered (weighted automata and logics), be it in transforming verification or diagnosis verdict into probabilistic statements (probabilistic diagnosis, statistical model checking), or within the recently started SystemX cooperation on supervision in multi-modal transport systems. This trend is certain to continue over the next couple of years, along with the growing importance of diagnosis and control issues.

In another development, the theory and use of partial order semantics has gained momentum in the past four years, and we intend to further strengthen our efforts and contacts in this domain to further develop and apply partial-order based deduction methods.

When no complete model of the underlying dynamic system is available, the analysis of logs may allow to reconstruct such a model, or at least to infer some properties of interest; this activity, which has emerged over the past 10 years on the international level, is referred to as process mining. In this emerging activity, we have contributed to unfolding-based process discovery [CI-146], and the study of process alignments [CI-121, CI-96, CI-83, CI-60, CI-33].

Finally, over the past years biological challenges have come to the center of our work, in two different directions:

1. (Re-)programming in discrete concurrent models. Cellular regulatory networks exhibit highly complex concurrent behaviours that is influenced by a high number of perturbations such as mutations. We are in particular investigating discrete models, both in the form of boolean networks and of Petri nets, to harness this complexity, and to obtain viable methods for two interconnected and central challenges:
   • find attractors, i.e. long-run stable states or sets of states, that indicate possible phenotypes of the organism under study, and
   • determine reprogramming strategies that apply perturbations in such a way as to steer the cell's long-run behaviour into some desired phenotype, or away from an undesired one.

2. Process mining @ MExICo The use of process models has increased in the last decade due to the advent of the process mining field. Process mining techniques aim at discovering, analyzing and enhancing formal representations of the real processes executed in any digital environment. These processes can only be observed by the footprints of their executions, stored in form of event logs. An event log is a collection of traces and is the input of process mining techniques. The derivation of an accurate formalization of an underlying process opens the door to the continuous improvement and analysis of the processes within an information system.

   Process models often use true concurrency to represent actions that appear in logs with different permutations.

   Among the important challenges in process mining, conformance checking is a crucial one: to assess the quality of a model (automatically discovered or manually designed) in describing the observed behavior, i.e., the event log.

   MExICo contributes to process mining, a field which discovers and manipulates true concurrency models and questions about their conformance to recorded event logs.

3. Distributed Algorithms in wild or synthetic biological systems. Since the arrival of Matthias Fuegger in the team, we also work, on the multi-cell level, with a distributed algorithms' view on microbiological systems, both with the goal to model and analyze existing microbiological systems as distributed systems, and to design and implement distributed algorithms in synthesized microbiological systems. Major long-term goals are drug production and medical treatment via synthesized bacterial colonies.

3.1 Concurrency

Keywords: Concurrency; Semantics; Automatic Control ; Diagnosis ; Verification.

Participants: Thomas Chatain, Philippe Dague, Stefan Haar, Serge Haddad, Stefan Schwoon.

3.2 Management of Quantitative Behavior

Participants: Thomas Chatain, Stefan Haar, Serge Haddad.

3.3 Probabilistic distributed Systems

Participants: Stefan Haar, Serge Haddad.

3.4 Large scale probabilistic systems

Addressing large-scale probabilistic systems requires to face state explosion, due to both the discrete part and the probabilistic part of the model. In order to deal with such systems, different approaches have been proposed:

• Restricting the synchronization between the components as in queuing networks allows to express the steady-state distribution of the model by an analytical formula called a product-form  43.
• Some methods that tackle with the combinatory explosion for discrete-event systems can be generalized to stochastic systems using an appropriate theory. For instance symmetry based methods have been generalized to stochastic systems with the help of aggregation theory  47.
• At last simulation, which works as soon as a stochastic operational semantic is defined, has been adapted to perform statistical model checking. Roughly speaking, it consists to produce a confidence interval for the probability that a random path fulfills a formula of some temporal logic  59 .

We want to contribute to these three axes: (1) we are looking for product-forms related to systems where synchronization are more involved (like in Petri nets 6); (2) we want to adapt methods for discrete-event systems that require some theoretical developments in the stochastic framework and, (3) we plan to address some important limitations of statistical model checking like the expressiveness of the associated logic and the handling of rare events.

3.5 Real time distributed systems

Nowadays, software systems largely depend on complex timing constraints and usually consist of many interacting local components. Among them, railway crossings, traffic control units, mobile phones, computer servers, and many more safety-critical systems are subject to particular quality standards. It is therefore becoming increasingly important to look at networks of timed systems, which allow real-time systems to operate in a distributed manner.

Timed automata are a well-studied formalism to describe reactive systems that come with timing constraints. For modeling distributed real-time systems, networks of timed automata have been considered, where the local clocks of the processes usually evolve at the same rate 5246. It is, however, not always adequate to assume that distributed components of a system obey a global time. Actually, there is generally no reason to assume that different timed systems in the networks refer to the same time or evolve at the same rate. Any component is rather determined by local influences such as temperature and workload.

4.1 Telecommunications

Participants: Stefan Haar, Serge Haddad.

• In the domain of service oriented computing, it is often necessary to insert some Web service into an existing orchestrated business process, e.g. to replace another component after failures. This requires to ensure, often actively, conformance to the interaction protocol. One therefore needs to synthesize adaptators for every component in order to steer its interaction with the surrounding processes.
• Still in the domain of telecommunications, the supervision of a network tends to move from out- of-band technology, with a fixed dedicated supervision infrastructure, to in-band supervision where the supervision process uses the supervised network itself. This new setting requires to revisit the existing supervision techniques using control and diagnosis tools.

Currently, we have no active cooperation on these subjects.

4.2 Biological Regulation Networks

Participants: Thomas Chatain, Matthias Fuegger, Stefan Haar, Serge Haddad, Juraj Kolcak, Hugues Mandon, Stefan Schwoon.

In the context of the Escape project (PhD thesis of G.K. Aguirre Samboni, started in October 2020) we are now extending our research on causal analysis of complex biological networks to the domain of ecosystems.

4.3 Transportation Systems

Participants: Thomas Chatain, Stefan Haar, Serge Haddad, Stefan Schwoon.

• Autonomous Vehicles. The validation of safety properties is a crucial concern for the design of computer guided systems, in particular for automated transport systems. Our approach consists in analyzing the interactions of a randomized environment (roads, cross-sections, etc.) with a vehicle controller.
• Multimodal Transport Networks. We are interested in predicting and harnessing the propagation of perturbations across different transportation modes.

5.1 Footprint of research activities

The carbon footprint of our activities is generic for office work, and probably strongest in traveling. While the latter came essential to a halt in 2020 because of the Covid pandemic, we believe that even in the future, intelligent use of online cooperation and communication can help limit the inevitable footprint of travel to the crucial activities of cooperation and networking, avoiding physical meetings when possible.

5.2 Impact of research results

With our Project ESCAPE, we are hoping for a strong impact on ecosystem analysis and management. Further, the research on biological regulation networks has the potential for enabling e.g. evaluation and design of medical therapies in epigenetic contexts.

7.1 New software

8.1 Active Prediction for Discrete Event Systems

A central task in partially observed controllable system is to detect or prevent the occurrence of certain events called faults. Systems for which one can design a controller avoiding the faults are called actively safe. Otherwise, one may require that a fault is eventually detected, which is the task of diagnosis. Systems for which one can design a controller detecting the faults are called actively diagnosable; we can build here on our past work in 5. An intermediate requirement is prediction, which consists in determining that a fault will occur whatever the future behaviour of the system. When a system is not predictable, one may be interested in designing a controller to make it so. Here we study the latter problem, called active prediction, and its associated property, active predictability. In other words, we investigate in 24 how to determine whether or not a system enjoys the active predictability property, i.e., there exists an active predictor for the system. Our contributions are threefold. From a semantical point of view, we refine the notion of predictability by adding two quantitative requirements: the minimal and maximal delay before the occurence of the fault, and we characterize the requirements fulfilled by a controller that performs predictions. Then we show that active predictability is EXPTIME-complete where the upper bound is obtained via a game-based approach. Finally we establish that active predictability is equivalent to active safety when the maximal delay is beyond a threshold depending on the size of the system, and we show that this threshold is accurate by exhibiting a family of systems fulfilling active predictability but not active safety.

8.2 Philosophers may dine - definitely !

In 32, we refine and extend the theory of Communicating Sequential Processes (CSP) of Hoare and Roscoe, whose denotational semantics of the Failure/Divergence Model was first formalized in Isabelle/HOL in 1997 to cope with infinite alphabets. We analyse a family of refinement notions, including some new ones. Better definitions allow us to clarify a number of obscure points in the classical literature, for example concerning the relationship between deadlock freeness and livelock freeness. As a result, we have a modern environment for formal proofs of concurrent systems that allow to combine general infinite processes with locally finite ones in a logically safe way. We demonstrate a number of verification-techniques for two examples: the Copy Buffer and Dijkstra’s Dining Philosopher Problem of an arbitrary size.

8.3 A Coloured Petri Nets Based Attack Tolerance Framework

It is well-known that web services become very vulnerable when being attacked, especially in the situation where service continuity is one of the most important requirements. In 33, we propose a Coloured Petri Nets based method for attack tolerance by modelling and analysing basic behaviours of attack-network interaction, attack detectors and their tolerance solutions. Furthermore, for complex attacks composed from basic ones, their corresponding tolerance solutions can be constructed from the corresponding basic solutions. The validity of our method is demonstrated through a case study on attack tolerance in cloud-based medical information storage.

8.4 Property-Directed Verification of Recurrent Neural Networks

We present in  35 a property-directed approach to verifying recurrent neural networks (RNNs). To this end, we learn a deterministic finite automaton as a surrogate model from a given RNN using active automata learning. This model may then be analyzed using model checking as verification technique. The term property-directed reflects the idea that our procedure is guided and controlled by the given property rather than performing the two steps separately. We show that this not only allows us to discover small counterexamples fast, but also to generalize them by pumping towards faulty flows hinting at the underlying error in the RNN

8.5 Guarded Autonomous Transitions Increase Conciseness and Expressiveness of Timed Automata

Timed Automata (TA) are an appropriate model for specifying timed requirements for Continuous Time Markov Chains (CTMC). However in order to keep tractable the model checking of a TA over a CTMC, temporal logics based on TA, like CSLTA, restrict TA to have a single clock and to be deterministic (DTA). Different variants of DTAs have been proposed to address the issue of their expressiveness and conciseness. In 30 we study the effect of two possible features: (1) autonomous transitions which are triggered by time elapsing in addition to synchronized transitions and (2) transitions guarded by propositional formulas instead of propositional formulas guarding locations. We first show that autonomous guarded transitions increase the expressiveness of DTAs (as already shown for guarded locations). Then we identify a hierarchy of DTAs subclasses all equivalent to DTAs without guarded autonomous transitions and we analyze their respective conciseness. In particular we show that eliminating resets in autonomous transitions implies an exponential blow-up, while eliminating autonomous transitions without reset can be performed in polynomial time if decision diagrams are used. Finally we compare TA with guarded transitions to TA with guarded locations showing that the former model is exponentially more concise than the latter one.

8.6 Dynamic Recursive Petri Nets

n the early two-thousands, Recursive Petri nets (RPN) have been introduced in order to model distributed planning of multi-agent systems for which counters and recursivity were necessary. While having a great expressive power, RPN suffer two limitations: (1) they do not include more general features for transitions like reset arcs, transfer arcs, etc. (2) the initial marking associated the recursive "call" only depends on the calling transition and not on the current marking of the caller. Here we introduce Dynamic Recursive Petri nets (DRPN) which address these issues. We show in 27 that the standard extensions of Petri nets for which decidability of the coverability problem is preserved are particular cases of DPRN. Then we establish that w.r.t. coverability languages, DRPN are strictly more expressive than RPN. Finally we prove that the coverability problem is still decidable for DRPN.

8.7 Concurrency in Boolean networks

Boolean networks (BNs) are widely used to model the qualitative dynamics of biological systems. Besides the logical rules determining the evolution of each component with respect to the state of its regulators, the scheduling of component updates can have a dramatic impact on the predicted behaviours. In 3, 14, we explore the use of Read (contextual) Petri Nets (RPNs) to study dynamics of BNs from a concurrency theory perspective. After showing bi-directional translations between RPNs and BNs and analogies between results on synchronism sensitivity, we illustrate that usual updating modes for BNs can miss plausible behaviours, i.e., incorrectly conclude on the absence/impossibility of reaching specific configurations. We propose an encoding of BNs capitalizing on the RPN semantics enabling more behaviour than the generalized asynchronous updating mode. The proposed encoding ensures a correct abstraction of any multivalued refinement, as one may expect to achieve when modelling biological systems with no assumption on its time features.

8.8 Minimal coverability tree construction made complete and efficient.

Downward closures of Petri net reachability sets can be finitely represented by their set of maximal elements called the minimal coverability set or Clover. Many properties (coverability, boundedness, ...) can be decided using Clover, in a time proportional to the size of Clover. So it is crucial to design algorithms that compute it efficiently. We present in 23 a simple modification of the original but incomplete Minimal Coverability Tree algorithm (MCT), computing Clover, which makes it complete: it memorizes accelerations and fires them as ordinary transitions. Contrary to the other alternative algorithms for which no bound on the size of the required additional memory is known, we establish that the additional space of our algorithm is at most doubly exponential. Furthermore we have implemented a prototype MinCov which is already very competitive: on benchmarks it uses less space than all the other tools and its execution time is close to the one of the fastest tool.

8.9 Expressiveness and Conciseness of Timed Automata for the Verification of Stochastic Models.

Timed Automata are a well-known formalism for specifying timed behaviours. In 26 we are concerned with Timed Automata for the specification of timed behaviour of Continuous Time Markov Chains (CTMC), as used in the stochastic temporal logic CSLTA. A timed path formula of CSLTA is specified by a Deterministic Timed Automaton (DTA) that features two kinds of transitions: synchronizing transitions (triggered by CTMC transitions) and autonomous transitions (triggered when a clock reaches a given threshold). Other definitions of CSLTA are based on DTAs that do not include autonomous transitions. This raises the natural question: do autonomous transitions enhance expressiveness and/or conciseness of DTAs? We prove in 26 that this is the case and we provide a syntactical characterization of DTAs for which autonomous transitions do not add expressive power, but allow one to define exponentially more concise DTAs.

8.10 Commodification of accelerations for the Karp and Miller Construction.

Karp and Miller’s algorithm is based on an exploration of the reachability tree of a Petri net where, the sequences of transitions with positive incidence are accelerated. The tree nodes of Karp and Miller are labeled with ω-markings representing (potentially infinite) coverability sets. This set of ω-markings allows us to decide several properties of the Petri net, such as whether a marking is coverable or whether the reachability set is finite. The edges of the Karp and Miller tree are labeled by transitions but the associated semantic is unclear which yields to a complex proof of the algorithm correctness. In this work we introduce three concepts: abstraction, acceleration and exploration sequence. In particular, we generalize the definition of transitions to ω-transitions in order to represent accelerations by such transitions. The notion of abstraction makes it possible to greatly simplify the proof of the correctness. On the other hand, for an additional cost in memory, which we theoretically evaluated, we propose in 15 n “accelerated” variant of the Karp and Miller algorithm with an expected gain in execution time. Based on a similar idea we have accelerated (and made complete) the minimal coverability graph construction, implemented it in a tool and performed numerous promising benchmarks issued from realistic case studies and from a random generator of Petri nets.

8.11 Diagnosis and Degradation Control for Probabilistic Systems

Systems prone to faults are often equipped with a controller whose aim consists in restricting the behaviour of the system in order to perform a diagnosis. Such a task is called active diagnosis. However to avoid that the controller degrades the system in view of diagnosis, a second objective in terms of quality of service is usually assigned to the controller. In the framework of stochastic systems, a possible specification, called safe active diagnosis requires that the probability of correctness of the infinite (random) run is non null. We introduce and study in 9 two alternative specifications that are in many contexts more realistic. The notion of (γ,v)-fault freeness associates with each run a value depending on the discounted length of its correct prefix where the discounting factor is γ. The controller has to ensure that the average of this value is above the threshold v. The notion of α-resiliency requires that asymptotically, at every time step, a proportion greater than α of correct runs remain correct. From a semantic point of view, we determine the equivalences and (non) implications between the three notions of degradations both for finite and infinite systems. From an algorithmic point of view, we establish the border between decidability and undecidability of the diagnosability problems. Furthermore in the positive case, we exhibit their precise complexity and propose a synthesis of the controller which may require an infinite memory.

8.12 Synchronizer-Free Digital Link Controller

In 12, we present a producer-consumer link between two independent clock domains. The link allows for metastability-free, low-latency, high-throughput communication by slight adjustments to the clock frequencies of the producer and consumer domains steered by a controller circuit. Any such controller cannot deterministically avoid, detect, nor resolve metastability. Typically, this is addressed by synchronizers, incurring a larger dead time in the control loop. We follow the approach of Friedrichs et al. (TC 2018) who proposed metastability-containing circuits. The result is a simple control circuit that may become metastable, yet deterministically avoids buffer underrun or overflow. More specifically, the controller output may become metastable, but this may only affect oscillator speeds within specific bounds. In contrast, communication is guaranteed to remain metastability-free. We formally prove correctness of the producer-consumer link and a possible implementation that has only small overhead. With SPICE simulations of the proposed implementation we further substantiate our claims. The simulation uses 65nm process running at roughly 2GHz.

8.13 Optimized SAT encoding of conformance checking artefacts

Conformance checking is a growing discipline that aims at assisting organizations in monitoring their processes. On its core, conformance checking relies on the computation of particular artefacts which enable reasoning on the relation between observed and modeled behavior. It is widely acknowledge that the computation of these artifacts is the lion’s share of conformance checking techniques. In 11, we show how important conformance artefacts like alignments, anti-alignments or multi-alignments, defined over the Levenshtein edit distance, can be efficiently computed by encoding the problem as an optimized SAT instance. From a general perspective, the work advocates for a unified family of techniques that can compute conformance artefacts in the same way. The implementation of the techniques presented in this paper show capabilities for dealing with both synthetic and real-life instances, which may open the door for a fresh way of applying conformance checking in the near future.

8.14 Anti-alignments – Measuring the precision of process models and event logs

Processes are a crucial artifact in organizations, since they coordinate the execution of activities so that products and services are provided. The use of models to analyze the underlying processes is a well-known practice. However, due to the complexity and continuous evolution of their processes, organizations need an effective way of analyzing the relation between processes and models. Conformance checking techniques assess the suitability of a process model in representing an underlying process, observed through a collection of real executions. One important metric in conformance checking is to assess the precision of the model with respect to the observed executions, i.e., characterize the ability of the model to produce behavior unrelated to the one observed. In 13, we present the notion of anti-alignment as a concept to help unveiling runs in the model that may deviate significantly from the observed behavior. Using anti-alignments, a new metric for precision is proposed. The proposed anti-alignment based precision metric satisfies most of the required axioms highlighted in a recent publication. Moreover, a complexity analysis of the problem of computing anti-alignments is provided, which sheds light into the practicability of using anti-alignment to estimate precision. Experiments are provided that witness the validity of the concepts introduced.

8.15 Model-based trace variant analysis of event logs

The comparison of trace variants of business processes opens the door for a fine-grained analysis of the distinctive features inherent in the executions of a process in an organization. The current approaches for trace variant analysis do not consider the situation where a process model is present, and therefore, it can guide the derivation of the trace variants by considering high-level structures present in the process model. In 10, we propose a fresh alternative to trace variant analysis, which proposes a generalized notion of trace variant that incorporates concurrency and iteration. This way, the analyst may be relieved from analyzing trace variants that are essentially the same, if these aspects are disregarded. We propose a general algorithm for model based trace variant analysis which is grounded in encoding the problem into SAT, and a family of heuristic alternatives including a very light sampling technique that represents a good trade-off between quality of the trace variants identified, and the complexity of the analysis. All the techniques of the paper are implemented in two open-source tools, and experiments with publicly available benchmarks are reported.

8.16 Reconciling Qualitative, Abstract, and Scalable Modeling of Biological Networks

Predicting biological systems’ behaviors requires taking into account many molecular and genetic elements for which limited information is available past a global knowledge of their pairwise interactions. Logical modeling, notably with Boolean Networks (BNs), is a well-established approach that enables reasoning on the qualitative dynamics of networks. Several dynamical interpretations of BNs have been proposed. The synchronous and (fully) asynchronous ones are the most prominent, where the value of either all or only one component can change at each step. In 8, 18, we prove that, besides being costly to analyze, these usual interpretations can preclude the prediction of certain behaviors observed in quantitative systems. We introduce an execution paradigm, the Most Permissive Boolean Networks (MPBNs), which offers the formal guarantee not to miss any behavior achievable by a quantitative model following the same logic. Moreover, MPBNs significantly reduce the complexity of dynamical analysis, enabling to model genome-scale networks.

8.17 The Involution Tool for Accurate Digital Timing and Power Analysis

In 17, we introduce the prototype of a digital timing simulation and power analysis tool for integrated circuits that supports the involution delay model (Függer et al. 2019). Unlike the pure and inertial delay models typically used in digital timing analysis tools, the involution model faithfully captures short pulse propagation and related effects. Our Involution Tool facilitates experimental accuracy evaluation of variants of involution models, by comparing their timing and power predictions to those from SPICE and standard timing analysis tools. The tool is easily customizable w.r.t. instances of the involution model and circuits, and supports automatic test case generation and parameter sweeping.

We demonstrate the capabilities of the Involution Tool by providing timing and power analysis results for three different circuits, namely, an inverter tree, the clock tree of an open-source processor, and a combinational circuit that involves multi-input NAND gates. Our evaluation uses two different technologies (15 nm and 65 nm CMOS), and three different variants of involution channels (Exp, Hill and SumExp-channels). It turns out that the timing and power predictions of all involution models are significantly better than the predictions obtained by standard digital simulations for the inverter tree and the clock tree, with the SumExp-channel channel clearly outperforming the others. For the NAND circuit, the performance of any involution model is generally comparable but not significantly better than that of standard models, however, which reveals some shortcomings of the existing involution channels for modeling multi-input gates.

8.18 On the Radius of Nonsplit Graphs and Information Dissemination in Dynamic Networks

In 16, we A nonsplit graph is a directed graph where each pair of nodes has a common incoming neighbor. In 16, we show that the radius of such graphs is in O(log log n), where n is the number of nodes. This is an exponential improvement on the previously best known upper bound of O(log n). We then generalize the result to products of nonsplit graphs. The analysis of nonsplit graph products has direct implications in the context of distributed systems, where processes operate in rounds and communicate via message passing in each round: communication graphs in several distributed systems naturally relate to nonsplit graphs and the graph product concisely represents relaying messages in such networks. Applying our results, we obtain improved bounds on the dynamic radius of such networks, i.e., the maximum number of rounds until all processes have received a message from a common process, if all processes relay messages in each round. We finally connect the dynamic radius to lower bounds for achieving consensus in dynamic networks.

8.19 An Alignment Cost-Based Classification of Log Traces Using Machine-Learning

Conformance checking is an important aspect of process mining that identifies the differences between the behaviors recorded in a log and those exhibited by an associated process model. Machine learning and deep learning methods perform extremely well in sequence analysis. In 20, we successfully apply both a Recurrent Neural Network and a Random Forest classifiers to the problem of evaluating whether the alignment cost of a log trace to a process model is below an arbitrary threshold, and provide a lower bound for the fitness of the process model based on the classification.

8.20 Synthesis in the Presence of Dynamic Links

. The problem of distributed synthesis is to automatically generate a distributed algorithm, given a target communication network and a specification of the algorithm's correct behavior. Previous work has focused on static networks with an apriori fixed message size. This approach has two shortcomings: Recent work in distributed computing is shifting towards dynamically changing communication networks rather than static ones, and an important class of distributed algorithms are so-called full-information protocols, where nodes piggy-pack previously received messages onto current messages. In 19, we consider the synthesis problem for a system of two nodes communicating in rounds over a dynamic link whose message size is not bounded. Given a network model, i.e., a set of link directions, in each round of the execution, the adversary choses a link from the network model, restricted only by the specification, and delivers messages according to the current link's directions. Motivated by communication buses with direct acknowledge mechanisms we further assume that nodes are aware of which messages have been delivered. We show that the synthesis problem is decidable for a network model if and only if it does not contain the empty link that dismisses both nodes' messages.

8.21 Distributed Computation with Continual Population Growth

Computing with synthetically engineered bacteria is a vibrant and active field with numerous applications in bio-production, bio-sensing, and medicine. Motivated by the lack of robustness and by resource limitation inside single cells, distributed approaches with communication among bacteria have recently gained in interest. In 22, we focus on the problem of population growth happening concurrently, and possibly interfering, with the desired bio-computation. Specifically, we present a fast protocol in systems with continuous population growth for the majority consensus problem and prove that it correctly identifies the initial majority among two inputs with high probability if the initial difference is $\Omega \left(\sqrt{nlogn}\right)$ where n is the total initial population. We also present a fast protocol that correctly computes the NAND of two inputs with high probability. We demonstrate that combining the NAND gate protocol with the continuous-growth majority consensus protocol, using the latter as an amplifier, it is possible to implement circuits computing arbitrary Boolean functions.

8.22 Drawing the Line: Basin Boundaries in Safe Petri Nets

Attractors of network dynamics represent the long-term behaviours of the modelled system. Understanding the basin of an attrac-tor, comprising all those states from which the evolution will eventually lead into that attractor, is therefore crucial for understanding the response and differentiation capabilities of a dynamical system. Building on our previous results 2 allowing to find attractors via Petri net Un-foldings, we exploit in  25 further the unfolding technique for a backward exploration of the state space, starting from a known attractor, and show how all strong or weak basins of attractions can be explicitly computed.

8.23 PALS: Plesiochronous and Locally Synchronous Systems

Consider an arbitrary network of communicating modules on a chip, each requiring a local signal telling it when to execute a computational step. There are three common solutions to generating such a local clock signal: (i) by deriving it from a single, central clock source, (ii) by local, free-running oscillators, or (iii) by handshaking between neighboring modules. Conceptually, each of these solutions is the result of a perceived dichotomy in which (sub)systems are either clocked or fully asynchronous, suggesting that the designer's choice is limited to deciding where to draw the line between synchronous and asynchronous design. In contrast, we take the view in 21 that the better question to ask is how synchronous the system can and should be. Based on a distributed clock synchronization algorithm, we present a novel design providing modules with local clocks whose frequency bounds are almost as good as those of corresponding free-running oscillators, yet neighboring modules are guaranteed to have a phase offset substantially smaller than one clock cycle. Concretely, parameters obtained from a 15 nm ASIC implementation running at 2 GHz yield mathematical worst-case bounds of 30 ps on phase offset for a 32 × 32 node grid network.

9.1 National initiatives

• Thomas Chatain, Stefan Haar, Serge Haddad and Stefan Schwoon are participating in the ANR Project ALGORECELL.
• Matthias Függer participates in the ANR project FREDDA on verification and synthesis of distributed algorithms.

9.2 Regional initiatives

• Matthias Függer co-organizes the Digicosme working group HicDiesMeus.
• Matthias Függer co-leads the CARE & U. Paris-Saclay project ETSHI on efficient test strategies for SARS-CoV-2 in healthcare institutions.
• Stefan Haar co-organizes the Digicosme working group TheoBioR2, and leads the Digicosme doctoral research projectESCAPE with Franck Pommereau.

10.1 Promoting scientific activities

10.2 Teaching - Supervision - Juries

11.1 Major publications

• 1 articleBéatriceB. Bérard, StefanS. Haar, SylvainS. Schmitz and StefanS. Schwoon. The Complexity of Diagnosability and Opacity Verification for Petri NetsFundamenta Informaticae16142018, 317-349
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• 2 inproceedingsThomasT. Chatain, StefanS. Haar, LoïgL. Jezequel, LoïcL. Paulevé and StefanS. Schwoon. Characterization of Reachable Attractors Using Petri Net UnfoldingsCMSB 20148859LNCS/LNBIManchester, United KingdomSpringer International PublishingNovember 2014, 14
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• 3 article ThomasT. Chatain, StefanS. Haar, JurajJ. Kolčák, LoïcL. Paulevé and AalokA. Thakkar. Concurrency in Boolean networks Natural Computing 2019
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• 4 article StephanS. Friedrichs, MatthiasM. Függer and ChristophC. Lenzen. Metastability-Containing Circuits IEEE Transactions on Computers 67 8 2018
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• 5 articleStefanS. Haar, SergeS. Haddad, TarekT. Melliti and StefanS. Schwoon. Optimal constructions for active diagnosisJournal of Computer and System Sciences8312017, 101-120
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• 6 articleSergeS. Haddad, JeanJ. Mairesse and Hoang-ThachH.-T. Nguyen. Synthesis and Analysis of Product-form Petri NetsFundamenta Informaticae1221-22013, 147-172
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• 7 articleJurajJ. Kolčák, DavidD. Šafránek, StefanS. Haar and LoïcL. Paulevé. Parameter Space Abstraction and Unfolding Semantics of Discrete Regulatory NetworksTheoretical Computer Science7652019, 120-144
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• 8 article LoïcL. Paulevé, JurajJ. Kolčák, ThomasT. Chatain and StefanS. Haar. Reconciling Qualitative, Abstract, and Scalable Modeling of Biological Networks Nature Communications 11 2020
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11.2 Publications of the year

11.3 Cited publications

• 38 articleA.~Benveniste, ÉricÉ. Fabre and S.~Haar. Markov Nets: Probabilistic Models for distributed and concurrent SystemsIEEE Transactions on Automatic Control48 (11)Extended version: IRISA Research Report 15382003, 1936-1950
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• 39 inproceedingsS. Akshay, NathalieN. Bertrand, SergeS. Haddad and LoicL. Helouet. The steady-state control problem for Markov decision processesQest 20138054Buenos Aires, ArgentinaSpringerSeptember 2013, 290-304
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• 40 articleR. Alur, K. Etessami and M. Yannakakis. Realizability and Verification of MSC GraphsTheor. Comput. Sci.33112005, 97--114
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• 41 articleP. Baldan, ThomasT. Chatain, StefanS. Haar and BarbaraB. König. Unfolding-based Diagnosis of Systems with an Evolving TopologyInformation and Computation20810October 2010, 1169-1192URL: http://www.lsv.ens-cachan.fr/Publis/PAPERS/PDF/BCHK-icomp10.pdf
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• 42 articlePaoloP. Baldan, AndreaA. Corradini, BarbaraB. König and StefanS. Schwoon. McMillan's complete prefix for contextual netsTransactions on Petri Nets and Other Models of Concurrency1Volume 5100 of Lecture Notes in Computer ScienceNovember 2008, 199--220
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• 43 articleForestF. Baskett, K. ManiK. Chandy, Richard R.R. Muntz and Fernando G.F. Palacios. Open, Closed, and Mixed Networks of Queues with Different Classes of CustomersJ. ACM222April 1975, 248--260URL: http://doi.acm.org/10.1145/321879.321887
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• 44 inproceedingsPuneetP. Bhateja, P. Gastin, M. Mukund and K. Narayan Kumar. Local testing of message sequence charts is difficultProceedings of the 16th International Symposium on Fundamentals of Computation Theory (FCT'07)4639Lecture Notes in Computer ScienceBudapest, HungarySpringerAugust 2007, 76-87URL: http://www.lsv.ens-cachan.fr/Publis/PAPERS/PDF/BGMN-fct07.pdf
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• 45 inproceedingsAnneA. Bouillard, StefanS. Haar and SidneyS. Rosario. Critical paths in the Partial Order Unfolding of a Stochastic Petri NetProceedings of the 7th International Conference on Formal Modelling and Analysis of Timed Systems (FORMATS'09)5813Lecture Notes in Computer ScienceBudapest, HungarySpringerSeptember 2009, 43-57URL: http://www.lsv.ens-cachan.fr/Publis/PAPERS/PDF/BHR-formats09.pdf
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• 46 inproceedingsP. Bouyer, SergeS. Haddad and Pierre-AlainP.-A. Reynier. Timed Unfoldings for Networks of Timed AutomataProceedings of the 4th International Symposium on Automated Technology for Verification and Analysis (ATVA'06)4218Lecture Notes in Computer ScienceBeijing, ROCSpringerOctober 2006, 292-306URL: http://www.lsv.ens-cachan.fr/Publis/PAPERS/PDF/BHR-atva06.pdf
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• 47 articleGiovanniG. Chiola, ClaudeC. Dutheillet, GiulianaG. Franceschinis and SergeS. Haddad. Stochastic Well-Formed Colored Nets and Symmetric Modeling ApplicationsIEEE Transactions on Computers4211November 1993, 1343-1360URL: http://www.lsv.ens-cachan.fr/Publis/PAPERS/PS/CDFH-toc93.ps
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• 48 articleRamiR. Debouk and DemosthenisD. Teneketzis. Coordinated decentralized protocols for failure diagnosis of discrete-event systemsJournal of Discrete Event Dynamical Systems: Theory and Application102000, 33--86
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• 49 book JavierJ. Esparza and KeijoK. Heljanko. Unfoldings - A Partial-Order Approach to Model Checking EATCS Monographs in Theoretical Computer Science Springer 2008
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• 50 articleÉricÉ. Fabre, A.~Benveniste, C.~Jard and S.~Haar. Distributed monitoring of concurrent and asynchronous systemsDiscrete Event Dynamic Systems: theory and application15 (1)Preliminary version: Proc.~CONCUR 2003, LNCS 2761, pp.1--28, Springer2005, 33-84
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• 51 articleS. Lafortune, Y. Wang and T.-S. Yoo. Diagnostic Decentralisé Des Systèmes A Evénements DiscretsJournal Europeen des Systèmes Automatisés (RS-JESA)9999August 2005, 95--110
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• 52 inproceedingsK. GK. Larsen, P. Pettersson and W. Yi. Compositional and symbolic model-checking of real-time systemsProc. of RTSS 1995IEEE Computer Society1995, 76-89
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• 53 inproceedingsP.~Baldan, S.~Haar and B.~Koenig. Distributed Unfolding of Petri NetsProc.FOSSACS 20063921LNCSExtended version: Technical Report CS-2006-1. Department of Computer Science, University Ca' Foscari of VeniceSpringer2006, 126-141
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• 54 articleLaurieL. Ricker and KarenK. Rudie. Know Means No: Incorporating Knowledge into Discrete-Event Control SystemsIEEE Transactions on Automatic Control459September 2000, 1656--1668
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• 55 articleLaurieL. Ricker and KarenK. Rudie. Knowledge Is a Terrible Thing to Waste: Using Inference in Discrete-Event Control ProblemsIEEE Transactions on Automatic Control523MarchSeptember 2007, 428--441
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• 56 inproceedingsCésarC. Rodríguez, StefanS. Schwoon and VictorV. Khomenko. Contextual Merged Processes34th International Conference on Applications and Theory of Petri Nets (ICATPN'13)7927Lecture Notes in Computer ScienceItalySpringer2013, 29-48
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• 57 inproceedings S.~Abbes, A.~Benveniste and S.~Haar. A Petri net model for distributed estimation Proc.~MTNS 2004, Sixteenth International Symposium on Mathematical Theory of Networks and Systems, Louvain (Belgium), ISBN 90-5682-517-8 2004
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• 58 articleS.~Haar. Probabilistic Cluster UnfoldingsFundamenta Informaticae53 (3-4)2003, 281-314
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• 59 articleH\aakan L. S.H. Younes and Reid G.R. Simmons. Statistical probabilistic model checking with a focus on time-bounded propertiesInf. Comput.2049September 2006, 1368--1409URL: http://dl.acm.org/citation.cfm?id=1182767.1182770
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