# Selected Papers of the 40th International Conference on Formal Techniques for Distributed Objects, Components, and Systems (FORTE 2020)

Editors: Ana Sokolova and Alexey Gotsman

This volume contains selected papers presented at the 40th IFIP WG 6.1 International Conference on Formal Techniques for Distributed Objects, Components, and Systems (FORTE 2020). The conference was held as one of three main conferences of the 15th International Federated Conference on Distributed Computing Techniques (DisCoTec 2020) during June 15–19, 2020, online due to the coronavirus pandemic.

FORTE is a well-established forum for fundamental research on theory, models, tools, and applications for distributed systems, with special interest in:

• Software quality, reliability, availability, and safety
• Security, privacy, and trust in distributed and/or communicating systems
• Service-oriented, ubiquitous, and cloud computing systems
• Component- and model-based design
• Object technology, modularity, and software adaptation
• Self-stabilization and self-healing/organizing
• Verification, validation, formal analysis, and testing of the above

This special issue features five articles:

• Eleni Bila, John Derrick, Simon Doherty, Brijesh Dongol, Gerhard Schellhorn, and Heike Wehrheim. Modularising Verification Of Durable Opacity.
• Sebastian Biewer, Rayna Dimitrova, Michael Fries, Maciej Gazda, Thomas Heinze, Holger Hermanns, and Mohammad Reza Mousavi. Conformance Relations and Hyperproperties for Doping Detection in Time and Space.
• Jeremy Sproston. Probabilistic Timed Automata with One Clock and Initialised Clock-Dependent Probabilities.
• Adam Shimi, Aurélie Hurault, and Philippe Queinnec. Characterization and Derivation of Heard-Of Predicates for Asynchronous Message-Passing Models.
• Igor Konnov, Marijana Lazi?<87>, Ilina Stoilkovska, and Josef Widder. Survey on Parameterized Verification with Threshold Automata and the Byzantine Model Checker.

We wish to thank all the authors of submitted papers, all the members of the Program Committee for their thorough evaluations of the submissions, and the external reviewers who assisted the evaluation process and helped the authors to improve their contributions. We also thank the editors of LMCS for their support during the editorial process.

February 2023

Alexey Gotsman and Ana Sokolova
Program Chairs FORTE 2020
Guest Editors of the Special Issue

### 1. Characterization and Derivation of Heard-Of Predicates for Asynchronous Message-Passing Models

In distributed computing, multiple processes interact to solve a problem together. The main model of interaction is the message-passing model, where processes communicate by exchanging messages. Nevertheless, there are several models varying along important dimensions: degree of synchrony, kinds of faults, number of faults... This variety is compounded by the lack of a general formalism in which to abstract these models. One way to bring order is to constrain these models to communicate in rounds. This is the setting of the Heard-Of model, which captures many models through predicates on the messages sent in a round and received on time. Yet, it is not easy to define the predicate that captures a given operational model. The question is even harder for the asynchronous case, as unbounded message delay means the implementation of rounds must depend on details of the model. This paper shows that characterising asynchronous models by heard-of predicates is indeed meaningful. This characterization relies on delivered predicates, an intermediate abstraction between the informal operational model and the heard-of predicates. Our approach splits the problem into two steps: first extract the delivered model capturing the informal model, and then characterize the heard-of predicates that are generated by this delivered model. For the first part, we provide examples of delivered predicates, and an approach to derive more. It uses the intuition that complex models are a composition of […]

### 2. Probabilistic Timed Automata with One Clock and Initialised Clock-Dependent Probabilities

Clock-dependent probabilistic timed automata extend classical timed automata with discrete probabilistic choice, where the probabilities are allowed to depend on the exact values of the clocks. Previous work has shown that the quantitative reachability problem for clock-dependent probabilistic timed automata with at least three clocks is undecidable. In this paper, we consider the subclass of clock-dependent probabilistic timed automata that have one clock, that have clock dependencies described by affine functions, and that satisfy an initialisation condition requiring that, at some point between taking edges with non-trivial clock dependencies, the clock must have an integer value. We present an approach for solving in polynomial time quantitative and qualitative reachability problems of such one-clock initialised clock-dependent probabilistic timed automata. Our results are obtained by a transformation to interval Markov decision processes.

### 3. Conformance Relations and Hyperproperties for Doping Detection in Time and Space

We present a novel and generalised notion of doping cleanness for cyber-physical systems that allows for perturbing the inputs and observing the perturbed outputs both in the time- and value-domains. We instantiate our definition using existing notions of conformance for cyber-physical systems. As a formal basis for monitoring conformance-based cleanness, we develop the temporal logic HyperSTL*, an extension of Signal Temporal Logics with trace quantifiers and a freeze operator. We show that our generalised definitions are essential in a data-driven method for doping detection and apply our definitions to a case study concerning diesel emission tests.

### 4. Modularising Verification Of Durable Opacity

Non-volatile memory (NVM), also known as persistent memory, is an emerging paradigm for memory that preserves its contents even after power loss. NVM is widely expected to become ubiquitous, and hardware architectures are already providing support for NVM programming. This has stimulated interest in the design of novel concepts ensuring correctness of concurrent programming abstractions in the face of persistency and in the development of associated verification approaches. Software transactional memory (STM) is a key programming abstraction that supports concurrent access to shared state. In a fashion similar to linearizability as the correctness condition for concurrent data structures, there is an established notion of correctness for STMs known as opacity. We have recently proposed durable opacity as the natural extension of opacity to a setting with non-volatile memory. Together with this novel correctness condition, we designed a verification technique based on refinement. In this paper, we extend this work in two directions. First, we develop a durably opaque version of NOrec (no ownership records), an existing STM algorithm proven to be opaque. Second, we modularise our existing verification approach by separating the proof of durability of memory accesses from the proof of opacity. For NOrec, this allows us to re-use an existing opacity proof and complement it with a proof of the durability of accesses to shared state.

### 5. Survey on Parameterized Verification with Threshold Automata and the Byzantine Model Checker

Threshold guards are a basic primitive of many fault-tolerant algorithms that solve classical problems in distributed computing, such as reliable broadcast, two-phase commit, and consensus. Moreover, threshold guards can be found in recent blockchain algorithms such as, e.g., Tendermint consensus. In this article, we give an overview of techniques for automated verification of threshold-guarded fault-tolerant distributed algorithms, implemented in the Byzantine Model Checker (ByMC). These threshold-guarded algorithms have the following features: (1) up to $t$ of processes may crash or behave Byzantine; (2) the correct processes count messages and make progress when they receive sufficiently many messages, e.g., at least $t+1$; (3) the number $n$ of processes in the system is a parameter, as well as the number $t$ of faults; and (4) the parameters are restricted by a resilience condition, e.g., $n > 3t$. Traditionally, these algorithms were implemented in distributed systems with up to ten participating processes. Nowadays, they are implemented in distributed systems that involve hundreds or thousands of processes. To make sure that these algorithms are still correct for that scale, it is imperative to verify them for all possible values of the parameters.