Operational Semantics Research Articles

Formal foundations of serverless computing

Serverless computing (also known as functions as a service) is a new cloud computing abstraction that makes it easier to write robust, large-scale web services. In serverless computing, programmers write what are called serverless functions, which are programs that respond to external events. When demand for the serverless function spikes, the platform automatically allocates additional hardware and manages load-balancing; when demand falls, the platform silently deallocates idle resources; and when the platform detects a failure, it transparently retries affected requests. In 2014, Amazon Web Services introduced the first serverless platform, AWS Lambda, and similar abstractions are now available on all major cloud computing platforms. Unfortunately, the serverless computing abstraction exposes several low-level operational details that make it hard for programmers to write and reason about their code. This paper sheds light on this problem by presenting λ λ , an operational semantics of the essence of serverless computing. Despite being a small (half a page) core calculus, λ λ models all the low-level details that serverless functions can observe. To show that λ λ is useful, we present three applications. First, to ease reasoning about code, we present a simplified naive semantics of serverless execution and precisely characterize when the naive semantics and λ λ coincide. Second, we augment λ λ with a key-value store to allow reasoning about stateful serverless functions. Third, since a handful of serverless platforms support serverless function composition, we show how to extend λ λ with a composition language and show that our implementation can outperform prior work.

Modelling concurrent objects running on the TSO and ARMv8 memory models

• Presents a method for using a standard linearizability proof method on hardware weak memory models. • Generates a transition system, which reflects on weak memory model behaviour, from a given piece of code. • Covers the widely used TSO memory model, as well as the latest version of the ARM processor, ARMv8. • Enables the use of a model checker to check correctness of code, taking into account TSO/ARMv8 weak memory behaviour. Hardware weak memory models, such as TSO and ARM, are used to increase the performance of concurrent programs by allowing program instructions to be executed on the hardware in a different order to that specified by the software. This places a challenge on the verification of concurrent objects used in these programs since the variations in the executions need to be considered. Many approaches exist for verifying concurrent objects along with associated tool support. In particular, we focus on a thread-local approach to checking linearizability , the standard correctness condition for concurrent objects, using a model checker. This approach, like most others, does not support weak memory models. In order to reuse this existing approach, therefore, we show how to use the semantics of a weak memory model to directly derive a transition system of concurrent objects running under it. We do this for both TSO and the latest version of ARM, ARMv8. Since there is a straightforward implementation of TSO, we reflect this in our transition system which includes a buffer of writes to memory mirroring the store buffer of TSO. We illustrate linearizability checking using model checking on a transition system generated by this approach. The implementation of the significantly more complex ARMv8 architecture is less obvious. We derive our transition system in this case from an existing operational semantics that is consistent with the results of thousands of litmus test run on ARM hardware.

BigData Applications from Graph Analytics to Machine Learning by Aggregates in Recursion

In the past, the semantic issues raised by the non-monotonic nature of aggregates often prevented their use in the recursive statements of logic programs and deductive databases. However, the recently introduced notion of Pre-mappability (PreM) has shown that, in key applications of interest, aggregates can be used in recursion to optimize the perfect-model semantics of aggregate-stratified programs. Therefore we can preserve the declarative formal semantics of such programs while achieving a highly efficient operational semantics that is conducive to scalable implementations on parallel and distributed platforms. In this paper, we show that with PreM, a wide spectrum of classical algorithms of practical interest, ranging from graph analytics and dynamic programming based optimization problems to data mining and machine learning applications can be concisely expressed in declarative languages by using aggregates in recursion. Our examples are also used to show that PreM can be checked using simple techniques and templatized verification strategies. A wide range of advanced BigData applications can now be expressed declaratively in logic-based languages, including Datalog, Prolog, and even SQL, while enabling their execution with superior performance and scalability.

Towards Gradually Typed Capabilities in the Pi-Calculus

Gradual typing is an approach to integrating static and dynamic typing within the same language, and puts the programmer in control of which regions of code are type checked at compile-time and which are type checked at run-time. In this paper, we focus on the pi-calculus equipped with types for the modeling of input-output capabilities of channels. We present our preliminary work towards a gradually typed version of this calculus. We present a type system, a cast insertion procedure that automatically inserts run-time checks, and an operational semantics of a pi-calculus that handles casts on channels. Although we do not claim any theoretical results on our formulations, we demonstrate our calculus with an example and discuss our future plans.

An Operational Semantics of Graph Transformation Systems Using Symmetric Nets

Graph transformation systems (GTS) have been successfully proposed as a general, theoretically sound model for concurrency. Petri nets (PN), on the other side, are a central and intuitive formalism for concurrent or distributed systems, well supported by a number of analysis techniques/tools. Some PN classes have been shown to be instances of GTS. In this paper, we change perspective presenting an operational semantics of GTS in terms of Symmetric Nets, a well-known class of Coloured Petri nets featuring a structured syntax that outlines model symmetries. Some practical exploitations of the proposed operational semantics are discussed. In particular, a recently developed structural calculus for SN is used to validate graph rewriting rules in a symbolic way.

Toward a Unified Executable Formal Automobile OS Kernel and Its Applications

In automobile industry, it is a common approach to develop automobile real-time operating systems under some standards. For instance, OSEK/VDX is a world-wide adopted open standard. Traditional workflow is to first understand the standard, design and develop a system, then test its conformance to the standard, and finally deploy. There are several issues with the traditional workflow, e.g., ambiguities in standards may lead to incorrect design and implementation of real-world systems; the conformance of real-world systems to standards is difficult to check; and bug fixing after implementation is costly. To remedy the situation, in this paper, we present a unified executable formal automobile kernel under OSEK/VDX standard by defining the operational semantics of the system services in the standard using a rewrite-based executable semantic framework called $\mathbb {K}$ . The formal kernel is unified in that it serves multiple purposes such as: 1) formal modeling of the OSEK/VDX standard helps detect ambiguities in the standard; 2) the executable kernel is essentially a formal model of the standard, which can be used to verify the correctness of automobile applications; and 3) verified applications can be used as test cases to check the conformance of a real-world automobile operating system against the OSEK/VDX standard. Using the formal kernel, we identify several ambiguities in the OSEK/VDX standard and a potential deadlock vulnerability in an industrial automobile application.

A calculus for collective-adaptive systems and its behavioural theory

We propose a process calculus, named AbC, to study the behavioural theory of interactions in collective-adaptive systems by relying on attribute-based communication. An AbC system consists of a set of parallel components each of which is equipped with a set of attributes. Communication takes place in an implicit multicast fashion, and interaction among components is dynamically established by taking into account “connections” as determined by predicates over their attributes. The structural operational semantics of AbC is based on Labelled Transition Systems that are also used to define bisimilarity between components. Labelled bisimilarity is in full agreement with a barbed congruence, defined by relying on simple basic observables and context closure. The introduced equivalence is used to study the expressiveness of AbC in terms of encoding aspects of broadcast channel-based interactions and to establish formal relationships between system descriptions at different levels of abstraction.

Proceedings Combined 26th International Workshop on Expressiveness in Concurrency and 16th Workshop on Structural Operational Semantics

Proceedings Combined 26th International Workshop on Expressiveness in Concurrency and 16th Workshop on Structural Operational Semantics

Cellular Monads from Positive GSOS Specifications

We give a leisurely introduction to our abstract framework for operational semantics based on cellular monads on transition categories. Furthermore, we relate it for the first time to an existing format, by showing that all Positive GSOS specifications generate cellular monads whose free algebras are all compositional. As a consequence, we recover the known result that bisimilarity is a congruence in the generated labelled transition system.

Ensuring liveness properties of distributed systems: Open problems

Often fairness assumptions need to be made in order to establish liveness properties of distributed systems, but in many situations they lead to false conclusions.This document presents a research agenda aiming at laying the foundations of a theory of concurrency that is equipped to ensure liveness properties of distributed systems without making fairness assumptions. This theory will encompass process algebra, temporal logic and semantic models. The agenda also includes the development of a methodology and tools that allow successful application of this theory to the specification, analysis and verification of realistic distributed systems.Contemporary process algebras and temporal logics fail to make distinctions between systems of which one has a crucial liveness property and the other does not, at least when assuming justness, a strong progress property, but not assuming fairness. Setting up an alternative framework involves giving up on identifying strongly bisimilar systems, inventing new induction principles, developing new axiomatic bases for process algebras and new congruence formats for operational semantics, and creating matching treatments of time and probability.Even simple systems like fair schedulers or mutual exclusion protocols cannot be accurately specified in standard process algebras (or Petri nets) in the absence of fairness assumptions. Hence the work involves the study of adequate language or model extensions, and their expressive power.

Theoretical and Practical Aspects of Linking Operational and Algebraic Semantics for MDESL

Verilog is a hardware description language (HDL) that has been standardized and widely used in industry. Multithreaded discrete event simulation language (MDESL) is a Verilog-like language. It contains interesting features such as event-driven computation and shared-variable concurrency. This article considers how the algebraic semantics links with the operational semantics for MDESL. Our approach is from both the theoretical and practical aspects. The link is proceeded by deriving the operational semantics from the algebraic semantics. First, we present the algebraic semantics for MDESL. We introduce the concept of head normal form. Second, we present the strategy of deriving operational semantics from algebraic semantics. We also investigate the soundness and completeness of the derived operational semantics with respect to the derivation strategy. Our theoretical approach is complemented by a practical one, and we use the theorem proof assistant Coq to formalize the algebraic laws and the derived operational semantics. Meanwhile, the soundness and completeness of the derived operational semantics is also verified via the mechanical approach in Coq. Our approach is a novel way to formalize and verify the correctness and equivalence of different semantics for MDESL in both a theoretical approach and a practical approach.

Axiomatization and characterization of BSP algorithms

bulk-synchronous parallel (bsp ) is a bridging model for hpc (high performance computing) algorithm design. It provides a conceptual bridge between the physical implementation of the machine and the abstraction available to a programmer, while having portable and scalable performance predictions of bsp algorithms on most hpc systems. Two questions may come to mind. What are formally bsp algorithms? And how to ensure that the programmer can effectively program every bsp algorithm with a bsp language, especially with the right cost? gurevich proved that three convincing postulates for the sequential algorithms are equivalent to what is called abstract state machines (asms ), and thus that asms capture the sequential algorithms. Firstly, we extend these sequential postulates and asms in order to intuitively and realistically capture the bsp algorithms (and not more). Secondly, by using an operational semantics and an algorithmic simulation, we prove that ▪ is equivalent to ▪, a minimal imperative bsp programming language. Therefore, bsp programming languages (extending at least ▪) are bsp algorithmically complete, involving the definition of a class model of the bsp algorithms.

Simply RaTT: a fitch-style modal calculus for reactive programming without space leaks

Functional reactive programming (FRP) is a paradigm for programming with signals and events, allowing the user to describe reactive programs on a high level of abstraction. For this to make sense, an FRP language must ensure that all programs are causal, and can be implemented without introducing space leaks and time leaks. To this end, some FRP languages do not give direct access to signals, but just to signal functions. Recently, modal types have been suggested as an alternative approach to ensuring causality in FRP languages in the synchronous case, giving direct access to the signal and event abstractions. This paper presents Simply RaTT , a new modal calculus for reactive programming. Unlike prior calculi, Simply RaTT uses a Fitch-style approach to modal types, which simplifies the type system and makes programs more concise. Echoing a previous result by Krishnaswami for a different language, we devise an operational semantics that safely executes Simply RaTT programs without space leaks. We also identify a source of time leaks present in other modal FRP languages: The unfolding of fixed points in delayed computations. The Fitch-style presentation allows an easy way to rules out these leaks, which appears not to be possible in the more traditional dual context approach.

Semantic variation operators for multidimensional genetic programming.

Multidimensional genetic programming represents candidate solutions as sets of programs, and thereby provides an interesting framework for exploiting building block identification. Towards this goal, we investigate the use of machine learning as a way to bias which components of programs are promoted, and propose two semantic operators to choose where useful building blocks are placed during crossover. A forward stagewise crossover operator we propose leads to significant improvements on a set of regression problems, and produces state-of-the-art results in a large benchmark study. We discuss this architecture and others in terms of their propensity for allowing heuristic search to utilize information during the evolutionary process. Finally, we look at the collinearity and complexity of the data representations that result from these architectures, with a view towards disentangling factors of variation in application.

An optimized cognitive-assisted machine translation approach for natural language processing

Currently, computer-aided machine translation (MT) processes play a significant role in natural language processing used to translate a specified language into another language like English to Spanish, Latin to French. During the translation process, and particularly during phrase composition, MT systems may exhibit several issues, including failure to produce high quality translations, increased time consumption, diminished linguistic precision and complexity. This research introduces an optimized cognitive assisted statistical MT process that is intended to reduce these difficulties. This process uses a supervised machine learning technique (OCSMT-SMT) for natural language processing that is intended to translate phrases with a higher degree of precision than other Support Vector Machine, Linear Regression, Decision Trees, Naive Bayes, and K-Nearest Neighbor MT techniques. The method introduced here uses semantic operations to examine the collected messages that are processed in network and obtained results have been stored in the memory to get the exact translation which helps to learn phrase semantics using MT. The OCSMT-SMT approach enables smarter, faster decision making regarding phrase translations, which greatly reduces translation time. The efficiency of this approach is evaluated using the bilingual evaluation understudy (BLEU) and Better Evaluation as Ranking (BEER) metrics for English language phrase datasets in English language. This ensures the high precision while performing the MT process.

A denotational semantics of textually aligned SPMD programs

We discuss the benefit of enforcing textual alignment in programming languages proposing unstructured spmd-like collective operations. Our study is based on a simple language which provides support for global synchronization barriers. A formal definition of textual alignment, based on an operational semantics, is considered. We prove that this property entails the absence of deadlocks.. Finally, we provide a compositional denotational semantics which is equivalent to the operational semantics for textually aligned programs.

An intermediate representation for optimizing machine learning pipelines

Machine learning (ML) pipelines for model training and validation typically include preprocessing, such as data cleaning and feature engineering, prior to training an ML model. Preprocessing combines relational algebra and user-defined functions (UDFs), while model training uses iterations and linear algebra. Current systems are tailored to either of the two. As a consequence, preprocessing and ML steps are optimized in isolation. To enable holistic optimization of ML training pipelines, we present Lara, a declarative domain-specific language for collections and matrices. Lara's inter-mediate representation (IR) reflects on the complete program, i.e., UDFs, control flow, and both data types. Two views on the IR enable diverse optimizations. Monads enable operator pushdown and fusion across type and loop boundaries. Combinators provide the semantics of domain-specific operators and optimize data access and cross-validation of ML algorithms. Our experiments on preprocessing pipelines and selected ML algorithms show the effects of our proposed optimizations on dense and sparse data, which achieve speedups of up to an order of magnitude.

Towards Dynamically Reconfigurable IoT Camera Virtualization for Video Analytics Edge Cloud Services

Video analytics edge computing exploiting IoT cameras has gained high attention. Running such tasks on the network edge is very challenging since video and image processing are bandwidth-hungry and computationally intensive. IoT cameras are heavily dependent on environmental factors such as the brightness of the view. In this paper, we propose an edge IoT camera virtualization architecture that enables an IoT camera to accommodate multiple application operation semantics and dynamically adjust its configuration to preserve them in the presence of environment context changes. For this, we develop an ontology-based application description model, a virtualization architecture with the container technology, and a context-aware dynamic reconfiguration scheme.

К ВОПРОСУ О ЗВУКЕ В ПОЭЗИИ: ФОНОСЕМАНТИКА ЕГОРА ЛЕТОВА

Статья посвящена звукосимволизму в поэзии Егора Летова. Доказывается, что звуковой принцип подбора слов в поэзии Летова является ее важнейшим организующим началом исследуются типы связей между звуком и значением выявляются функции звука на разных уровнях поэтического текста (синтаксическом, семантическом, образносимволическом) определяются психолингвистические основы звукоизобразительности поэзии Летова In this study we provide an analysis of Yegor Letovs phonopoethics in the semantic aspect. The material of the study is Letovs texts corpus. The main method of the research is phonosemantic analysis, that reveals semantic, emotional and psycholinguistic factors of poetic iconicity. As a result of the study the typology of iconic words in Letovs texts is de ned. It is shown that there are three types of relations between sound and denotatum: 1) onomatopoetic relation, implying the reproduction of acoustic denotatum 2) iconic articulation relation, that recreates internal bodily states through articulation and muscular sense 3) iconic articulation relation, that patterns images of external objects through articulation, resulting in the formation of sound image, which is a response to the sensory impression.The basic scenarios for working with sound are revealed: the zaum scenario that assumes analytical decomposition of a word into sound elements and the distant scenario that assumes working with sound within the syntagma (sentence text). It is shown that the iteration of sound combinations at the level of the text and the corpus is connected with the poetics of key words, which in Letovs texts are related not so much to the traditional semantic parameter as to the sound one. The key word in this case turns out to be the center of a vast phonosemantic network, the soundandsemantic concentrate, and perhaps it predetermines the process of text generation. Correlation of iconic words with repetitive syntactic structures (syntactical automatism) is revealed. The possible psycholinguistic factor of this correlation is de ned and that is the weakening of the logical semantic operator, which leads to the intensi cation of the sound principle of word choice and the appearance of the parataxis. Some typological correspondences between Letovs poetics and modernist poetics that use similar phonosemantic mechanisms are found. The aesthetic function of the sound image is de ned: the modeling of the inner reality image, in which the original image is not transmitted, but is sensed by the body. Achieving such direct sensory impact occurs through the use of articulatorysound mechanisms, which literally embed emotion in the body praxis. In conclusion, it is shown that Letovs iconic speech has the following features: iconicity, dominance of sound, di use semantics, synesthesia, connection with the gestural component, a ectivity, syntactic automatism. Structurally similar speech accompanies the altered states of consciousness, which ts into the avantgarde myth associated with the regressive scenario of a return to the root

A fully abstract semantics for value-passing CCS for trees

We propose a fully abstract semantics for value-passing CCS for trees (VCCTS) with the feature that processes are located at the vertices of a graph whose edges describe possible interaction capabilities. The operational semantics is given both in terms of a reduction semantics and in terms of a labelled transition semantics. We develop a theory of behavioral equivalences by introducing both weak barbed congruence and weak bisimilarity. In particular, we show that, on image-finite processes, weak barbed congruence coincides with weak bisimilarity. To illustrate potential applications and the powerful expressiveness of VCCTS, we formally compare VCCTS with some well-known models, e.g., dynamic pushdown networks, top-down tree automata and value-passing CCS.