Execution Semantics Research Articles

Hierarchical and Concurrent ECCs for IEC 61499 Function Blocks

IEC 61499 enables component-oriented descriptions of complex industrial processes facilitating model-driven engineering. One aspect that is lacking, however, is the ability to directly express Statecharts-like hierarchy and concurrency within basic function blocks (BFBs). Such features can significantly enhance function blocks and help create more succinct and readable specifications. We propose a new syntactic extension to the standard called hierarchical and concurrent execution control chart (HCECC). A major roadblock for any suggested changes to the standard is the need for compliance. Our approach extends the synchronous execution semantics of IEC 61499, where HCECCs are purely syntactic sugar. Using a revised synchronous semantics, our compiler generates standard compliant C code from HCECCs. Benchmarking and usability studies reveal the relative superiority of the proposed approach over existing approaches.

An Algebraic Method for Analysing Control Flow of BPMN Models

This paper introduces an approach for formal verification of BPMN models. The incompatible constructs of the BPMN patterns can lead to wrong or incomplete semantics which resulting the behavioral errors such as deadlock and multiple termination. This research is motivated by the need to create a correct business process and in order to generate a more complete formalization of BPMN semantics than existing formalizations. We first introduce the chosen patterns which are the most used in the modelisation of the service-based business processes. Then, we illustrate a definition of the execution semantics of these patterns by using the rules of Max+ Algebra formulas, which have important benefits.

Executable Semantics for the Formal Specification and Verification of E-agents

Background: Software agents are expected to work autonomously and deal with unfamiliar situations astutely. Achieving cent percent test case coverage for these agents has always been a problem due to limited resources. Also a high degree of dependability is expected from autonomous software agents. Formal verification of these systems can be done by specifying the possible actions of the agents to increase the confidence in the correctness of such systems. Methods: We have used social approach of agent communication where actions of the agents are described as social obligation between the participants. Formal specification of e-agents was done using μ-calculus as it is more expressive than Linear Temporal Logic (LTL). An algorithm has been given for syntactic conversion of μ-calculus formulas to TAPA model checker environment. Results: Using converstion algorithm syntactically modified μ-calculus formulas will be executed on TAPA model checker to verify the system being modelled for the specified properties. Application: The study will help to provide future directions for formal modelling of e-agents for their correct functioning.

ITSAT: An Efficient SAT-Based Temporal Planner

Planning as satisfiability is known as an efficient approach to deal with many types of planning problems. However, this approach has not been competitive with the state-space based methods in temporal planning. This paper describes ITSAT as an efficient SAT-based (satisfiability based) temporal planner capable of temporally expressive planning. The novelty of ITSAT lies in the way it handles temporal constraints of given problems without getting involved in the difficulties of introducing continuous variables into the corresponding satisfiability problems. We also show how, as in SAT-based classical planning, carefully devised preprocessing and encoding schemata can considerably improve the efficiency of SAT-based temporal planning. We present two preprocessing methods for mutex relation extraction and action compression. We also show that the separation of causal and temporal reasoning enables us to employ compact encodings that are based on the concept of parallel execution semantics. Although such encodings have been shown to be quite effective in classical planning, ITSAT is the first temporal planner utilizing this type of encoding. Our empirical results show that not only does ITSAT outperform the state-of-the-art temporally expressive planners, it is also competitive with the fast temporal planners that cannot handle required concurrency.

Improvement of type declaration of the IEC 61499 basic function block for developing applications of cyber-physical system

The cyber-physical system relies on a network of physical devices. The IEC 61499 standard entails a systematic solution to distributed system development. The basic function block (BFB) is the essential construct of the IEC 61499 architecture. However, the execution semantics of the BFB is not well defined by the standard, which leads to a part of the semantic ambiguity. In this paper, we contribute to improve BFB type declaration by proposing a compact interface model and a strict execution control chart (ECC) model. The improved BFB exhibits less semantic ambiguity, is easier to be created, and is more convenient to be applied in a function block network than the standard BFB.

Timing Semantics for Abstraction and Execution of Synthesized High-Level Robot Control

The use of formal methods for synthesis has recently enabled the automated construction of verifiable high-level robot control. Most approaches use a discrete abstraction of the underlying continuous domain, and make assumptions about the physical execution of actions given a discrete implementation; examples include when actions will complete relative to each other, and possible changes in the robot's environment while it is performing various actions. Relaxing these assumptions give rise to a number of challenges during the continuous implementation of automatically synthesized hybrid controllers. This paper presents several distinct timing semantics for controller synthesis, and compares them with respect to the assumptions they make on the execution of actions. It includes a discussion of when each set of assumptions is reasonable, and the computational tradeoffs inherent in relaxing them at synthesis time.

Efficient execution of dynamically controllable simple temporal networks with uncertainty

A simple temporal network with uncertainty (STNU) is a data structure for representing and reasoning about temporal constraints where the durations of certain temporal intervals--the contingent links--are only discovered during execution. The most important property of an STNU is whether it is dynamically controllable (DC)--that is, whether there exists a strategy for executing its time-points that will guarantee that all of its constraints will be satisfied no matter how the durations of the contingent links turn out. The literature on STNUs includes a variety of DC-checking algorithms and execution algorithms. The fastest DC-checking algorithm reported so far is the $$O(N^3)$$O(N3)-time algorithm due to Morris (Integration of AI and OR techniques in constraint programming--11th international conference, CPAIOR 2014, volume 8451 of Lecture Notes in Computer Science. Springer, Berlin, pp 464---479, 2014). The fastest execution algorithm for dynamically controllable STNUs is the $$O(N^3)$$O(N3)-time algorithm due to Hunsberger (Proceedings of the 20th international symposium on temporal representation and reasoning (TIME-2013). IEEE Computer Society, Washington, 2013). This paper begins by providing the first comprehensive, rigorous, and yet streamlined treatment of the theoretical foundations of STNUs, including execution semantics, dynamic controllability, and a set of results that have been collected into what has recently been called the fundamental theorem of STNUs. The paper carefully argues from basic definitions to proofs of the major theorems on which all of the important algorithmic work on STNUs depends. Although many parts of this presentation have appeared in various forms, in various papers, the scattered nature of the STNU literature has allowed too many holes in the theory to persist, and has relied all too often on proof sketches that leave important details unexamined. The presentation combines results from many sources, while also introducing novel approaches and proofs. The paper concludes by presenting a modified version of a recent algorithm for managing the execution of dynamically controllable STNUs, the fastest reported so far in the literature. The modified version organizes its computations more efficiently and corrects an oversight in the original algorithm.

Executable rewriting logic semantics of Orc and formal analysis of Orc programs

The Orc calculus is a simple, yet powerful theory of concurrent computations with great versatility and practical applicability to a very wide range of applications, as it has been amply demonstrated by the Orc language, which extends the Orc calculus with powerful programming constructs that can be desugared into the underlying formal calculus. This means that for: (i) theoretical, (ii) program verification, and (iii) language implementation reasons, the formal semantics of Orc is of great importance. Furthermore, having a semantics of Orc that is executable is essential to provide: (i) a formally-defined interpreter against which language implementations can be validated, and (ii) a (semi-)automatic way of generating a wide range of semantics-based program verification tools, including model checkers and theorem provers.This work proposes a formal executable semantics for Orc in rewriting logic, to support formal verification of Orc programs and to make possible semantics-based correct-by-construction Orc implementations. While being a very simple calculus, Orc has a quite subtle semantics, so that fully capturing all its semantic aspects is highly nontrivial. The two main sources of subtlety are: (i) its real-time semantics, and (ii) the priority of internal computations within an Orc expression over external computations that process responses from external sites. In this paper, we show a simple and elegant way of handling these two sources of subtlety in rewriting logic using an order-sorted type system supporting subtypes and subtype polymorphism, and “tick” rewrite rules for capturing time. Moreover, our rewriting semantics incorporates useful semantic equivalences between Orc programs as equations and equational attributes, making the semantics both more abstract and more efficient. The semantics of Orc is given in two different styles: (i) an SOS style, which is directly based on the original SOS of Orc, whose correctness follows immediately by construction, and (ii) a reduction semantics, which is much more efficiently executable and analyzable, as shown through several experiments, and whose correctness is proved using a strong bisimulation theorem. The paper also presents MOrc, a simulator and model checking tool based on the rewriting semantics of Orc and Real-Time Maude. MOrc facilitates formal verification of Orc programs, and allows for user-defined state predicates and LTL formulas, with no need for any prior knowledge of Maude or its rewriting logic foundations.

A Full R/I-Net Construct Lexicon for Declare Constraints

Recently, declarative process languages are gaining ground as the procedural way of capturing activities in a fixed work-flow is deemed to be inflexible. Declare, one of the prime languages of the declarative process modeling paradigm, is composed of logic-based activity constraints which are event-driven. In this paper, a template-based conversion is proposed of every Declare constraint into a single Petri net fragment with weighted, reset and inhibitor arcs, i.e. a weighted R/I-net. As such, a formalization of the execution semantics of Declare is obtained, similar to linear temporal logic or regular expressions, but now expressed in the form of Petri nets. Equivalence of Declare constraints and the respective Petri net templates are analyzed at the theoretical level and by means of a simulation experiment.

K-Java

This paper presents K-Java, a complete executable formal semantics of Java 1.4. K-Java was extensively tested with a test suite developed alongside the project, following the Test Driven Development methodology. In order to maintain clarity while handling the great size of Java, the semantics was split into two separate definitions -- a static semantics and a dynamic semantics. The output of the static semantics is a preprocessed Java program, which is passed as input to the dynamic semantics for execution. The preprocessed program is a valid Java program, which uses a subset of the features of Java. The semantics is applied to model-check multi-threaded programs. Both the test suite and the static semantics are generic and ready to be used in other Java-related projects.

Compiler/Runtime Framework for Dynamic Dataflow Parallelization of Tiled Programs

Task-parallel languages are increasingly popular. Many of them provide expressive mechanisms for intertask synchronization. For example, OpenMP 4.0 will integrate data-driven execution semantics derived from the StarSs research language. Compared to the more restrictive data-parallel and fork-join concurrency models, the advanced features being introduced into task-parallel models in turn enable improved scalability through load balancing, memory latency hiding, mitigation of the pressure on memory bandwidth, and, as a side effect, reduced power consumption. In this article, we develop a systematic approach to compile loop nests into concurrent, dynamically constructed graphs of dependent tasks. We propose a simple and effective heuristic that selects the most profitable parallelization idiom for every dependence type and communication pattern. This heuristic enables the extraction of interband parallelism (cross-barrier parallelism) in a number of numerical computations that range from linear algebra to structured grids and image processing. The proposed static analysis and code generation alleviates the burden of a full-blown dependence resolver to track the readiness of tasks at runtime. We evaluate our approach and algorithms in the PPCG compiler, targeting OpenStream, a representative dataflow task-parallel language with explicit intertask dependences and a lightweight runtime. Experimental results demonstrate the effectiveness of the approach.

Supporting Information Systems Analysis Through Conceptual Model Query – The Diagramed Model Query Language (DMQL)

Analyzing conceptual models such as process models, data models, or organizational charts is useful for several purposes in information systems engineering (e.g., for business process improvement, compliance management, model driven software development, and software alignment). To analyze conceptual models structurally and semantically, so-called model query languages have been put forth. Model query languages take a model pattern and conceptual models as input and return all subsections of the models that match this pattern. Existing model query languages typically focus on a single modeling language and/or application area (such as analysis of execution semantics of process models), are restricted in their expressive power of representing model structures, and/or abstain from graphical pattern specification. Because these restrictions may hamper query languages from propagating into practice, we close this gap by proposing a modeling language-spanning structural model query language based on flexible graph search that, hence, provides high structural expressive power. To address ease-of-use, it allows one to specify model queries using a diagram. In this paper, we present the syntax and the semantics of the diagramed model query language (DMQL), a corresponding search algorithm, an implementation as a modeling tool prototype, and a performance evaluation.

Formal Specification of the Assurance Point Web Service Composition Model

This paper presents a formal specification of the Assurance Point (AP) web service composition model. The AP model provides a flexible way of checking constraints and responding to execution errors in service composition. An AP is a combined logical and physical checkpoint, providing an execution milestone that stores critical data and interacts with integration rules (IRs) to alter program flow and to invoke different forms of recovery depending on the execution status. In this paper, the execution and recovery semantics of assurance points have been fully defined in the context of if-else, parallel, and loop control structures. The activities and complex control structures of the AP model have been formalized and tested in the Yet Another Workflow Language (YAWL) engine. By doing so, the correctness of the execution and recovery semantics in the AP model has been verified, demonstrating that YAWL nets of the AP model satisfy the soundness property. Different from existing web service composition models, the AP model presented by this research provides multiple levels of protection against service execution failure with a combination of forward and backward recovery techniques.

UML with meaning

To most in the software community, "modeling" is drawing pictures, something much different than "coding". While programming languages must be specified precisely enough to be executable, this has not necessarily been the case for modeling languages. Indeed, the well-known Unified Modeling Language (UML) has for years had a specification that is far from precise. However, precise models are quite common in other engineering disciplines, and there has been a minority even in software engineering that have created models that are precise enough to be, themselves, executable. Over the last decade, the UML community has more generally come to realize the benefits of having at least a subset of UML specified with precise execution semantics. While not all models need to be executable, executable models can support early testing and analysis of the design of critical software systems, as well as supporting simulation for communities using UML for modeling beyond just software (such as SysML for systems engineering). As a result, there has been a great deal of work in recent years toward the standardization of precise, executable semantics for UML models - the "meaning" behind the pictures: Foundational UML (fUML), the Action Language for fUML (Alf) and the recently completed Precise Semantics for UML Composite Structures. These new specifications provide the promise of widespread support for executable modeling across the many UML tools now on the market.

Symbolic execution of multithreaded programs from arbitrary program contexts

We describe an algorithm to perform symbolic execution of a multithreaded program starting from an arbitrary program context. We argue that this can enable more efficient symbolic exploration of deep code paths in multithreaded programs by allowing the symbolic engine to jump directly to program contexts of interest. The key challenge is modeling the initial context with reasonable precision - an overly approximate model leads to exploration of many infeasible paths during symbolic execution, while a very precise model would be so expensive to compute that computing it would defeat the purpose of jumping directly to the initial context in the first place. We propose a context-specific dataflow analysis that approximates the initial context cheaply, but precisely enough to avoid some common causes of infeasible-path explosion. This model is necessarily approximate - it may leave portions of the memory state unconstrained, leaving our symbolic execution unable to answer simple questions such as "which thread holds lock A?". For such cases, we describe a novel algorithm for evaluating symbolic synchronization during symbolic execution. Our symbolic execution semantics are sound and complete up to the limits of the underlying SMT solver. We describe initial experiments on an implementation in Cloud 9.

Specifying and Executing Optimizations for Parallel Programs

Compiler optimizations, usually expressed as rewrites on program graphs, are a core part of all modern compilers. However, even production compilers have bugs, and these bugs are difficult to detect and resolve. The problem only becomes more complex when compiling parallel programs; from the choice of graph representation to the possibility of race conditions, optimization designers have a range of factors to consider that do not appear when dealing with single-threaded programs. In this paper we present PTRANS, a domain-specific language for formal specification of compiler transformations, and describe its executable semantics. The fundamental approach of PTRANS is to describe program transformations as rewrites on control flow graphs with temporal logic side conditions. The syntax of PTRANS allows cleaner, more comprehensible specification of program optimizations; its executable semantics allows these specifications to act as prototypes for the optimizations themselves, so that candidate optimizations can be tested and refined before going on to include them in a compiler. We demonstrate the use of PTRANS to state, test, and refine the specification of a redundant store elimination optimization on parallel programs.

Verification of Change in a Fragmented Event-Based Process Coordination Environment

To allow the distribution of control and visibility of cross-organizational process models and to increase availability and performance of the processes, a process model can be fragmented into logically different parts and distributed in the enterprise architecture. Fragmentation algorithms and execution environments which connect the fragmented process model parts together, recreating the original process execution semantics, have been proposed in earlier works. However, a critical challenge that is left open is the ability to independently change the control structure of a process fragment. This is not trivial as changing the control structure of a specific fragment could break the global process execution. The global process overview is also not available anymore because it is fragmented in the enactment environment and many independent control structure changes could have already been done. In this paper we describe an approach based on state reconstruction which enables to check the admissibility of changes made to the control structure of a process fragment in a fragmented event-based process enactment environment.

Using Timed Game Automata to Synthesize Execution Strategies for Simple Temporal Networks with Uncertainty

A Simple Temporal Network with Uncertainty (STNU) is a structure for representing and reasoning about temporal constraints in domains where some temporal durations are not controlled by the executor. The most important property of an STNU is whether it is dynamically controllable (DC) whether there exists a strategy for executing the controllable time-points that guarantees that all constraints will be satisfied no matter how the uncontrollable durations turn out. This paper provides a novel mapping from STNUs to Timed Game Automata (TGAs) that: (1) explicates the deep theoretical relationships between STNUs and TGAs; and (2) enables the memoryless strategies generated from the TGA to be transformed into equivalent STNU execution strategies that reduce the real-time computational burden for the executor. The paper formally proves that the STNU-to-TGA encoding properly captures the execution semantics of STNUs.

Pathway Specification and Comparative Queries: A High Level Language with Petri Net Semantics

Understanding biological pathways is an important activity in the biological domain for drug development. Due to the parallelism and complexity inherent in pathways, computer models that can answer queries about pathways are needed. A researcher may ask `what-if' questions comparing alternate scenarios, that require deeper understanding of the underlying model. In this paper, we present overview of such a system we developed and an English-like high level language to express pathways and queries. Our language is inspired by high level action and query languages and it uses Petri Net execution semantics.

CASM

In this paper we present CASM, a language based on Abstract State Machines (ASM), and its optimizing compiler. ASM is a well-defined (formal) method based on algebraic concepts. A distinct feature of ASM is its combination of parallel and sequential execution semantics. This makes it an excellent choice to formally specify and verify micro-architectures. We present a compilation scheme and an implementation of a runtime system supporting efficient execution of ASM. After introducing novel analysis techniques we present optimizations allowing us to eliminate many costly operations. Benchmark results show that our baseline compiler is 2-3 magnitudes faster than other ASM implementations. The optimizations further increase the performance of the compiled programs up to 264%. The achieved performance allows our ASM implementation to be used with industry-size applications.