Execution Semantics Research Articles

Provenance-Based Reproducibility in the Semantic Web

Reproducibility is a crucial property of data since it allows users to understand and verify how data was derived, and therefore allows them to put their trust in such data. Reproducibility is essential for science, because the reproducibility of experimental results is a tenet of the scientific method, but reproducibility is also beneficial in many other fields, including automated decision making, visualization, and automated data feeds. To achieve the vision of reproducibility, the workflow-based community has strongly advocated the use of provenance as an underpinning mechanism for reproducibility, since a rich representation of provenance allows steps to be reproduced and all intermediary and final results checked and validated. Concurrently, multiple ontology-based representations of provenance have been devised, to be able to describe past computations, uniformly across a variety of technologies. However, such Semantic Web representations of provenance do not have any formal link with execution. Even assuming a faithful and non-malicious environment, how can we claim that an ontology-based representation of provenance enables reproducibility, since it has not been given any execution semantics, and therefore has no formal way of expressing the reproduction of computations? This is the problem that this paper tackles by defining a denotational semantics for the Open Provenance Model, which is referred to as the reproducibility semantics. This semantics is used to implement a reproducibility service, leveraging multiple Semantic Web technologies, and offering a variety of reproducibility approaches, found in the literature. A series of empirical experiments were designed to exhibit the range of reproducibility capabilities of our approach; in particular, we demonstrate the ability to reproduce computations involving multiple technologies, as is commonly found on the Web.

SECRET

There are many academic and commercial stream processing engines (SPEs) today, each of them with its own execution semantics. This variation may lead to seemingly inexplicable differences in query results. In this paper, we present SECRET, a model of the behavior of SPEs. SECRET is a descriptive model that allows users to analyze the behavior of systems and understand the results of window-based queries for a broad range of heterogeneous SPEs. The model is the result of extensive analysis and experimentation with several commercial and academic engines. In the paper, we describe the types of heterogeneity found in existing engines, and show with experiments on real systems that our model can explain the key differences in windowing behavior.

Peer coordination through distributed triggers

This is a demonstration of data coordination in a peer data management system through the employment of distributed triggers. The latter express in a declarative manner individual security and consistency requirements of peers, that cannot be ensured by default in the P2P environment. Peers achieve to handle in a transparent way data changes that come from local and remote actions and events. The distributed triggers are implemented as an extension of the active functionality of a centralized commercial DBMS. The language and execution semantics of distributed triggers are integrated in the kernel of the DBMS such that the latter handles transparently and simultaneously both centralized and distributed triggers. Moreover, the management of distributed triggers is associated with a set of peer acquaintance and termination protocols which are incorporated in the centralized DBMS.

FORMALIZATION OF TEXTUAL USE CASES BASED ON PETRI NETS

A use case is a specification of interactions involving a system and external actors of that system. The intuitive, user centered nature of textual use cases is one of the reasons for the success of the use case approach. A certain level of formalization is however needed to automate use case based system development, including tasks such as design synthesis, verification and validation. In this paper, a mapping from textual use cases to a formal model (Petri nets) is proposed. Use cases are described in a restricted-form of natural language. The abstract syntax of the language is formally defined using a tuple structure. The mapping from use cases to Petri nets considers use cases sequencing constraints defined at the syntactic-level, and provides a definition of execution semantics to use cases.

Interaction-centric modeling of process choreographies

With the rise of electronic integration between organizations, the need for a precise specification of interaction behavior increases. Information systems, replacing interaction previously carried out by humans via phone, faxes and emails, require a precise specification for handling all possible situations. Such interaction behavior is described in process choreographies. While many proposals for choreography languages have already been made, most of them fall into the category of interconnection models, where the observable behavior of the different partners is described and then related via message flow. As this article will show, this modeling approach fails to support fundamental design principles of choreographies and typically leads to modeling errors. This motivates an alternative modeling style, namely interaction modeling, for overcoming these limitations. While the main concepts are independent of a concrete modeling language, iBPMN is introduced as novel interaction modeling language. Formal execution semantics are provided and a comprehensive toolset implementing the approach is presented.

On Formal Analysis of IEC 61499 Applications, Part B: Execution Semantics

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> IEC 61499 is a standard architecture, based on function blocks, for developing distributed control and measurement applications. However, the standard has no formal semantics and different interpretations of the standard have emerged. As a consequence, the execution behavior of applications running on different platforms may exhibit different behavior, thus making it harder to transfer applications between the platforms. This paper shows how three different execution semantics, buffered sequential execution model (BSEM), nonpreempted multithreaded (NPMTR), and cyclic buffered execution model (CBEM) can be mathematically defined. The mathematical definitions can be used to analyze an application's behavior when executed using those execution semantics. The mathematical definitions have been used as a basis for implementation of a runtime environment and a software tool that generates formal models suitable for formal verification. Formal verification can be used to help discover execution errors before the application is executed on the factory floor. </para>

On Formal Analysis of IEC 61499 Applications, Part A: Modeling

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> IEC 61499 is a standard architecture, based on function blocks, for developing distributed control and measurement applications. However, the standard has no formal semantics and different interpretations of the standard have emerged. As a consequence, it is harder to transfer applications between different standard compliant platforms. This paper presents a formal framework for mathematical modeling and comparison of different execution semantics. The framework provides definitions that allow modeling of applications and execution semantics separately. Together, the models can be used to analyze and compare how an application would behave when executed using different execution semantics. In addition, a mathematical model made possible by the framework has been used as a basis for implementation of a runtime environment that can execute applications and a software tool that generates formal models suitable for formal verification, both assuming different execution semantics. </para>

An executable object-oriented semantics and its application to firewall verification

This paper presents a formal executable semantics of object-oriented models. We made it possible to conduct both simulation and theorem proving on the semantics by implementing it within the expressive intersection of the functional programming language ML and the theorem prover HOL. In this paper, we present the definition and implementation of the semantics. We also present a prototype verification tool ObjectLogic which supports simulation and theorem proving on the semantics. As a case study, we show the verification of a practical firewall system.

An algebraic semantics for MOF

AbstractIn model-driven development, software artifacts are represented as models in order to improve productivity, quality, and cost effectiveness. In this area, the meta-object facility (MOF) standard plays a crucial role as a generic framework within which a wide range of modeling languages can be defined. The MOF standard aims at offering a good basis for model-driven development, providing some of the building concepts that are needed: what is a model, what is a metamodel, what is reflection in the MOF framework, and so on. However, most of these concepts are not yet fully formally defined in the current MOF standard. In this paper we define a reflective, algebraic, executable framework for precise metamodeling based on membership equational logic (mel) that supports the MOF standard. Our framework provides a formal semantics of the following notions:metamodel,model, andconformanceof a model to its metamodel. Furthermore, by using the Maude language, which directly supportsmelspecifications, this formal semantics isexecutable. This executable semantics has been integrated within the Eclipse modeling framework as a plugin tool called MOMENT2. In this way, formal analyses, such as semantic consistency checks, model checking of invariants and LTL model checking, become available within Eclipse to provide formal support for model-driven development processes.

Beyond soundness: on the verification of semantic business process models

The verification of control-flow soundness is well understood as an important step before deploying business process models. However, the control flow does not capture what the process activities actually do when they are executed. Semantic annotations offer the opportunity to take this into account. Inspired by semantic Web service approaches such as OWL-S and WSMO, we consider process models in which the individual activities are annotated with logical preconditions and effects, specified relative to an ontology that axiomatizes the underlying business domain. Verification then addresses the overall process behavior, arising from the interaction between control-flow and behavior of individual activities. To this end, we combine notions from the workflow community with notions from the AI actions and change literature. We introduce a formal execution semantics for annotated business processes. We point out four verification tasks that arise, concerning precondition/effect conflicts, reachability, and executability. We examine the borderline between classes of processes that can, or cannot, be verified in polynomial time. For precondition/effect conflicts, we show that the borderline is the same as that of the logic underlying the ontology axioms. For reachability and executability, we identify a class of processes that can be verified in polynomial time by a fixpoint algorithm which we design for that purpose. We show that this class of processes is maximal in the sense that, when generalizing it in any of the most relevant directions, the validation tasks become computationally hard.

A domain-specific language for defining static structure of database applications

In this paper we present DOMMLite - an extensible domain specific language (DSL) for static structure definition of data base oriented applications. The model-driven engineering (MDE) approach, an emerging software development paradigm, has been used. The language structure is defined by the means of a meta model supplemented by validation rules based on Check language and extensions based on Extend language, which are parts of the openArchitectureWare framework [1]. The meta model has been defined along with the textual syntax, which enables creation, update and persistence of DOMMLite models using a common text editor. DSL execution semantics has been defined by the specification and implementation of the source code generator for a target platform with an already defined execution semantics. In order to enable model editing, a textual Eclipse editor has also been developed. DSL, defined in this way, has the capability of generating complete source code for GUI forms with CRUDS (Create-Read-Update-Delete-Search) and navigation operations [2,3,4,5].

Model-Based Development: A New Approach to Engineering Medical Software

With an increasing number of medical device features being implemented in code, the amount of software that is present in a modern device as well as its complexity and criticality has grown sharply over the years. Existing quality-control regimes for software, dependent as they are on traditional inspection and ad-hoc testing techniques, has been unable to meet many of these challenges. The numbers tell the story. In 1998, close to 8% of device failures could be traced to software errors. Currently, the number of device recalls due to software problems is believed by some to be about 18%. Model-based development (MBD) is often suggested as a candidate solution, a novel way of doing software development and quality control. So what is model-based development and what it does it mean for professioals working in the medical instrumentation field? Read on.

Concurrent Collections

We introduce the Concurrent Collections (CnC) programming model. CnC supports flexible combinations of task and data parallelism while retaining determinism. CnC is implicitly parallel, with the user providing high-level operations along with semantic ordering constraints that together form a CnC graph. We formally describe the execution semantics of CnC and prove that the model guarantees deterministic computation. We evaluate the performance of CnC implementations on several applications and show that CnC offers performance and scalability equivalent to or better than that offered by lower-level parallel programming models.

Formal executable semantics for conformance in the MDE framework

In the MDE framework, a metamodel is a language referring to some kind of metadata whose elements formalize concepts and relations providing a modeling language. An instance of this modeling language which adheres to its concepts and relations is called a valid model, i.e., a model satisfying structural conformance to its metamodel. However, a metamodel frequently imposes additional constraints to its valid instances. These conditions are usually written in OCL and are called well-formedness rules. In presence of these constraints, a valid model must adhere to the concepts and relations of its metamodel and fullfill its constraints, i.e., a valid model is a model satisfying semantical conformance to its metamodel. In this work, we provide a formal semantics to the notions of structural and semantical conformance between models and metamodels building on our previous work. Our definitions can be automatically checked using the ITP/OCL tool.

Pervasive Load-Time Transformation for Transparently Distributed Java

The transformation of large, off-the-shelf Java applications to support complex new functionality essentially requires generation of an entirely new application that retains the execution semantics of the original. We describe such a whole-program modification in the context of RuggedJ, a dynamic transparent Java distribution system.We discuss the proxy-based object model that allows remote Java objects to be referenced in the same way as those residing on the current virtual machine, the optimizations that allow us to bypass proxies in the case of purely local or remote object, and the mechanisms needed to guarantee that static data remain unique in a distributed system. We then detail some of the more interesting features involved when implementing this object model in rewritten bytecode, including transformations required within method bodies and coordination between bytecode and the run-time system that distributes an application across the network.

SAT-Based Parallel Planning Using a Split Representation of Actions

Planning based on propositional SAT(isfiability) is a powerful approach to computing step-optimal plans given a parallel execution semantics. In this setting: (i) a solution plan must be minimal in the number of plan steps required, and (ii) non-conflicting actions can be executed instantaneously in parallel at a plan step. Underlying SAT-based approaches is the invocation of a decision procedure on a SAT encoding of a bounded version of the problem. A fundamental limitation of existing approaches is the size of these encodings. This problem stems from the use of a direct representation of actions — i.e. each action has a corresponding variable in the encoding. A longtime goal in planning has been to mitigate this limitation by developing a more compact split — also termed lifted — representation of actions in SAT encodings of parallel step-optimal problems. This paper describes such a representation. In particular, each action and each parallel execution of actions is represented uniquely as a conjunct of variables. Here, each variable is derived from action pre and post-conditions. Because multiple actions share conditions, our encoding of the planning constraints is factored and relatively compact. We find experimentally that our encoding yields a much more efficient and scalable planning procedure over the state-of-the-art in a large set of planning benchmarks.

Module superimposition: a composition technique for rule-based model transformation languages

As the application of model transformation becomes increasingly commonplace, the focus is shifting from model transformation languages to the model transformations themselves. The properties of model transformations, such as scalability, maintainability and reusability, have become important. Composition of model transformations allows for the creation of smaller, maintainable and reusable transformation definitions that together perform a larger transformation. This paper focuses on composition for two rule-based model transformation languages: the ATLAS Transformation Language (ATL) and the QVT Relations language. We propose a composition technique called module superimposition that allows for extending and overriding rules in transformation modules. We provide executable semantics as well as a concise and scalable implementation of module superimposition based on ATL.

Interacting process classes

Many reactive control systems consist of classes of active objects involving both intraclass interactions (i.e., objects belonging to the same class interacting with each other) and interclass interactions. Such reactive control systems appear in domains such as telecommunication, transportation and avionics. In this article, we propose a modeling and simulation technique for interacting process classes. Our modeling style uses standard notations to capture behavior. In particular, the control flow of a process class is captured by a labeled transition system, unit interactions between process objects are described as transactions , and the structural relations are captured via class diagrams. The key feature of our approach is that our execution semantics leads to an abstract simulation technique which involves (i) grouping together active objects into equivalence classes according their potential futures, and (ii) keeping track of the number of objects in an equivalence class rather than their identities. Our simulation strategy is both time and memory efficient and we demonstrate this on well-studied nontrivial examples of reactive systems. We also present a case study involving a weather-update controller from NASA to demonstrate the use of our simulator for debugging realistic designs.

On compliance checking for clausal constraints in annotated process models

Compliance management is important in several industry sectors where there is a high incidence of regulatory control. It must be ensured that business practices, as reflected in business processes, comply with the rules. Such compliance checks are challenging due to (1) the different life cycles of rules and processes, and (2) their disparate representations. (1) requires retrospective checking of process models. To address (2), we herein devise a framework where processes are annotated to capture the semantics of task execution, and compliance is checked against a set of constraints posing restrictions on the desirable process states. Each constraint is a clause, i.e., a disjunction of literals. If a process can reach a state that falsifies all literals of one of the constraints, then that constraint is violated in that state, and indicates non-compliance. Naively, such compliance can be checked by enumerating all reachable states. Since long waiting times are undesirable, it is important to develop efficient (low-order polynomial time) algorithms that (a) perform exact compliance checking for restricted cases, or (b) perform approximate compliance checking for more general cases. Herein, we observe that methods of both kinds can be defined as a natural extension of our earlier work on semantic business process validation. We devise one method of type (a), and we devise two methods of type (b); both are based on similar restrictions to the processes, where the restrictions made by methods (b) are a subset of those made by method (a). The approximate methods each guarantee either of soundness (finding only non-compliances) or completeness (finding all non-compliances). We describe how one can trace the state evolution back to the process activities which caused the (potential) non-compliance, and hence provide the user with an error diagnosis.

A semantic framework for metamodel-based languages

In the model-based development context, metamodel-based languages are increasingly being defined and adopted either for general purposes or for specific domains of interest. However, meta-languages such as the MOF (Meta Object Facility)--combined with the OCL (Object Constraint Language) for expressing constraints--used to specify metamodels focus on structural and static semantics but have no built-in support for specifying behavioral semantics. This paper introduces a formal semantic framework for the definition of the semantics of metamodel-based languages. Using metamodelling principles, we propose several techniques, some based on the translational approach while others based on the weaving approach, all showing how the Abstract State Machine formal method can be integrated with current metamodel engineering environments to endow language metamodels with precise and executable semantics. We exemplify the use of our semantic framework by applying the proposed techniques to the OMG metamodelling framework for the behaviour specification of the Finite State Machines provided in terms of a metamodel.