Shahar Maoz

Shahar Maoz, Rafi Shalom

One of the main challenges of reactive synthesis, an automated procedure to obtain a correct-by-construction reactive system, is to deal with unrealizable specifications. One means to deal with unrealizability, in the context of GR(1), an expressive assume-guarantee fragment of LTL that enables efficient synthesis, is the computation of an unrealizable core, which can be viewed as a fault-localization approach. Existing solutions, however, are computationally costly, are limited to computing a single core, and do not correctly support specifications with constructs beyond pure GR(1) elements. In this work we address these limitations. First, we present QuickCore, a novel algorithm that accelerates unrealizable core computations by relying on the monotonicity of unrealizability, on an incremental computation, and on additional properties of GR(1) specifications. Second, we present Punch, a novel algorithm to efficiently compute all unrealizable cores of a specification. Finally, we present means to correctly handle specifications that include higher-level constructs beyond pure GR(1) elements. We implemented our ideas on top of Spectra, an open-source language and synthesis environment. Our evaluation over benchmarks from the literature shows that QuickCore is in most cases faster than previous algorithms, and that its relative advantage grows with scale. Moreover, we found that most specifications include more than one core, and that Punch finds all the cores significantly faster than a competing naive algorithm.

Shahar Maoz, Jan Oliver Ringert

Spectra is a new specification language for reactive systems, specifically tailored for the context of reactive synthesis. The meaning of Spectra is defined by a translation to a kernel language. Spectra comes with the Spectra Tools, a set of analyses, including a synthesizer to obtain a correct-by-construction implementation, several means for executing the resulting controller, and additional analyses aimed at helping engineers write higher-quality specifications. We present the language and give an overview of the tool set.

Shahar Maoz, Jan Oliver Ringert

Reactive synthesis is an automated procedure to obtain a correct-by-construction reactive system from a given specification. GR(1) is a well-known fragment of linear temporal logic (LTL) where synthesis is possible using a polynomial symbolic algorithm. We conducted a case study to learn about the challenges that software engineers may face when using GR(1) synthesis for the development of a reactive robotic system. In the case study we developed two variants of a forklift controller, deployed on a Lego robot. The case study employs LTL specification patterns as an extension of the GR(1) specification language, an examination of two specification variants for execution scheduling, traceability from the synthesized controller to constraints in the specification, and generated counter strategies to support understanding reasons for unrealizability. We present the specifications we developed, our observations, and challenges faced during the case study.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

Models are heavily used in software engineering and together with their systems they evolve over time. Thus, managing their changes is an important challenge for system maintainability. Existing approaches to model differencing concentrate on heuristics matching between model elements and on finding and presenting differences at a concrete or abstract syntactic level. While showing some success, these approaches are inherently limited to comparing syntactic structures. This paper is a manifesto for research on semantic model differencing. We present our vision to develop semantic diff operators for model comparisons: operators whose input consists of two models and whose output is a set of diff witnesses, instances of one model that are not instances of the other. In particular, if the models are syntactically different but there are no diff witnesses, the models are semantically equivalent. We demonstrate our vision using two concrete diff operators, for class diagrams and for activity diagrams. We motivate the use of semantic diff operators, brie y discuss the algorithms to compute them, list related challenges, and show their application and potential use as new fundamental building blocks for change management in model-driven engineering.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

This document defines an operational semantics for activity diagrams (ADs) using a translation to SMV. The translation is inspired by the work of Eshuis [Esh06] and extends it with support for data. Each execution step of the SMV module obtained from an AD represents an executed action of this AD with interleaved execution of concurrent branches. An implementation of the given translation was used in the context of semantic differencing for ADs [MRR11]. We define the translation and give two examples, showing ADs and their complete representation in SMV.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

Class diagrams (CDs), which specify classes and the relationships between them, are widely used for modeling the structure of object-oriented systems. As models, programs, and systems evolve over time, during the development lifecycle and beyond it, effective change management is a major challenge in software development, which has attracted much research efforts in recent years. In this paper we present cddiff, a semantic diff operator for CDs. Unlike most existing approaches to model comparison, which compare the concrete or the abstract syntax of two given diagrams and output a list of syntactical changes or edit operations, cddiff considers the semantics of the diagrams at hand and outputs a set of diff witnesses, each of which is an object model that is possible in the first CD and is not possible in the second. We motivate the use of cddiff, formally define it, and show how it is computed. The computation is based on a reduction to Alloy. The work is implemented in a prototype Eclipse plug-in. Examples show the unique contribution of our approach to the state-of-the-art in version comparison and evolution analysis.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

While object diagrams (ODs) are widely used as a means to document object-oriented systems, they are expressively weak, as they are limited to describe specific possible snapshots of the system at hand. In this paper we introduce modal object diagrams (MODs), which extend the classical OD language with positive/negative and example/invariant modalities. The extended language allows the designer to specify not only positive example models but also negative examples, ones that the system should not allow, positive invariants, ones that all system's snapshots should include, and negative invariants, ones that no system snapshot is allowed to include. Moreover, as a primary application of the extended language we provide a formal verification technique that decides whether a given class diagram satisfies (i.e., models) a multi-modal object diagrams specification. In case of a negative answer, the technique outputs relevant counterexample object models, as applicable. The verification is based on a reduction to Alloy. The ideas are implemented in a prototype Eclipse plug-in. Examples show the usefulness of the extended language in specifying structural requirements of object-oriented systems in an intuitive yet expressive way.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

Activity diagrams (ADs) have recently become widely used in the modeling of workflows, business processes, and web-services, where they serve various purposes, from documentation, requirement definitions, and test case specifications, to simulation and code generation. As models, programs, and systems evolve over time, understanding changes and their impact is an important challenge, which has attracted much research efforts in recent years. In this paper we present addiff, a semantic differencing operator for ADs. Unlike most existing approaches to model comparison, which compare the concrete or the abstract syntax of two given diagrams and output a list of syntactical changes or edit operations, addiff considers the Semantics of the diagrams at hand and outputs a set of diff witnesses, each of which is an execution trace that is possible in the first AD and is not possible in the second. We motivate the use of addiff, formally define it, and show two algorithms to compute it, a concrete forward-search algorithm and a symbolic xpoint algorithm, implemented using BDDs and integrated into the Eclipse IDE. Empirical results and examples demonstrate the feasibility and unique contribution of addiff to the state-of-the-art in version comparison and evolution analysis.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

We present CD2Alloy, a novel, powerful translation of UML class diagrams (CDs) to Alloy. Unlike existing translations, which are based on a shallow embedding strategy, and are thus limited to checking consistency and generating conforming object models of a single CD, and support a limited set of CD language features, CD2Alloy uses a deeper embedding strategy. Rather than mapping each CD construct to a semantically equivalent Alloy construct, CD2Alloy defines (some) CD constructs as new concepts within Alloy. This enables solving several analysis problems that involve more than one CD and could not be solved by earlier works, and supporting an extended list of CD language features. The ideas are implemented in a prototype Eclipse plug-in. The work advances the state-of-the-art in CD analysis, and can also be viewed as an interesting case study for the different possible translations of one modeling language to another, their strengths and weaknesses.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

Checking consistency between an object diagram (OD) and a class diagram (CD) is an important analysis problem. However, several variations in the semantics of CDs and ODs, as used in different contexts and for different purposes, create a challenge for analysis tools. To address this challenge in this paper we investigate semantically configurable model analysis. We formalize the variability in the languages semantics using a feature model: each configuration that the model permits induces a different semantics. Moreover, we develop a parametrized analysis that can be instantiated to comply with every legal configuration of the feature model. Thus, the analysis is semantically congured and its results change according to the semantics induced by the selected feature configuration. The ideas are implemented using a parametrized transformation to Alloy. The work can be viewed as a case study example for a formal and automated approach to handling semantic variability in modeling languages.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

Fundamental building blocks for managing and understanding software evolution in the context of model-driven engineering are differencing operators one can use for model comparisons. Semantic model differencing deals with the definition and computation of semantic diff operators for model comparison, operators whose input consists of two models and whose output is a set of diff witnesses, instances of one model that are not instances of the other. However, in many cases the complete set of diff witnesses is too large to be efficiently computed and effectively presented. Moreover, many of the witnesses are very similar and hence not interesting. Thus, an important challenge of semantic differencing relates to witness selection and presentation. In this paper we propose to address this challenge using a summarization technique, based on a notion of equivalence that partitions the set of diff witnesses. The result of the computation is a summary set, consisting of a single representative witness from each equivalence class. We demonstrate our ideas using two concrete diff operators, for class diagrams and for activity diagrams, where the computation of the summary set is efficient and does not require the enumeration of all witnesses.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

This position paper provides an interim summary on the goals and current state of our ongoing research project on semantic model differencing for software evolution. We describe the basics of semantic model differencing, give two examples from our recent work, and discuss future challenges in taking full advantage of the potential of semantic differencing techniques in the context of models' evolution.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

We present component and connector (C&C) views, which specify structural properties of component and connector models in an expressive and intuitive way. C&C views provide means to abstract away direct hierarchy, direct connectivity, port names and types, and thus can crosscut the traditional boundaries of the implementation-oriented hierarchical decomposition of systems and sub-systems, and reflect the partial knowledge available to different stakeholders involved in a system's design. As a primary application for C&C views we investigate the synthesis problem: given a C&C views specification, consisting of mandatory, alternative, and negative views, construct a concrete satisfying C&C model, if one exists. We show that the problem is NP-hard and solve it, in a bounded scope, using a reduction to SAT, via Alloy. We further extend the basic problem with support for library components, specification patterns, and architectural styles. The result of synthesis can be used for further exploration, simulation, and refinement of the C&C model or, as the complete, final model itself, for direct code generation. A prototype tool and an evaluation over four example systems with multiple specifications show promising results and suggest interesting future research directions towards a comprehensive development environment for the structure of component and connector designs.

Shahar Maoz, Jan Oliver Ringert, Bernhard Rumpe

The structure of component and connector (C&C) models, which are used in many application domains of software engineering, consists of components at different containment levels, their typed input and output ports, and the connectors between them. C&C views, presented in [24], can be used to specify structural properties of C&C models in an expressive and intuitive way. In this work we address the verification of a C&C model against a C&C view and present efficient (polynomial) algorithms to decide satisfaction. A unique feature of our work, not present in existing approaches to checking structural properties of C&C models, is the generation of witnesses for satisfaction/non-satisfaction and of short naturallanguage texts, which serve to explain and formally justify the verification results and point the engineer to its causes. A prototype tool and an evaluation over four example systems with multiple views, performance and scalability experiments, as well as a user study of the usefulness of the witnesses for engineers, demonstrate the contribution of our work to the state-of-the-art in component and connector modeling and analysis.