Patrick Heymans

Feature Diagrams: A Survey and a Formal Semantics

Feature diagrams (FD) are a family of popular modelling languages used for engineering requirements in software product lines. FD were first introduced by Kang as part of the FODA (feature oriented domain analysis) method back in 1990, Since then, various extensions of FODA FD were devised to compensate for a purported ambiguity and lack of precision and expressiveness. However, they never received a proper formal semantics, which is the hallmark of precision and unambiguity as well as a prerequisite for efficient and safe tool automation, In this paper, we first survey FD variants. Subsequently, we generalize the various syntaxes through a generic construction called free feature diagrams (FFD). Formal semantics is defined at the FFD level, which provides unambiguous definition for ail the surveyed FD variants in one shot. All formalisation choices found a clear answer in the original FODA FD definition, which proved that although informal and scattered throughout many pages, it suffered no ambiguity problem. Our definition has several additional advantages: it is formal, concise and generic. We thus argue that it contributes to improve the definition, understanding, comparison and reliable implementation of FD languages

Generic semantics of feature diagrams

Feature Diagrams (FDs) are a family of popular modelling languages used to address the feature interaction problem, particularly in software product lines, FDs were first introduced by Kang as part of the FODA (Feature-Oriented Domain Analysis) method back in 1990. Afterwards, various extensions of FODA FDs were introduced to compensate for a purported ambiguity and lack of precision and expressiveness. However, they never received a formal semantics, which is the hallmark of precision and unambiguity and a prerequisite for efficient and safe tool automation. The reported work is intended to contribute a more rigorous approach to the definition, understanding, evaluation, selection and implementation of FD languages. First, we provide a survey of FD variants. Then, we give them a formal semantics, thanks to a generic construction that we call Free Feature Diagrams (FFDs). This demonstrates that FDs can be precise and unambiguous. This also defines their expressiveness. Many variants are expressively complete, and thus the endless quest for extensions actually cannot be justified by expressiveness. A finer notion is thus needed to compare these expressively complete languages. Two solutions are well-established: succinctness and embeddability, that express the naturalness of a language. We show that the expressively complete FDs fall into two succinctness classes, of which we of course recommend the most succinct. Among the succinct expressively complete languages, we suggest a new, simple one that is not harmfully redundant: Varied FD (VFD). Finally, we study the execution time that tools will need to solve useful problems in these languages.

A proposal for a scenario classification framework

The requirements engineering, information systems and software engineering communities recently advocated scenario-based approaches which emphasise the user/system interaction perspective in developing computer systems. Use of examples, scenes, narrative descriptions of contexts, mock-ups and prototypes-all these ideas can be called scenario-based approaches, although exact definitions are not easy beyond stating that these approaches emphasise some description of the real world. Experience seems to tell us that people react to ‘real things’ and that this helps in clarifying requirements. Indeed, the widespread acceptance of prototyping in system development points to the effectiveness of scenario-based approaches. However, we have little understanding about how scenarios should be constructed, little hard evidence about their effectiveness and even less idea about why they work.The paper is an attempt to explore some of the issues underlying scenario-based approaches in requirements engineering and to propose a framework for their classification. The framework is a four-dimensional framework which advocates that a scenario-based approach can be well defined by itsform, content, purpose andlife cycle. Every dimension is itself multifaceted and a metric is associated with each facet. Motivations for developing the framework are threefold: (a) to help in understanding and clarifying existing scenario-based approaches; (b) to situate the industrial practice of scenarios; and (c) to assist researchers develop more innovative scenario-based approaches.

Model checking lots of systems: efficient verification of temporal properties in software product lines

In product line engineering, systems are developed in families and differences between family members are expressed in terms of features. Formal modelling and verification is an important issue in this context as more and more critical systems are developed this way. Since the number of systems in a family can be exponential in the number of features, two major challenges are the scalable modelling and the efficient verification of system behaviour. Currently, the few attempts to address them fail to recognise the importance of features as a unit of difference, or do not offer means for automated verification. In this paper, we tackle those challenges at a fundamental level. We first extend transition systems with features in order to describe the combined behaviour of an entire system family. We then define and implement a model checking technique that allows to verify such transition systems against temporal properties. An empirical evaluation shows substantial gains over classical approaches.

Disambiguating the Documentation of Variability in Software Product Lines: A Separation of Concerns, Formalization and Automated Analysis

Feature diagrams are a popular means for documenting variability in software product line engineering. When examining feature diagrams in the literature and from industry, we observed that the same modelling concepts are used for documenting two different kinds of variability: (1) product line variability, which reflects decisions of product management on how the systems that belong to the product line should vary, and (2) software variability, which reflects the ability of the reusable product line artefacts to be customized or configured. To disambiguate the documentation of variability, we follow previous suggestions to relate orthogonal variability models (OVMs) to feature diagrams. This paper reuses an existing formalization of feature diagrams, but introduces a formalization of OVMs. Then, the relationships between the two kinds of models are formalized as well. Besides a precise definition of the languages and the links, the important benefit of this formalization is that it serves as a foundation for a tool supporting automated reasoning on variability. This tool can, e.g., analyse whether the product line artefacts are flexible enough to build all the systems that should belong to the product line.

Symbolic model checking of software product lines

We study the problem of model checking software product line (SPL) behaviours against temporal properties. This is more difficult than for single systems because an SPL with n features yields up to 2n individual systems to verify. As each individual verification suffers from state explosion, it is crucial to propose efficient formalisms and heuristics. We recently proposed featured transition systems (FTS), a compact representation for SPL behaviour, and defined algorithms for model checking FTS against linear temporal properties. Although they showed to outperform individual system verifications, they still face a state explosion problem as they enumerate and visit system states one by one. In this paper, we tackle this latter problem by using symbolic representations of the state space. This lead us to consider computation tree logic (CTL) which is supported by the industry-strength symbolic model checker NuSMV. We first lay the foundations for symbolic SPL model checking by defining a feature-oriented version of CTL and its dedicated algorithms. We then describe an implementation that adapts the NuSMV language and tool infrastructure. Finally, we propose theoretical and empirical evaluations of our results. The benchmarks show that for certain properties, our algorithm is over a hundred times faster than model checking each system with the standard algorithm.

Featured Transition Systems: Foundations for Verifying Variability-Intensive Systems and Their Application to LTL Model Checking

The premise of variability-intensive systems, specifically in software product line engineering, is the ability to produce a large family of different systems efficiently. Many such systems are critical. Thorough quality assurance techniques are thus required. Unfortunately, most quality assurance techniques were not designed with variability in mind. They work for single systems, and are too costly to apply to the whole system family. In this paper, we propose an efficient automata-based approach to linear time logic (LTL) model checking of variability-intensive systems. We build on earlier work in which we proposed featured transitions systems (FTSs), a compact mathematical model for representing the behaviors of a variability-intensive system. The FTS model checking algorithms verify all products of a family at once and pinpoint those that are faulty. This paper complements our earlier work, covering important theoretical aspects such as expressiveness and parallel composition as well as more practical things like vacuity detection and our logic feature LTL. Furthermore, we provide an in-depth treatment of the FTS model checking algorithm. Finally, we present SNIP, a new model checker for variability-intensive systems. The benchmarks conducted with SNIP confirm the speedups reported previously.

On extracting feature models from product descriptions

In product line engineering, domain analysis is the process of analyzing related products to identify their common and variable features. This process is generally carried out by experts on the basis of existing product descriptions, which are expressed in a more or less structured way. Modeling and reasoning about product descriptions are error-prone and time consuming tasks. Feature models (FMs) constitute popular means to specify product commonalities and variabilities in a compact way, and to provide automated support to the domain analysis process. This paper aims at easing the transition from product descriptions expressed in a tabular format to FMs accurately representing them. This process is parameterized through a dedicated language and high-level directives (e.g., products/features scoping). We guarantee that the resulting FM represents the set of legal feature combinations supported by the considered products and has a readable tree hierarchy together with variability information. We report on our experiments based on public data and characterize the properties of the derived FMs.

Feature model extraction from large collections of informal product descriptions

Feature Models (FMs) are used extensively in software product line engineering to help generate and validate individual product configurations and to provide support for domain analysis. As FM construction can be tedious and time-consuming, researchers have previously developed techniques for extracting FMs from sets of formally specified individual configurations, or from software requirements specifications for families of existing products. However, such artifacts are often not available. In this paper we present a novel, automated approach for constructing FMs from publicly available product descriptions found in online product repositories and marketing websites such as SoftPedia and CNET. While each individual product description provides only a partial view of features in the domain, a large set of descriptions can provide fairly comprehensive coverage. Our approach utilizes hundreds of partial product descriptions to construct an FM and is described and evaluated against antivirus product descriptions mined from SoftPedia.

Visual syntax does matter: improving the cognitive effectiveness of the i * visual notation

Goal-oriented modelling is one of the most important research developments in the requirements engineering (RE) field. This paper conducts a systematic analysis of the visual syntax of i*, one of the leading goal-oriented languages. Like most RE notations, i* is highly visual. Yet surprisingly, there has been little debate about or modification to its graphical conventions since it was proposed more than a decade ago. We evaluate the i* visual notation using a set of principles for designing cognitively effective visual notations (the Physics of Notations). The analysis reveals some serious flaws in the notation together with some practical recommendations for improvement. The results can be used to improve its effectiveness in practice, particularly for communicating with end users. A broader goal of the paper is to raise awareness about the importance of visual representation in RE research, which has historically received little attention.