July 6 – 11 , 2014, Dagstuhl Seminar 14281
Feature Interactions: The Next Generation
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A major goal of software and systems engineering is to construct systems from reusable parts, which we call features (end-user--visible units of behavior or increments in system functionality). Such a compositional approach can decrease time to market, improve product quality, and diversify the product portfolio. However, the success of a compositional approach depends on the modularity of the reusable parts. The quest for modularity has a long tradition in software and systems engineering, programming languages research, and even in newer fields such as synthetic biology.
In the early days of software and systems engineering, the feature-interaction problem was identified (and coined) as a major threat to modularity [8,31,25]. A feature interaction occurs when the behavior of one feature is affected by the presence of another feature. Often the interaction cannot be deduced easily from the intended behaviors of the individual features involved. A canonical example is the inadvertent interaction between the call-forwarding and call-waiting features of a telephony system : If both features are active, the system can reach an undefined possibly unsafe state when it receives a call on a busy line, because it is not specified whether the call should be suspended or forwarded. Alternatively, a feature interaction can be planned: for example, advanced cruise-control features are designed to interact with and extend basic cruise control.
To be safe, software developers must analyze the consequences of all possible feature interactions, in order to find and fix the undesired interactions. The feature-interaction problem is that the number of potential interactions to consider is exponential in the number of features. As a result, software developers find that their work in developing new features is dominated by the tasks to detect, analyze, and verify interactions.
The feature-interaction problem is deeply rooted in the fact that the world is often not compositional [25,20]. That is, a feature is not an island. It communicates and cooperates with other features and the environment, so it cannot be completely isolated. Insights from complex-systems research suggest that feature interactions are a form of emergent behavior that is inherent to any system that consists of many, mutually interacting parts. So, emergent system behavior - which is not deducible from the individual parts of a system - can be observed in many situations including in quantum systems (e.g., superconductivity), biological systems (e.g., swarm intelligence), and economical systems (e.g., trading market crashes). The challenge is to foster and manage desired interactions and to detect, resolve, and even avoid undesired feature interactions - in a scalable manner.
The feature-interaction problem became a crisis in the telecommunications industry in the late 1980s . To handle complexity, there was the strong desire to compose systems from independently developed features, but there was no means to detect, express, and reason about feature interactions. Researchers responded with formalisms that enable automatic detection of feature interactions [4, 7, 15, 14, 21, 26], architectures that avoid classes of interactions [17, 29, 18, 28, 31], and techniques for resolving interactions at run-time [16, 27]. Architectural solutions have been the most successful because they impose general coordination strategies (i.e., serial execution) that apply to all features that are 'plugged' into the architecture, thereby, addressing the scalability issue at the heart of the feature-interaction problem. In coordination-based approaches, such as BIP [2, 3] or Composition Patterns , the interactions among a set of features are specified explicitly and can be specialized for subsets of features.
While the pioneering work on the feature-interaction problem in telecommunication systems was foundational and very successful , it is limited in the sense that it is based on assumptions that hold for telecommunication systems, but that do not hold in other domains. For example, architecture-based approaches take advantage of the fact that communication takes place over a mostly serial connection between communicating parties -- which is not the case in systems made up of parallel components (e.g., service systems, automotive software) or software product lines (e.g., features implemented via conditional compilation such as the Linux kernel). Specifying interactions explicitly is not a general solution either. When facing systems composed of thousands of features, attempting to identify and model a possibly exponential number of feature interactions is elusive. Furthermore, the highly dynamic nature of feature (or service) composition in self-adaptive systems, dynamic product lines, cloud computing, and systems of systems imposes a new class of challenges to solving the feature-interaction problem [24, 9, 1]. So, it is not surprising that different notions of feature interactions have emerged in different communities . Instances of the feature-interaction problem have been observed and addressed in Internet applications , service systems , automotive systems , software product lines , requirements engineering , computational biology , and in many other fields outside of computer science. While all instances of the problem are rooted in the nature of modularity and compositionality [25, 20] the individual views, interpretations, and possible solutions differ considerably. For example, the view on feature interactions taken in program synthesis  differs significantly from the view in automotive systems engineering : there are structural vs. behaviour views, static vs. dynamic views, sequential vs. parallel views, functional vs. non-functional, coordinated vs. emergent-behaviour views, and so on. It turns out that feature interactions are a much more general concept than investigated in the past in the context of telecommunication systems, but a classification, comparison, and generalization of the multitude of different views is missing.
The feature-interaction problem is still of pivotal importance in various industrial applications, but, despite significant efforts, it is far from being solved. The underlying hypothesis of organizing a Dagstuhl seminar on this topic was that the time is ripe to gather researchers and practitioners from different areas of computer science and other disciplines to compare, discuss, and consolidate their views, experience, and domain-specific solutions to the feature-interaction problem. To make progress, scientific discourse on the feature-interaction problem must be based on a broader foundation to be able to join forces of different communities. Can other domains learn from the success of domain-specific solutions for telecommunication systems? Are there key principles, patterns, and strategies to represent, identify, manage, and resolve feature interactions that are domain-independent, that are valid and useful across domains? Or, should we strive for domain-specific solutions that are only loosely related to solutions from other domains? Can we develop a unified terminological and conceptual framework for feature-interaction research? Is that even possible or meaningful, given that interactions in telecommunication systems and emergent behavior and phase transitions in swarm systems are, although related, quite different views?
Goals of the Seminar and Further Activities
It is our goal and firm belief that the feature-interaction problem needs to be viewed from a broader perspective. While feature interactions are still a major challenge in software and systems engineering, both in academia and industry, research on the feature-interaction problem has diversified and diverged in the last decade. Researchers working on similar problems, but in different contexts, are largely disconnected and unaware of related work. A major goal of the seminar was to (re)launch a sustained research community that embraces researchers and practitioners from different fields within and outside computer science. We firmly believe that we reached this goal with our seminar. In particular, a subset of the participants is going to organize a follow-up seminar that directly builds on this seminar's results. The next major milestone will be -- now as we gained a better understanding of the similarities and differences between the different notions of feature interactions -- to establish a catalog on feature-interaction patterns and solutions thereof. The idea for this pattern catalog arose from the final panel session of the seminar. It is inspired by work on patterns in architecture (of buildings). Such a catalog will be the necessary basis for further research on leveraging patterns for detecting, managing, and resolving feature interactions in different kinds of systems.
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Creative Commons BY 3.0 Unported license
Sven Apel and Joanne M. Atlee and Luciano Baresi and Pamela Zave
Related Dagstuhl Seminar
- 11021: "Feature-Oriented Software Development (FOSD)" (2011)
- Modelling / Simulation
- Semantics / Formal Methods
- Software Engineering
- Feature interactions
- Feature-interaction problem
- Feature orientation
- Product lines