Abstract: The problem of mitigating uncertainty in self-adaptation has driven much of the research proposed in the area of software engineering for self-adaptive systems in the last decade. Although many solutions have already been proposed, most of them tend to tackle specific types, sources, and dimensions of uncertainty (e.g., in goals, resources, adaptation functions) in isolation. A special concern are the aspects associated with uncertainty modeling in an integrated fashion. Different uncertainties are rarely independent and often compound, affecting the satisfaction of goals and other system properties in subtle and often unpredictable ways. Hence, there is still limited understanding about the specific ways in which uncertainties from various sources interact and ultimately affect the properties of self-adaptive, software-intensive systems. In this SoSym expert voice, we introduce the Uncertainty Interaction Problem as a way to better qualify the scope of the challenges with respect to representing different types of uncertainty while capturing their interaction in models employed to reason about self-adaptation. We contribute a characterization of the problem and discuss its relevance in the context of case studies taken from two representative application domains. We posit that the Uncertainty Interaction Problem should drive future research in software engineering for autonomous and self-adaptive systems, and therefore, contribute to evolving uncertainty modeling towards holistic approaches that would enable the construction of more resilient self-adaptive systems.

Abstract: Self-adaptive systems have the ability to adapt their behavior to dynamic operating conditions. In reaction to changes in the environment, these systems determine the appropriate corrective actions based in part on information about which action will have the best impact on the system. Existing models used to describe the impact of adaptations are either unable to capture the underlying uncertainty and variability of such dynamic environments, or are not compositional and described at a level of abstraction too low to scale in terms of specification effort required for non-trivial systems. In this paper, we address these shortcomings by describing an approach to the specification of impact models based on architectural system descriptions, which at the same time allows us to represent both variability and uncertainty in the outcome of adaptations, hence improving the selection of the best corrective action. The core of our approach is a language equipped with a formal semantics defined in terms of Discrete Time Markov Chains that enables us to describe both the impact of adaptation tactics, as well as the assumptions about the environment. To validate our approach, we show how employing our language can improve the accuracy of predictions used for decision-making in the Rainbow framework for architecture-based self-adaptation.

Abstract: This paper argues that change alone is sufficiently difficult; simply, handling change, i.e., anticipated changes, remains the ultimate challenge of self-adaptation.

Abstract: The question of whether “handling unanticipated changes is the ultimate challenge for self-adaptation” is impossible to evaluate without looking closely at what “unanticipated” means. In this position paper I try to bring a little clarity to this issue by arguing that the common distinction between “known unknowns” and “unknown unknowns” is too crude: for most systems there are changes that are not directly handled by “first-order” adaptation, but can, with appropriate engineering, be addressed naturally through “second-order” adaptation. I explain what I mean by this and consider ways in which such systems might be engineered.

Abstract: Self-adaptive systems depend on models of themselves and their environment to decide whether and how to adapt, but these models are often affected by uncertainty. While current adaptation decision approaches are able to model and reason about this uncertainty, they do not consider ways to reduce it. This presents an opportunity for improving decision-making in self-adaptive systems, because reducing uncertainty results in a better characterization of the current and future states of the system and the environment (at some cost), which in turn supports making better adaptation decisions. We propose uncertainty reduction as the natural next step in uncertainty management in the field of self-adaptive systems. This requires both an approach to decide when to reduce uncertainty, and a catalog of tactics to reduce different kinds of uncertainty. We present an example of such a decision, examples of uncertainty reduction tactics, and describe how uncertainty reduction requires changes to the different activities in the typical self-adaptation loop.

Abstract: Self-Adaptive systems are expected to adapt to unanticipated run-time events using imperfect information about their environment. This entails handling the effects of uncertainties in decision-making, which are not always considered as a first-class concern. This paper contributes a formal analysis technique that explicitly considers uncertainty in sensing when reasoning about the best way to adapt. We illustrate our approach on a Denial of Service (DoS) attack scenario and present some preliminary results that show the benefits of uncertainty-aware decision-making with respect to using an uncertainty-ignorant approach.

Abstract: In this paper, we argue that the reality of today's software systems requires us to consider uncertainty as a first-class concern in the design, implementation, and deployment of those systems. We further argue that this induces a paradigm shift, and a number of research challenges that must be addressed.

Abstract: Autonomic, or self-adaptive, systems are increasingly important. One of the most prevalent techniques is to adopt a control systems view of the solution: adding a runtime, separate control unit that monitors and adapts the system under consideration. A problem with this paradigm for system engineering is that the control and the system are loosely coupled, introducing a variety of sources of uncertainty. In this paper we describe three specific sources of uncertainty, and briefly explain how we address those in the Rainbow Project.

Abstract: Dynamic adaptive systems based on multiple concurrent applications typically employ optimization models to decide how to allocate scarce resources among the applications and how to tune their runtime settings for optimal quality-of-service according to the preferences of an end user.

Traditionally, such systems have avoided dealing with uncertainty by assuming that current snapshots of the relevant inputs are precise and by solving for an optimal system point. To achieve dynamic behavior, a system performs an optimization loop upon discovering changes in the input variables (e.g. changes in the available level of resources) and adapts the applications according to the new optimal solution. Unfortunately, when certain adaptation actions incur costs, such reactive adaptation strategies suffer from a significant shortcoming: several locally optimal decisions over time may often be less than optimal globally.

By using predictive information about the future values of the problem inputs, we can model and implement an anticipatory adaptation strategy that helps improve the global behavior of the system in many situations. However, modeling predictions requires representing and dealing with uncertainty from different sources. In this paper, we describe our proposed approach to represent multiple sources of uncertainty and outline algorithms for solving the anticipatory configuration problem with predictive inputs.

Abstract: Increasingly software systems operate in environments of change and uncertainty, where the system's ability to satisfy its quality objectives depends on its ability to adapt. Self-adaptation allows these systems to manage this challenge by autonomously adapting to changes in their environments. While self-* systems are designed precisely to manage uncertainty, unexpected changes may violate design assumptions, resulting in the system failing to satisfy its quality attribute requirements. When this occurs, the planner must generate a new plan, an expensive operation for large systems. As autonomous systems increase in size, interconnectedness, and complexity, this cost can quickly become prohibitive.

This thesis addresses this problem by leveraging information contained in prior plans to reduce the replanning necessary to respond to an unexpected change. Even in the face of an unexpected change, some of the insights contained in existing plans are likely to remain applicable. For example, an autonomous aerial vehicle encountering an unexpected obstacle will need to replan to avoid the obstacle, but the drone may be able to return to its prior plan after this maneuver. A larger change will reduce the amount of reuse that is possible, for example changing the drone's mission to fly to a new location, but still, the takeoff and landing procedures may be reused. This thesis reuses existing adaptation plans by seeding a genetic algorithm with these plans. This enables a scalable self-* planner that can replan in complex systems with large search spaces.

While the idea of plan reuse is intuitive, in practice plan reuse is difficult and may even be worse than replanning from scratch if not performed carefully. This dissertation provides reuse enhancing approaches to reduce the evaluation time of candidate plans, an approach for building reusable repertoires of plans and identifying generalizable plan fragments, and a co-evolutionary extension to enable plan reuse for security. The thesis is evaluated on three simulated case study systems, including a cloud-based web service provider, a team of autonomous aerial vehicles, and an enterprise business system under a cyber attack. Ultimately, plan reuse will enable large self-* systems to replan even after unexpected changes.

Abstract: Self-Adaptive systems are expected to adapt to unanticipated run-time events using imperfect information about their environment. This entails handling the effects of uncertainties in decision-making, which are not always considered as a first-class concern. This technical report summarizes a set of existing techniques and insights into addressing uncertainty in self-adaptive systems and outlines a future research agenda on uncertainty management in self-adaptive systems. The material in this report is strongly informed by our own research in the area, and is therefore not necessarily representative of other works.