ASCENS Project Blog

Is your software adaptive?

As business processes and software architectures have started to extend beyond the boundaries of corporate systems through the Internet, computing systems have been facing a complete new set of expectations as well as market opportunities. Not only there has been an increasing complexity to be met for guaranteeing that the offered networks and services are trustworthy, but the need of designing and implementing open, loosely coupled services has come together with rapidly changing requirements to be met, with severe time-to-market constraints to be met and with unpredictable service compositions to be anticipated. In principle, autonomic systems are the best answer to these challenges, because they are capable to self-adapt to changing scenarios. In practice, there is no common agreement in the literature on what makes a system be adaptive, so that autonomic systems can be engineered under completely different and possibly conflicting strategies and operating guidelines.

Self-adaptive systems have been widely studied in several disciplines ranging from Biology to Economy and Sociology and there it is widely accepted that a system is self-adaptive if it can modify its behaviour as a reaction to a change in its context (e.g., the environment, the knowledge accumulated). But the problem with such definition emerges during the translation to the Computer Science setting, because it becomes too ambiguous to be helpful in discriminating adaptive systems from non-adaptive ones. In fact, most programs can change behaviour (as they can execute conditional statements, handle exceptions, etc.) as a reaction to a change in their context (as they can input data, modify parameters, etc.). And then, must an adaptive system always be capable to react to any change? Or does it suffice that it may adapt to some changes? Typically the behaviour is changed to improve some measure of performance or degree of satisfaction, but there can be no warranty about that, so what if the opposite happens? Does a system that decreases its performance deserve to be called adaptive?

As an example, consider the scenario of a speaker at a meeting that wants to connect her notebook to the beamer for given her presentation. The software of the notebook and of the beamer has been designed separately, without knowing which kind of beamer (resp. notebook) will be connected to, an agreement must be found for choosing the resolution at which the beamer will work and it is maybe the case that the resolution of the screen of the laptop will change as a consequence. Given these premises, one is lead to conclude that beamer and the notebook are adaptive systems. But what if we are told that the beamer is controlled by a loop that tries to synch with the notebook, starting from the highest resolution to lower ones until a common resolution is found? Isn't this just a default behaviour? Would you still call the beamer adaptive? And the notebook?

As a more interesting example, take the swarms of robots studied in the ASCENS case studies and one of the many challenging adaptation scenarios, like the hill crossing one, where the robots must self-assemble
if they encounter a hill to steep to climb alone. Looking at one of the video demo is impressive: the robots try and fail several time until they finally succeed (or maybe some robots is left alone behind). There is little doubt in calling the software of the robots (self-)adaptive. Still, a closer look at the so-called basic self-assembling strategy they run reveal a finite state machine controlling the adaptation strategy, not much different than a few conditional statement in a program in the end.

Puzzled by the above examples and many similar ones, we have come to the conclusion that the main problem with definition given above is that it relies on a black box view of the system, where an observer tries to classify the system without looking at the software it runs. This is reasonable in many other fields where adaptive systems have been studied, like Biology, because we cannot have a direct look at the "software" and we need to guess it. But the assumption is no longer valid in software systems, especially when the designer perspective is taken, not the user one.

The ambiguity becomes less striking when we take a white boxview of the system. In fact, many architectural approaches to autonomic systems take radically different guidelines on the way they can be structured or composed and on the way they operate. However, in most cases, such approaches are prescriptive: they take an underlying principle and explain how to build autonomic system according to that principle. For example, in the FORMS approach (FOrmal Reference Model for Self-adaptation) reflection is taken as a necessary criterion for any self-adaptive system: roughly, it must be capable to represent and manipulate a meta-level description of itself / of its behaviour. In the MAPE-K approach a self-adaptive system is made of a component implementing the application logic, equipped with a control loop that Monitors the execution through sensors, Analyses the collected data, Plans an adaptation strategy, and finally Executes the adaptation of the managed component through effectors; all the phases of the control loop access a shared Knowledge repository. While FORMS and MAPE-K are not necessarily conflicting, they suggest diverging architectures: in the case of MAPE-K, a strictly hierarchical structure is recommended, where the application logic of one layer implements the adaptation logic of the layer below and the two are to be kept well separated; in the case of FORMS it is favoured a mixture of application logic and adaptation logic by the use of reflection at the level of the application logic.

In a recent work we have proposed an architecture-agnostic, conceptual framework for adaptation, where the white box view of the system is essential. The crux of the framework is that the decision of whether a system is adaptive or not is a relative matter, not an absolute one.

According to a traditional paradigm, a program governing the behaviour of a component is made of control and data : these are two conceptual ingredients that in presence of sufficient resources (like computing power, memory or sensors) determine the behaviour of the component. In order to introduce adaptivity in this framework, we require to make explicit the fact that the behaviour of a component depends on some well identified control data, without being prescriptive on the way it is marked as such. Then: we define adaptation just as the run-time modification of the control data; we say that a system is adaptive if its control data are modified at run-time, at least in some of its executions; and we say that a system is self-adaptive if it is able to modify its own control data at run-time.

Ideally, a sensible collection of control data should be chosen to enforce a separation of concerns, allowing to distinguish neatly, if possible, the activities relevant for adaptation (those that affect the control data) from those relevant for the application logic only (that should not modify the control data). Then, any computational model or programming language can be used to implement an adaptive system, just by identifying the part of the data governing the behaviour. Consequently, the nature of control data can greatly vary depending on the degree of adaptivity of the system and on the computational formalisms used to implement it. Examples of control data include configuration variables, rules (in rule-based programming), contexts (in context-oriented programming), interactions (in connector-centred approaches), policies (in policy-driven languages), aspects (in aspect-oriented languages), monads and effects (in functional languages), and even entire programs (in models of computation exhibiting higher-order or reflective features).

So, coming back to the question posed in the title of this post, if you need to decide if you software system is adaptive or not, you need to name your control data first, then the rest will follow.





Posted by bruni

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