Flexible Formality Practical Experience with Agile Formal Methods

Abstract

Agile software development and Formal Methods are traditionally seen as being in conflict. From an Agile perspective, there is pressure to deliver quickly, building vertical prototypes and doing many iterations/sprints, refining the requirements; from a Formal Methods perspective, there is pressure to deliver correctly and any change in requirements often necessitates changes in the formal specification and might even impact all arguments of correctness. Over the years, the need to "be agile" has become a kind of mantra in software development management, and there is a prevalent prejudice that using formal methods was an impediment to being agile. In this paper, we contribute to the refutation of this stereotype, by providing a real-world example of using good practices from formal methods and agile software engineering to deliver software that is simultaneously reliable, effective, testable, and that can also be iterated and delivered rapidly. We thus present how a lightweight software engineering methodology, drawing from appropriate formal methods techniques and providing the benefits of agile software development, can look like. Our methodology is informed and motivated by practical experience. We have devised and adapted it in the light of experience in delivering a large-scale software system that needs to meet complex real-world requirements: the Cardano blockchain and its cryptocurrency ada. The cryptocurrency domain is a rather new application area for which no clear engineering habit exists, so it is fitting well for agile methods.

Mind Your Outcomes: Quality-Centric Systems Development

“…This paper has presented the ∆QSD systems development process, that is steered by performance predictability concerns, and that is supported by a rigorous formalism (Section 5). Our formalisation of ∆QSD is a part of a wider initiative both within PNSol and IOHK [10]. ∆QSD has been successfully used in a wide range of industries, including telecommunications, avionics, space and defence, and cryptocurrency.…”

“…For compositional properties, what is "true" about subsystems (e.g., their timeliness, their resource consumption) is also "true" about their (appropriate) combination: there exists an invariant (e.g., timeliness, aspects of functional correctness) that must hold over the reified components of the system. In the broader software development space, functional programming techniques are improving the compositionality of functional aspects of software systems, and can deliver high assurance of functional correctness when combined with appropriate formal methods [10]. The paradigm presented here, which we call ∆QSD, represents a similar step change in handling the "non-functional" aspects of performance and resource consumption.…”

Mind Your Outcomes: The ΔQSD Paradigm for Quality-Centric Systems Development and Its Application to a Blockchain Case Study

“…In the broader software development space, functional programming techniques are improving the compositionality of functional aspects of software systems, and they can deliver high assurance of functional correctness when combined with appropriate formal methods [9]. The ∆QSD paradigm represents a similar step change in handling the "nonfunctional" aspects of performance and resource consumption.…”

Mind Your Outcomes: The ∆Q approach to Quality-Centric Systems Development and Its Application to a Blockchain Case-Study

“…In the broader software development space, functional programming techniques are improving the compositionality of functional aspects of software systems, and can deliver high assurance of functional correctness when combined with appropriate formal methods [10]. The paradigm presented here, which we call ∆QSD, represents a similar step change in handling the "non-functional" aspects of performance and resource consumption.…”