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Flexible Formality: Practical Experience with Agile Formal Methods

August/2020, To appear in: TFP '20 (LNCS)

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. At the same time, there is a lot of real monetary value at stake, making it a good fit for using formal methods to ensure high quality and correctness. This paper reports on the issues that have been faced and overcome, and provides a number of real-world lessons that can be used to leverage the benefits of both agile and formal methods in other situations.