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

Abstract

This paper directly addresses a long-standing issue that affects the development of many1 complex distributed software systems: how to establish quickly, cheaply, and reliably whether2 they can deliver their intended performance before expending significant time, effort and money on3 detailed design and implementation. We describe ∆QSD, a novel metrics-based and quality-centric4 paradigm that uses formalised outcome diagrams to explore the performance consequences of design5 decisions, as a performance blueprint of the system. The distinctive feature of outcome diagrams is6 that they capture the essential observational properties of the system, independent of the details of7 system structure and behaviour. The ∆QSD paradigm derives bounds on performance expressed as8 probability distributions encompassing all possible executions of the system. The ∆QSD paradigm9 is both effective and generic: it allows values from various sources to be combined in a rigorous10 way, so that approximate results can be obtained quickly and subsequently refined. ∆QSD has been11 successfully used by Predictable Network Solutions for consultancy on large-scale applications in a12 number of industries, including telecommunications, avionics, and space and defence, resulting in13 cumulative savings worth billions of US dollars. The paper outlines the ∆QSD paradigm, describes14 its formal underpinnings, and illustrates its use via a topical real-world example taken from the15 blockchain/cryptocurrency domain. ∆QSD has enabled challenging throughput targets to be met for16 a globally distributed blockchain operating on the public internet.