An Empirical Study on the "Usage of Not" in Real-World JSON Schema Documents (Long Version).

In this paper, we study the usage of negation in JSON Schema data modeling. Negation is a logical operator that is rarely present in type systems and schema description languages, since it complicates decision problems. As a consequence, many software tools, but also formal frameworks for working with JSON Schema, do not fully support negation. As of today, the question whether covering negation is practically relevant, or a mainly theoretical exercise (albeit challenging), is open. This motivates us to study whether negation is really used in practice, for which aims, and whether it could be - in principle - replaced by simpler operators. We have collected the most diverse corpus of JSON Schema documents analyzed so far, based on a crawl of 90k open source schemas hosted on GitHub. We perform a systematic analysis, quantify usage patterns of negation, and also qualitatively analyze schemas. We show that negation is indeed used, following a stable set of patterns, with the potential to mature into design patterns.