  Several studies analyzed existing Web APIs against the constraints of REST to
estimate the degree of REST compliance among state-of-the-art APIs. These
studies revealed that only a small number of Web APIs are truly RESTful.
Moreover, identified mismatches between theoretical REST concepts and practical
implementations lead us to believe that practitioners perceive many rules and
best practices aligned with these REST concepts differently in terms of their
importance and impact on software quality. We therefore conducted a Delphi
study in which we confronted eight Web API experts from industry with a catalog
of 82 REST API design rules. For each rule, we let them rate its importance and
software quality impact. As consensus, our experts rated 28 rules with high, 17
with medium, and 37 with low importance. Moreover, they perceived usability,
maintainability, and compatibility as the most impacted quality attributes. The
detailed analysis revealed that the experts saw rules for reaching Richardson
maturity level 2 as critical, while reaching level 3 was less important. As the
acquired consensus data may serve as valuable input for designing a
tool-supported approach for the automatic quality evaluation of RESTful APIs,
we briefly discuss requirements for such an approach and comment on the
applicability of the most important rules.
