  While many maintainability metrics have been explicitly designed for
service-based systems, tool-supported approaches to automatically collect these
metrics are lacking. Especially in the context of microservices,
decentralization and technological heterogeneity may pose challenges for static
analysis. We therefore propose the modular and extensible RAMA approach
(RESTful API Metric Analyzer) to calculate such metrics from machine-readable
interface descriptions of RESTful services. We also provide prototypical tool
support, the RAMA CLI, which currently parses the formats OpenAPI, RAML, and
WADL and calculates 10 structural service-based metrics proposed in scientific
literature. To make RAMA measurement results more actionable, we additionally
designed a repeatable benchmark for quartile-based threshold ranges (green,
yellow, orange, red). In an exemplary run, we derived thresholds for all RAMA
CLI metrics from the interface descriptions of 1,737 publicly available RESTful
APIs. Researchers and practitioners can use RAMA to evaluate the
maintainability of RESTful services or to support the empirical evaluation of
new service interface metrics.
