  Web APIs may have constraints on parameters, such that not all parameters are
either always required or always optional. Moreover, the presence or value of
one parameter could cause another parameter to be required, or parameters could
have restrictions on what kinds of values are valid. Having a clear overview of
the constraints helps API consumers to integrate without the need for
additional support and with fewer integration faults. We made use of existing
documentation and code analysis approaches for identifying parameter
constraints in complex web APIs. In this paper, we report our case study of
several APIs at Adyen, a large-scale payment company that offers complex Web
APIs to its customers. Our results show that the documentation- and code-based
approach can identify 23% and 53% of the constraints respectively and, when
combined, 68% of them. We also reflect on the current challenges that these
approaches face. In particular, the absence of information that explicitly
describes the constraints in the documentation (in the documentation analysis),
and the engineering of a sound static code analyser that is sensitive to
data-flow, maintains longer parameter references throughout the API's code, and
that is able to symbolically execute the several libraries and frameworks used
by the API (in the static analysis).
