Microservice APIs represent the client perspective on microservice-based software architecture design and related practices. Major issues in API de-sign concern the quality aspects of the API. However, it is not well understoodtoday what the established practices related to those quality aspects are, how thesepractices are related, and what the major decision drivers are. This leads to greatuncertainty in the design process. In this paper, we report on a qualitative, in-depth study of 31 widely used APIs plus 24 API specifications, standards, andtechnologies. In our study we identified six recurring architectural design deci-sions in two API design contexts with a total of 40 decision options and a total of47 decision drivers. We modelled our findings in a formal, reusable architecturaldecision model. We measured the uncertainty in the resulting design space withand without use of our model, and found that a substantial uncertainty reductioncan be potentially achieved by applying our model.

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