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Usability is an important quality attribute for APIs. To create APIs with good usability, appropriate measurement methods are needed. But currently available methods are cost-and time-expensive and the results are not objective and therefore hard to quantify. API design guidelines give a good overview about important usability factors, but lack a scientific basis. When looking at scientific API usability studies, only a very small area of API design has been researched yet. Existing results don't give enough basis for a good API usability measurement method.In this paper we identify influencing usability factors for the two most common concepts of APIs: classes and methods. We therefore conduct a study with 20 programmers and 2 different API variants and evaluate how differences between the APIs influence usability when instantiating classes and calling methods. The results build a basis for API usability measurement methods and should help design more usable APIs.

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Abstract. Modern distributed software systems must integrate in neartime parallel processes and heterogeneous information sources provided by active, autonomous software systems. Such lively information sources are e.g. sensory data, weather data, traffic data, or booking data, operated by independent distributed sites. The complex integration requires the coordination of these data flows to guarantee consistent global semantics. Design, implementation, analysis and control of distributed concurrent systems are notoriously complex tasks. Petri Nets are widely used to model concurrent activities. However, a higher-level programming abstraction is needed. We propose a new programming model for modeling concurrent coordination patterns, which is based on the idea of "peer workers" that represent re-usable coordination and application components. These components encapsulate behavior, structure distributed data and control flow, and allow integration of pre-existing service functions. A domain-specific language is presented. The usability of the peerbased programming model is evaluated with the Split/Join pattern.