  Event-driven architecture has been widely adopted in the software industry,
emerging as an alternative to the development of enterprise applications based
on the REST architectural style. However, little is known about the effects of
event-driven architecture on modularity while enterprise applications evolve.
Consequently, practitioners end up adopting it without any empirical evidence
about its impacts on essential indicators, including separation of concerns,
coupling, cohesion, complexity and size. This article, therefore, reports an
exploratory study comparing event-driven architecture and REST style in terms
of modularity. A real-world application was developed using an event-driven
architecture and REST through five evolution scenarios. In each scenario, a
feature was added. The generated versions were compared using ten metrics. The
initial results suggest that the event-driven architecture improved the
separation of concerns, but was outperformed considering the metrics of
coupling, cohesion, complexity and size. The findings are encouraging and can
be seen as a first step in a more ambitious agenda to empirically evaluate the
benefits of event-driven architecture against the REST style.
