Service-based applications have to continuously and dynamically self-adapt in order to timely react to changes in their context, as well as to efficiently accommodate for deviations from their expected functionality or quality of service. Currently, self-adaptation is triggered by monitoring events. Yet, monitoring only observes changes or deviations after they have occurred. Therefore, self-adaptation based on monitoring is reactive and thus often comes too late, e.g., when changes or deviations already have led to undesired consequences. In this paper we present the PROSA framework, which aims to enable proactive self-adaptation. To this end, PROSA exploits online testing techniques to detect changes and deviations before they can lead to undesired consequences. This paper introduces and illustrates the key online testing activities needed to trigger proactive adaptation, and it discusses how those activities can be implemented by utilizing and extending existing testing and adaptation techniques.