Continuous (24/7) smartphone sensing applications are on the rise, especially in the field of health and wellness, e.g., to monitor physical activity, to quickly detect emergencies (e.g., falls), and to provide various context-specific services and tools. Smartphones are able to monitor a large array of human activities and patterns, including a user's mobility, physical activities, social interactions, mobile app usage, or communication events. However, building smartphone sensing applications that operate reliably and efficiently on a continuous basis is challenging. Specifically, in this paper, we describe our experience with building a sensing service for the iOS platform, that has been running on more than 400 devices continuously for more than 9 months to date. We present and discuss a variety of technical and non-technical obstacles and challenges and how they were addressed.
