Design a check-in service that automatically checks users out when they leave a defined proximity range.

To design an automatic check-in/check-out service, we'll focus on core functionalities like real-time location tracking, geofencing for check-ins/check-outs, and storing historical visit data. The system should allow users to check into nearby places and automatically mark them as checked out when they leave a defined proximity. We'll also include analytics to track check-in frequency per user and location, and identify popular spots. Key non-functional requirements include low latency for immediate updates, high availability to ensure continuous operation, and eventual consistency for data synchronization. The system needs to handle a high volume of daily active users and a significant amount of historical data, necessitating a scalable architecture. A potential high-level design could involve a user-facing application interacting with a location service, which in turn triggers check-in/check-out events and data storage.