Proximity marketing geofence

Design a proximity-marketing geofence system that triggers an action (push notification, order-ahead prompt) when a user approaches one of 200k store locations, for an app with 30M installs. The catch: it must work with minimal battery drain on the device and tolerate phones being offline/backgrounded. Cover where geofence evaluation happens, how you select which fences a device watches, and avoiding spammy/duplicate triggers.