Runner tracking in a city marathon's spectator app

Your city's marathon has 30,000 runners, and each wears a chip on their shoe. Timing mats read the chip at the start, every 5 km and at the finish line: 10 mats in all. A mat sends each read to your servers within a few seconds over mobile data. Each read holds the chip number, the mat and the time of the read. A runner crossing a mat is often read several times.

Friends and family follow runners in your app. About 200,000 people use it on race day, following 3 runners each on average. A few top runners have 20,000 followers each. Followers want a push notification within 30 seconds of a runner crossing a mat. The app also shows each followed runner's time at the last mat they crossed and their pace so far.

How does a mat read become a notification, and what does the app show for a runner?