Modeling Event Data

A fitness app sends a record to its server each time something happens: a workout starts, a workout ends, a user buys a plan, or a user earns a badge. Each record is an event, and the details sent with it, such as the workout type or its length in minutes, are its properties. The app's event format promises three fields on every event: an ID, the user, and the day it happened.

It promises nothing about the properties. Two event types may share none of them, and a property may change its name from one app version to the next