Event-Driven vs. Polling Architecture

There are two ways for one part of a system to find out that something changed somewhere else: the place where it changed announces it (event-driven), or the interested part keeps checking (polling).

This is close to the push and pull question, but the subject is different. That lesson was about delivering updates to a user's device. This one is about how the components inside your system learn about each other's work, and how that choice shapes the architecture.