Another very similar pattern is the Publish-Subscribe pattern, usually abbreviated as Pub-Sub. This pattern is almost exactly the same as the previous one. You have Subscribers that, as the name implies, subscribe to a specific event, or topic, or whatever you want to call it, and then you have Publishers that emit those events or send information to those topics:

The difference to the previous pattern may look very thin but is actually very important. The Pub-Sub pattern involves a third-party service and, unlike the Observer pattern, the Publishers have no knowledge of the Subscribers. This removes the need to handle and directly notify the Subscribers, thereby simplifying your code.

This pattern is quite useful for microservice communication. It involves a third party that abstracts the state change notification. Also, Publishers and Subscribers have no knowledge of each other.