In this section, we will implement the Observer design pattern that we showed previously on Behavioral patterns, but with a concurrent structure and thread safety.
If you remember from the previous explanation, the Observer pattern maintains a list of observers or subscribers that want to be notified of a particular event. In this case, each subscriber is going to run in a different Goroutine as well as the publisher. We will have new problems with building this structure:
Now, the access to the list of subscribers must be serialized. If we are reading the list with one Goroutine, we cannot be removing a subscriber from it or we will have a race.
When a subscriber is removed, the subscriber's Goroutine must be closed too, or it will keep iterating forever and we will run into Goroutine leaks.
When stopping the publisher, all subscribers must stop their Goroutines, too.