Understanding the Observer design pattern

In the Observer design pattern, an object (Subject) maintains a list of dependents (Observers) so that the Subject can notify all the Observers about the changes that it undergoes using any of the methods defined by the Observer.

In the world of distributed applications, multiple services interact with each other to perform a larger operation that a user wants to achieve. Services can perform multiple operations, but the operation they perform is directly or heavily dependent on the state of the objects of the service that it interacts with.

Consider a use case for user registration where the user service is responsible for user operations on the website. Let's say that we have another service called e-mail service that observes the state of the user and sends e-mails to the user. For example, if the user has just signed up, the user service will call a method of the e-mail service that will send an e-mail to the user for account verification. If the...