If services need to talk to each other, you have a quadratic problem in learning APIs. Each yellow box in the diagram represents a learning task. SQL had the same problem with data repositories. The solution in SQL was to make the communication protocol very small: Only SELECT, UPDATE and so on. Now you only have to learn the schema of the database, which is domain specific. REST is the same idea. Make the protocol very easy: Only things like GET or POST. Your REST-API can be modeled and used as a domain specific database.