I have a REST API. The fundamentals of the API takes care of two things.
Let's say I was about to do a small service (microservice whatever...) next to the API and have it defined in it's own scope which could be a DLL or another service endpoint perhaps.
Then I would go back to the REST API and define a set of routes for interacting with that other small service (DLL or service endpoint).
Now, let's say that the other service needs to know about the authenticated user. Perhaps a user id, because it wants to return all the data inside that specific service. Should I just pass the user id to the service?
The above is just a small example of simple data that can be passed to another small service. But what if more than 2 services need to know about each other? That could be an email service that need to know whom has opened the email store that via another service because we need statistics. Etc.
How do I define and "make rules" around the boundaries of these different services?
Bear in mind that I want to keep this as stupid-simple as possible :)