> A monolith sending requests to a database instance is already a distributed system.
True, of course.
And even a simple set like this brings in "distribution" issues for the app developper:
When do you connect ? When do you reconnect ?
Where do you get your connection credentials from ?
What should happen when those credentials have to change ?
Do you ever decide to connect to a backup db ?
Do you ever switch your application logic to a mode where you know the DB is down, but you still try to work without it anyway ?
Etc..
Those examples are specific to DBS, but in a distributed system any other services brings in the same questions.
With experience you get opinions and intuitions about how to attack each issues ; my question is still : "should you need to point a newcomer to some reference / book about those questions, where would you point to ?"
People opening a page in a browser that sends requests to a server is already a distributed system.
A monolith sending requests to a database instance is already a distributed system.
Having a metrics sidecar running along your monolith is already a distributed system.