[javanonymous]

There are tools for enforcing boundaries.

One name for this is "Modulith" where you use modules that have a clear enforced boundary. You get the same composability as micro-services without the complexity.

Here's how Spring solves it: https://www.baeldung.com/spring-modulith

It's basically a library that ensures strict boundaries. Communication has to go through an interface (similar to service api) and you are not allowed to leak internal logic such as database entities to the outer layer

If you later decide to convert the module into a separate service, you simply move the module to a new service and write a small API layer that uses the same interface. No other code changes are necessary.

This enables you to start with a single service (modulith) and split into microservices later if you see the need for it without any major refactoring